IMAGE ENHANCEMENT FRAMEWORK

Abstract

An image enhancement system includes an ambient light sensor, a circuit and a processor. The ambient light sensor is operational to measure an ambient light level. The circuit is operational to: generate a histogram based on an input video signal; export the histogram; receive a gray shade look up table; and generate an output video signal by converting a plurality of gray shades in the input video signal based on the gray shade look up table. The processor is operational to: receive the histogram from the circuit; develop the gray shade look up table based on the histogram and the ambient light level; and transfer the gray shade look up table to the circuit.

Description

TECHNICAL FIELD

The present disclosure generally relates to systems and methods for vehicle display image enhancement.

BACKGROUND

In automotive display applications, light sensors have been utilized to automatically control the display luminance as a function of the ambient lighting environment. As the ambient illumination increases, the display luminance increases to maintain visibility of the images. Automatic luminance control methods maintain a comfortable level of viewing brightness, and reduce a display power consumption as the ambient illumination decreases. Although automatic luminance control methods maintain visibility of the symbols for peak-white gray shades, the visibility of lower gray shades may be compromised.

SUMMARY

An image enhancement system is provided herein. The image enhancement system includes an ambient light sensor, a circuit and a processor. The ambient light sensor is operational to measure an ambient light level. The circuit is operational to: generate a histogram based on an input video signal; export the histogram; receive a gray shade look up table; and generate an output video signal by converting a plurality of gray shades in the input video signal based on the gray shade look up table. The processor is operational to: receive the histogram from the circuit; develop the gray shade look up table based on the histogram and the ambient light level; and transfer the gray shade look up table to the circuit.

A method for image enhancement is provided herein, The method includes measuring an ambient light level with an ambient light sensor, generating a histogram with a circuit based on an input video signal, exporting the histogram from the circuit to a processor, receive the histogram at the processor from the circuit, developing a gray shade look up table with the processor based on the histogram and the ambient light level, transferring the gray shade look up table from the processor to the circuit, receiving the gray shade look up table at the circuit from the processor, and generating an output video signal with the circuit by converting a plurality of gray shades in the input video signal based on the gray shade look up table.

A vehicle is provided herein. The vehicle includes an ambient light sensor, a control unit, and a display panel. The ambient light sensor is operational to measure an ambient light level. The control unit is coupled to the ambient light sensor and is operational to generate an input video signal, generate a histogram based on the input video signal, develop a gray shade look up table based on the histogram and the ambient light level, and generate an output video signal by converting a plurality of gray shades in the input video signal based on the gray shade look up table. The display panel is coupled to the control unit and is operational to generate a visible image based on the video output signal.

The above features and advantages and other features and advantages of the present teachings are readily apparent from the following detailed description of the best modes for carrying out the teachings when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a context of a platform.

FIG. 2 illustrates a graph of a histogram of gray shades in an image.

FIG. 3 illustrates a graph of a gray shade ratio.

FIG. 4 illustrates a graph for a first histogram detection.

FIG. 5 illustrates a graph for a second histogram detection.

FIG. 6 illustrates a graph of a first example offset gamma curve.

FIG. 7 illustrates a graph of a comparison between a constant contrast ratio method and a gamma method.

FIG. 8 illustrates a contrast ratio as a function of background luminance.

FIG. 9 illustrates an example illuminance inside an integrating hemisphere.

FIG. 10 illustrates a graph of an example end point slope gamma matching function.

FIG. 11 illustrates a graph of a display luminance with end point slope gamma matching function.

FIG. 12 illustrates a graph of a pupillary light reflex to a blue light stimulus.

FIG. 13 illustrates an intermediate functional block diagram of a system.

FIG. 14 illustrates a functional block diagram of an image enhancement architecture.

FIG. 15 illustrates a graph for a start point function definition.

FIG. 16 illustrates a graph of a gray shade histogram.

FIG. 17 illustrates a graph on an offset gray shade remapping.

The present disclosure may have various modifications and alternative forms, and some representative embodiments are shown by way of example in the drawings and will be described in detail herein. Novel aspects of this disclosure are not limited to the particular forms illustrated in the above-enumerated drawings. Rather, the disclosure is to cover modifications, equivalents, and combinations falling within the scope of the disclosure as encompassed by the appended claims.

DETAILED DESCRIPTION

Embodiments of the disclosure generally provide for a framework to determine gray scale transfer functions for display image enhancements. The gray scale transfer functions generally have three independent functions. A start point function based ambient lighting conditions is used to improve low gray shade visibility. An intermediate gray shade stretching function is used to ensure image gray shade visibility. An end point function is based on white shade image content. The three functions are stitched together using a slope matching gamma function.

In various embodiments, Adaptive Image Enhancement (AIE) and Low Video Enhancement (LVE) is provided in the framework. The adaptive image enhancement generally compensates for distorted linearity of grey level perception under ambient light conditions. The adaptive image enhancement may enhance and optimize perceived overall brightness of image content for given ambient conditions. Roll-off functions are implemented to minimize loss of image details. A local contrast preservation feature ensures legibility of contrast sensitive image information. A prevention of noise enhancement due to excessive gain is implemented for dark grey levels of the images. Robustness for all type of image content, especially for all kinds of Human Machine Interface (HMI) content is also provided.

FIG. 1 illustrates a context of a platform 90 in accordance with one or more exemplary embodiments. The platform 90 generally includes an instrument panel 92. The instrument panel 92 includes a control unit 94 and one or more display panels 100a-100c. The instrument panel 92 may be implemented as part of a vehicle 93. The vehicle 93 may include mobile vehicles such as automobiles, trucks, motorcycles, boats, trains and/or aircraft. In some embodiments, the instrument panel 92 may be part of a stationary object. The stationary objects may include, but are not limited to, billboards, kiosks, and/or marquees. Other types of platforms 90 may be implemented to meet the design criteria of a particular application.

The control unit 94 implements one or more display-drive circuits. The control unit 94 is generally operational to generate control signals that drive the display panels 100a-100c. In various embodiments, the control signals may be configured to provide instrumentation (e.g., speed, tachometer, fuel, temperature, etc.) to at least one display panel 100a-100c (e.g., 100a). In some embodiments, the control signals may also be configured to provide video (e.g., a rear-view camera video, a forward-view camera video, an onboard DVD player, etc.) to the display panels 100a-100c. In other embodiments, the control signals may be further configured to provide alphanumeric information shown on one or more of the display panels 100a-100c.

In various embodiments, the control unit 94 generally comprises at least one microcontroller. The at least one microcontroller may include one or more processors, each of which may be embodied as a separate processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or a dedicated electronic control unit.

The at least one microcontroller may be any sort of electronic processor (implemented in hardware, software executing on hardware, or a combination of both). The at least one microcontroller may also include tangible, non-transitory memory, (e.g., read-only memory in the form of optical, magnetic, and/or flash memory). For example, the at least one microcontroller may include application-suitable amounts of random-access memory, read-only memory, flash memory and other types of electrically-erasable programmable read-only memory, as well as accompanying hardware in the form of a high-speed clock or timer, analog-to-digital and digital-to-analog circuitry, and input/output circuitry and devices, as well as appropriate signal conditioning and buffer circuitry. The at least one microcontroller may be embedded as part of an FPGA or ASIC device.

Computer-readable and executable instructions embodying the present method may be recorded (or stored) in the memory and executed as set forth herein. The executable instructions may be a series of instructions employed to run applications on the at least one microcontroller (either in the foreground or background). The at least one microcontroller may receive commands and information, in the form of one or more input signals from various controls or components in the platform 90 and communicate instructions to the display panels 100a-100c through one or more control signals to control the displays panels 100a-100c.

The display panels 100a-100c are generally mounted to the instrument panel 92. In various embodiments, one or more of the display panels 100a-100c may be disposed inside the platform 90 (e.g., vehicle 93). In other embodiments, one or more of the display panels 100a-100c may be disposed exterior to the platform 90. One or more display panels 100a-100c may implement an active public/privacy viewing modes. One or more display panels 100a-100c may also implement the privacy mode. As illustrated, the display panel 100a may be a cluster display positioned for use by a driver. The display panel 100b may be a console display positioned for use by the driver and a passenger. The display panel 100c may be a passenger display positioned for use by the passenger and the driver.

FIG. 2 illustrates a graph of an example histogram 110 of gray shades in an image in accordance with one or more exemplary embodiments. The image histogram 110 illustrated in the figure is primarily used to determine if little higher shade content (e.g., on the right side of the histogram 110) exists in the image in order to allow the lower shades (e.g., on the left side of the histogram 110) to be stretched. Furthermore, low video enhancement basically offsets the image such that the lower gray shade levels may become visible.

In order to determine a correct function to offset the lower gray shades into the realm of visibility, past human factor studies provide insight. Maintaining a constant contrast ratio may not be a preferred approach because a contrast sensitivity of an observer generally increases as ambient illumination increases. In particular, relatively low contrast ratios may be used at high illumination levels. Furthermore, the forward-field-of view (FFOV) luminance may also be taken into consideration to correct for eye adaptation mismatch while the eye is looking at a bright scene through the windshield and the ambient light level on the display is low due to a shadowing condition.

A human perception adaptation may be considered a straight line on a log-log plot of display background luminance verses information display luminance. Most literature focuses on a legibility of primary information. Burnette studies indicate that a minimum legibility plot is also established that may be more germane as the function for lower luminance levels in a display image because the lower levels (such as shading enhancements) are often used to display secondary information. Additional details of the Burnette studies may be found in the paper “The Status of Human Perceptual Characteristics Data for Electronic Flight Display Design”, Proceedings of AGARD Conference No. 96 on Guidance and Control Displays, Paris, France, 1972, by K. T. Burnette, which is hereby incorporated by reference in its entirety. Additional details for human visual system utilization for display visibility may be found in the paper “The Development and Evaluation of Color Systems for Airborne Applications—Phase I: Fundamental Visual Perceptual, and Display Systems Considerations”, July 1985, by Dr. Louis Silverstein and Robin Hoerner, which is hereby incorporated by reference in its entirety.

Framework:

In various embodiments, the framework is based on a combination of a Silverstein fractional power function and use of constant luminance ratios between sequential gray shades taking into account an ambient reflective light. An ambient light sensor is provided to measure the ambient light level and is proportional to the background luminance seen by the viewer. In order to visibly perceive a first gray shade (GS1), the luminance level of GS1 may be increased to provide a sufficient contrast ratio between GS1 and a reflected background luminance (LBG). It may be noted that the black gray shade, GS0, is not be altered and may be assigned with the output GS0 value. Per the Silverstein visibility criteria, the Emitted Symbol Luminance (ESL) may be determined per Equation 1 as follows:

$\begin{array}{cc}ESL={B_O\left(L_{\mathrm{BG}}\right)}^C& \mathrm{Eq}.\left(1\right)\end{array}$

• • Where:
  • ESL=Emitted Symbol Luminance in cd/m².
  • B_O=Luminance Offset Constant.
  • L_BG=Display Background Luminance in cd/m².
  • c=Power Constant (e.g., a slope of the power function in logarithmic coordinates and is generally a fractional number).

By way of example, a minimum contrast ratio between symbols and the background may be 5:1 for night conditions, 3:1 for twilight conditions, 3:1 for day conditions, and 2:1 for direct sunlight conditions. Using Equation 1, the minimum luminance, L_Min, that the user sees when looking at GS1 may be devised per Equation 2 as follows:

$\begin{array}{cc}{L}_{Min}={B_O\left(L_{\mathrm{BG}}\right)}^C+L_{\mathrm{BG}}& \mathrm{Eq}.\left(2\right)\end{array}$

A maximum luminance that the user sees when looking at the maximum gray shade may be according to Equation 3 as follows:

$\begin{array}{cc}{L}_{M\mathrm{ax}}=L_{\mathrm{DMax}}+L_{\mathrm{BG}}& \mathrm{Eq}.\left(3\right)\end{array}$

• • Where L_DMax is the display maximum luminance value.

Note that in both Equations 2 and 3, the reflected display background luminance, LBG, is added because that is what the user actually sees.

To determine the gray shade luminance values between L_Min and L_Max the concept of constant luminance ratios is introduced. The eye basically sees equal luminance ratios as equal increments in luminance and therefore affords an ability to distinguish all gray shades equally. Equation 4 may be utilized to determine the desired intermediate luminance values as follows:

$\begin{array}{cc}{L}_{\mathrm{SEL}}=\frac{L_{M\mathrm{ax}}}{{\left[\frac{L_{M\mathrm{ax}}}{L_{Min}}\right]}^{\left(\frac{T-N}{T-1}\right)}}& \mathrm{Eq}.\left(4\right)\end{array}$

• • Where T is the total number of luminance levels and N is the gray shade number.

Substituting Equations 2 and 3 into Equation 4 yields Equation 5 as follows:

$\begin{array}{cc}{L}_{\mathrm{SEL}}=\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{\left[\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{B_O\left(L_{\mathrm{BG}}\right)}^C+L_{\mathrm{BG}}}\right]}^{\left(\frac{T-N}{T-1}\right)}}& \mathrm{Eq}.\left(5\right)\end{array}$

Equation 5 may be utilized to determine every Nth gray shade luminance value for 8 bit per color input (N=1 through 255 and T=255).

The gray shade luminance values determined by Equation 5 are decomposed into the new gray shade values, GS_N, based on using a larger number of gray shade levels, such as a 10-bits range. Equation 6 shows the relationship between the display luminance and the new gray shade number, GS_N, and the total number of bits, N_New as follows:

$\begin{array}{cc}{L}_{{\mathrm{GS}}_N}={L_{\mathrm{DMax}}\left(\frac{{\mathrm{GS}}_N}{2^{N_{\mathrm{New}}}-1}\right)}^{\gamma }& \mathrm{Eq}.\left(6\right)\end{array}$

• • Where the display gamma is notated as γ.

A next step is to recognize that the luminance values, L_SEL, determined in Equation 5 includes the reflected background luminance, L_BG. Therefore, the background luminance is subtracted to determine the display luminance. Consequently, using Equation 6, Equation 7 may be formulated as follows:

$\begin{array}{cc}{L}_{\mathrm{SEL}}-L_{\mathrm{BG}}={L_{\mathrm{DMax}}\left(\frac{{\mathrm{GS}}_N}{2^{N_{\mathrm{New}}}-1}\right)}^{\gamma }& \mathrm{Eq}.\left(7\right)\end{array}$

Solving Equation 7 for GS_N yields Equation 8 as follows:

$\begin{array}{cc}{\mathrm{GS}}_N={\left(2^{N_{\mathrm{New}}}-1\right)\left[\frac{L_{\mathrm{SEL}}-L_{\mathrm{BG}}}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{\gamma }\right)}& \mathrm{Eq}.\left(8\right)\end{array}$

Thereafter, Equation 5 may be substituted into equation 8 for L_SEL thereby yielding equation 9 as follows:

$\begin{array}{cc}{\mathrm{GS}}_N={\left(2^{N_{\mathrm{New}}}-1\right)\left[\frac{\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{\left[\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{B_O\left(L_{\mathrm{BG}}\right)}^C+L_{\mathrm{BG}}}\right]}^{\left(\frac{T-N}{T-1}\right)}}-L_{\mathrm{BG}}}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{\gamma }\right)}& \mathrm{Eq}.\left(9\right)\end{array}$

Therefore, Equation 9 may be used to input gray shade numbers (e.g., numbers 1 through 255) for N, and based on the background luminance determined by a light sensor, the new gray shade values, GS_N, may be determined. The gray shade determinations may be equally applied for red, green and blue to maintain a correct white color point. The above concept may also be applied to stretch the dynamic range of a low gray shade image.

FIG. 3 illustrates a graph 120 of a gray shade ratio for a simulation in accordance with one or more exemplary embodiments. The Algo LN ratio Line 122 is the ratio between successive gray shades of the display luminance with LVE compensation. It does not represent what the user will see. The Algo LN+LBG Ratio Line 124 is the ratio between successive gray shades of the display+background luminance with LVE compensation. It represents what the user will see. The γ Ratio Line 126 is the ratio between successive gray shades of the display luminance without LVE compensation. It represents what the user will see with LVE compensation. The γ+LBG Ratio Line 128 is the ratio between successive gray shades of the display+background luminance without LVE compensation. It represents what the user will see without the LVE compensation.

AIE Gray Shade Stretching:

A method to accomplish gray shade expansion or stretching is to use the constant ratio formula, as shown in Equation 9 above. For instance, if the frame histogram as shown in FIG. 2 has a detection threshold of N_Max gray shades, then Equation 9 may be modified to set T=N_Max as shown in Equation 10 as follows:

$\begin{array}{cc}{\mathrm{GS}}_N={\left(2^{N_{\mathrm{New}}}-1\right)\left[\frac{\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{\left[\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{B_O\left(L_{\mathrm{BG}}\right)}^C+L_{\mathrm{BG}}}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}-L_{\mathrm{BG}}}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(10\right)\end{array}$

Note that γ has been changed to γ_D to better convey that the gamma is the target display γ value. Any gray shades greater than T_NewMax is set to T_NewMax.

FIG. 4 illustrates a graph 140 for a first histogram detection in accordance with one or more exemplary embodiments. Using the parameters L_BG=100 nits and CR_GS1=1.12, if a detection threshold is determined to be gray shade 200, the result of setting N_Max=200 in Equation 10 are illustrated in FIG. 4 as an LN+LBG line 142 and a γ+LBG line 144.

FIG. 5 illustrates a graph 150 for a second histogram detection in accordance with one or more exemplary embodiments. If the image becomes even darker and the histogram detection threshold becomes N_Max=100, the resulting LN+LBG line 152 and a γ+LBG line 154 are illustrated in FIG. 5 using the parameters L_BG=100 nits and CR_GS1=1.12.

The results show that the general formula per Equation 10 may be used for both LVE and AIE. It also demonstrates the simplistic nature of the entire system when the constant luminance ratio approach is utilized in conjunction with the fractional power function offset.

Equation 10 may be further simplified by taking the interior term and assigning variables “A” and “B” values as shown in Equation 11 as follows:

$\begin{array}{cc}\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{\left[\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{B_O\left(L_{\mathrm{BG}}\right)}^C+L_{\mathrm{BG}}}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}=\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}& \mathrm{Eq}.\left(11\right)\end{array}$

For convenience, exponent laws are shown in Equations 12 through 16 as follows:

$\begin{array}{cc}{a}^{-n}=\frac{1}{a^n}\left(a\ne 0\right)& \mathrm{Eq}.\left(12\right)\end{array}$ $\begin{array}{cc}{a}^0=1\left(a\ne 0\right)& \mathrm{Eq}.\left(13\right)\end{array}$ $\begin{array}{cc}{a}^m{a}^n=a^{m+n}& \mathrm{Eq}.\left(14\right)\end{array}$ $\begin{array}{cc}{\left(a^m\right)}^n=a^{\mathrm{mn}}& \mathrm{Eq}.\left(15\right)\end{array}$ $\begin{array}{cc}{\left(\mathrm{ab}\right)}^n=a^n{b}^n& \mathrm{Eq}.\left(16\right)\end{array}$

Using Equation (exponent law) 12, Equation 11 may be rewritten as Equation 17 as follows:

$\begin{array}{cc}\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}={A\left[\frac{A}{B}\right]}^{-1\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(17\right)\end{array}$

Using Equation (exponent law) 12 again results in Equation 18 as follows:

$\begin{array}{cc}\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}={{A^1\left[A\right]}^{\left(\frac{N-N_{\mathrm{Max}}}{N_{\mathrm{Max}}-1}\right)}\left[B\right]}^{-1\left(\frac{N-N_{\mathrm{Max}}}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(18\right)\end{array}$

Using Equations (laws 15 and 16), Equation 18 is transformed into Equation 19 as follows:

$\begin{array}{cc}\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}={{A^1\left[A\right]}^{\left(\frac{N-N_{\mathrm{Max}}}{N_{\mathrm{Max}}-1}\right)}\left[B\right]}^{-1\left(\frac{N-N_{\mathrm{Max}}}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(19\right)\end{array}$

Next, using Equation 14, Equation 19 becomes Equation 20 as follows:

$\begin{array}{cc}\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}={{\left[A\right]}^{\left(1+\frac{N-N_{\mathrm{Max}}}{N_{\mathrm{Max}}-1}\right)}\left[B\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(20\right)\end{array}$

The exponent may be modified for a common denominator per Equation 21 as follows:

$\begin{array}{cc}\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}={{\left[A\right]}^{\left(\frac{N_{\mathrm{Max}}-1}{N_{\mathrm{Max}}-1}+\frac{N-N_{\mathrm{Max}}}{N_{\mathrm{Max}}-1}\right)}\left[B\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(21\right)\end{array}$

Adding the numerators for “A”, the exponential term yields Equation 22 as follows:

$\begin{array}{cc}\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}={{\left[A\right]}^{\left(\frac{N-1}{N_{\mathrm{Max}}-1}\right)}\left[B\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(22\right)\end{array}$

Equation 22 may be a best compromise because the results are in a reasonable range. However, Equation 22 may be manipulated further using Equation (law) 15 to yield Equation 23 as follows:

$\begin{array}{cc}\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}={\left[A^{\left(N-1\right)}{B}^{\left(N_{\mathrm{Max}}-1\right)}\right]}^{\left(\frac{1}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(23\right)\end{array}$

Finally using Equation (law) 16, Equation 23 may be manipulated to yield Equation 24 as follows:

$\begin{array}{cc}\frac{A}{{\left[\frac{A}{B}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}}={\left[A^{\left(N-1\right)}{B}^{\left(N_{\mathrm{Max}}-1\right)}\right]}^{\left(\frac{1}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(24\right)\end{array}$

Using the result in Equation 22, Equation 10 may be changed to Equation 25 as follows:

$\begin{array}{cc}G{S}_N={\left(2^{N_{New}}-1\right)\left[\frac{{{\left[A\right]}^{\left(\frac{N-1}{N_{\mathrm{Max}}-1}\right)}\left[B\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}-L_{BG}}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(25\right)\end{array}$

The L_DMax term in the denominator of Equation 25 may be brought outside of the parentheses as shown in Equation 26 as follows:

$\begin{array}{cc}G{S}_N=\left(2^{N_{New}}1\right){L_{\mathrm{DMax}}^{\left(-1/{\gamma }_D\right)}\left[{{\left[A\right]}^{\left(\frac{N-1}{N_{\mathrm{Max}}-1}\right)}\left[B\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}-L_{BG}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(26\right)\end{array}$ $\mathrm{Where}:$ $A=L_{\mathrm{Max}}=L_{\mathrm{DMax}}+L_{BG}.$ $B=L_{\mathrm{Min}}={B_O\left(L_{BG}\right)}^C+L_{BG}.$

• • N=input gray shade number 1-255.
  • N_Max=maximum input gray shade determined from the input video chronometric data.
  • γ_D=gamma of the target display.
  • L_DMax=maximum display luminance (nits).
  • L_BG=reflected background luminance on the display (nits).
  • B_O=Fechner offset constant.
  • c=Fechner slope constant.
  • N_New=new 10 bit or 11 bit value for the target display including FRC.
  • GS_N=new mapped gray level value.

Equation 26 may provide the basis for a simplified implementation compared to Equation 10. Equation 27 may be formulated from Equation 26 by substituting L_Max and L_Min for the variables “A” and “B” respectively.

$\begin{array}{cc}G{S}_N=\left(2^{N_{New}}-1\right){L_{\mathrm{DMax}}^{\left(-1/{\gamma }_D\right)}\left[{{\left[L_{\mathrm{Max}}\right]}^{\left(\frac{N-1}{N_{\mathrm{Max}}-1}\right)}\left[L_{\mathrm{Min}}\right]}^{\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)}-L_{BG}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(27\right)\end{array}$ $\mathrm{Where}:$ $L_{\mathrm{Max}}=L_{\mathrm{DMax}}+L_{BG}$ $L_{\mathrm{Min}}={B_O\left(L_{BG}\right)}^C+L_{BG}$

Another way to think about the system is that the maximum (L_Max) and minimum (L_Min) luminance end point values are determined based on the filtered light sensor value. Equation 26 then calculates all of the new 10-bit or 11-bit values between the minimum and maximum end points. The result of using equation 26 is that successive intermediate gray scale values may have a constant visual contrast ratio that includes the effect of the reflected ambient light (L_BG). A new look up table generated by using equation 27 may be used to convert 8-bit video input gray shades to 10-bit or 11-bit video gray shade output values.

An example of a partial look up table is shown in Table 1 using the following inputs.

• • N_Max=255
  • γ_D=2.2
  • L_DMax=1000 nits
  • L_BG=100 nits
  • B_O=11
  • c=0.273
  • N_New=10 bit

TABLE 1
Look up table example
N     GSN
0     230
1     233
2     237
3     240
4     243
5     246
6     249
7     252
8     255
9     258
10    261
. . . . . .
248   995
249   999
250   1003
251   1007
252   1012
253   1016
254   1020
255   1024

An alternative formula that may be utilized for equation 27 is based on calculating the ratio once and then multiplying the ratio N−1 times the L_Min term. The ratio between adjacent gray shade levels is given by equation 28 as follows:

$\begin{array}{cc}R={\left[\frac{L_{\mathrm{Max}}}{L_{\mathrm{Min}}}\right]}^{\left(\frac{1}{N_{\mathrm{Max}}-1}\right)}& \mathrm{Eq}.\left(28\right)\end{array}$

Therefore the selected gray shade luminance, L_sel, may be determined per equation 29 as follows:

$\begin{array}{cc}{L}_{sel}=R^{\left(N-1\right)}{L}_{\mathrm{Min}}& \mathrm{Eq}.\left(29\right)\end{array}$

However L_sel includes the background luminance, L_BG, that may be subtracted out and equated to the target display luminance and resolution per equation 30 as follows:

$\begin{array}{cc}{L_{\mathrm{DMax}}\left(\frac{G{S}_N}{2^{N}New-1}\right)}^{{\gamma }_D}=R^{\left(N-1\right)}{L}_{\mathrm{Min}}{L}_{BG}& \mathrm{Eq}.\left(30\right)\end{array}$

Solving for GS_N yields equation 31 as follows:

$\begin{array}{cc}G{S}_N=\left(2^{N_{New}}-1\right){L_{\mathrm{DMax}}^{\left(-1/{\gamma }_D\right)}\left[R^{\left(N-1\right)}{L}_{\mathrm{Min}}-L_{BG}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(31\right)\end{array}$

• • Where R is defined in equation 28.

Output Gray Shade Resolution:

An aspect to accomplish AIE-LVE compensation is to increase the display resolution in order to prevent banding due to rounding. For example, an 11-bit resolution may be appropriate to avoid seeing image artifacts.

AIE-LVE Gamma Equation:

Although the previously described concept of using user perceived constant contrast ratios is a desired method to make all shades equally visible to the user, an alternative method is to use an offset gamma function.

FIG. 6 illustrates a graph 160 of a first example offset gamma curve in accordance with one or more exemplary embodiments. As shown in the figure, a bottom of the gamma curve 164 is offset from zero so that black shades become visible and the rest of the shades are determined according to the gamma function. The offset between the curve 162 and the curve 164 is due to the reflected background luminance.

The gamma curve method may start based on equation 32 as follows:

$\begin{array}{cc}{L_{\mathrm{DMax}}\left(\frac{G{S}_N}{2^{N}New-1}\right)}^{{\gamma }_D}=\left(L_{\mathrm{Max}}-L_{\mathrm{Min}}\right){\left(\frac{N}{N_{\mathrm{Max}}}\right)}^{{\gamma }_G}+\left(L_{\mathrm{Min}}-L_{BG}\right)& \mathrm{Eq}.\left(32\right)\end{array}$

• • Where: N=0:N_Max (Input video gray shade numbers);
  • GS_N=new gray shade values.
  • N_Max=maximum input gray shade determined from the input video image gray shade histogram data;
  • N_New=Bit level of the output gray shades such as 8 or 10 bits;
  • Note that the gammas have different subscripts to convey that γ_D=target display gamma (usually 2.2); and
  • γ_G=new desired gamma.

Equation 32 may be solved for the new gray shade numbers GS_N as shown in equation 33 as follows:

$\begin{array}{cc}G{S}_N=\left(2^{N_{New}}-1\right){L_{\mathrm{DMax}}^{\left(-1/{\gamma }_D\right)}\left[\left(L_{\mathrm{Max}}-L_{\mathrm{Min}}\right){\left(\frac{N}{N_{\mathrm{Max}}}\right)}^{{\gamma }_G}+\left(L_{\mathrm{Min}}-L_{BG}\right)\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(33\right)\end{array}$

Using the L_Min definition in Equation 26, the term (L_Min−L_BG)=B_O(L_BG)^c, equation 33 becomes equation 34 as follows:

$\begin{array}{cc}G{S}_N=\left(2^{N_{New}}\right){L_{\mathrm{DMax}}^{\left(-1/{\gamma }_D\right)}\left[\left(L_{\mathrm{Max}}-L_{\mathrm{Min}}\right){\left(\frac{N}{N_{\mathrm{Max}}}\right)}^{{\gamma }_G}+{B_O\left(L_{BG}\right)}^c\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(34\right)\end{array}$

FIG. 7 illustrates a graph 170 of a comparison between the constant contrast ratio method (e.g., curve 172) and the gamma method (e.g., curve 174) in accordance with one or more exemplary embodiments.

B_O and c Constants Analysis:

The selection of the offset, B_O, and slope, c, constants may be approached with a further study of the Burnette data. Since increasing the LVE increases black levels to the visibility threshold and reduces the amount of image AIE stretching, the goal is to increase the lowest-black gray shades (but not pure black gray shade 0) to the level of visibility. Therefore, the analysis may start by investigating three categories on the Burnette plot: Minimum threshold legibility; 50% threshold legibility; and bargraph 99% legibility (note that lower gray shades are not normally used for numeric data).

Table 2 shows some estimated values. As an example, for the bargraph data, Burnette indicates that where the reflected background luminance is 100 fL, a display bargraph luminance of only 6.3 fL is visible 99% of the time.

TABLE 2
Data Points Estimated from Burnette Plot
	LBG (fL)	ESL (fL)
	Minimum threshold legibility	10	0.0794
	Minimum threshold legibility	100	0.5
	50% threshold legibiltiy	1	0.05
	50% threshold legibility	10	0.316
	Bargraph 99% legibility	10	1
	Bargraph 99% legibility	100	6.3

Minimum Threshold Legibility:

Equation 35 and equation 36 may be formulated for the data in Table 2 as follows:

$\begin{array}{cc}0.0794={B_O\left(10\right)}^C& \mathrm{Eq}.\left(35\right)\end{array}$ $\begin{array}{cc}0.5={B_O\left(100\right)}^C& \mathrm{Eq}.\left(36\right)\end{array}$

Dividing equation 36 by equation 35 yields equation 37 as follows:

$\begin{array}{cc}\frac{0.5}{0.0794}=\frac{100^C}{10^C}& \mathrm{Eq}.\left(37\right)\end{array}$

Equation 37 may be rewritten as equation 38 as follows:

$\begin{array}{cc}\frac{0.5}{0.0794}=100^{C}10^{-1\times C}={\left[100\times 10^{-1}\right]}^C={\left[\frac{100}{10}\right]}^C& \mathrm{Eq}.\left(38\right)\end{array}$

Taking the logarithm of equation 38 results in equation 39 as follows:

$\begin{array}{cc}\log \left(\frac{0.5}{0.0794}\right)=\log \left({\left[\frac{100}{10}\right]}^C\right)=C\times \log \left(\frac{100}{10}\right)& \mathrm{Eq}.\left(39\right)\end{array}$

Since the log of 10 is equal to 1, equation 39 simplifies to equation 40 as follows:

$\begin{array}{cc}c=\log \left(\frac{0.5}{0.0794}\right)=0.799& \mathrm{Eq}.\left(40\right)\end{array}$

Using the value of c determined in equation 40, equation 36 may be used to solve for the offset constant B_O per equation 41 as follows:

$\begin{array}{cc}{B}_O=\frac{0.5}{100^{0.799}}=0.0126& \mathrm{Eq}.\left(41\right)\end{array}$

Therefore equation 1 may be written as equation 42, as follows:

$\begin{array}{cc}ESL=0.0126{\left(L_{BG}\right)}^{0.799}& \mathrm{Eq}.\left(42\right)\end{array}$

50% Threshold Legibility:

Using the methods described above and the values from Table 2, a 50% threshold legibility equation 43 may be determined as follows:

$\begin{array}{cc}ESL=0.05{\left(L_{BG}\right)}^{0.8}& \mathrm{Eq}.\left(43\right)\end{array}$

Bargraph 99% Legibility:

Using methods described above and the values from Table 2, a bargraph 99% legibility equation 44 may be determined as follows:

$\begin{array}{cc}ESL=0.159{\left(L_{BG}\right)}^{0.799}& \mathrm{Eq}.\left(44\right)\end{array}$

A next step is to convert the equations 42, 43, and 44 from Imperial footlambert (fL) units to SI nit (cd/m²) units. The slope constant does not change between Imperial and SI units and only the offset constant B_O changes. To keep track of the unit system the labels B_OfL and B_ONit may be utilized. The conversion process starts with writing the general equation in Imperial fL units per equation 45 as follows. Note that equations 42, 43, and 44 are in imperial units.

$\begin{array}{cc}ES{L}_{\mathrm{Nit}}={B_{\mathrm{ONit}}\left(L_{\mathrm{BGNit}}\right)}^C& \mathrm{Eq}.\left(45\right)\end{array}$

A relationship between a footlambert and nits are shown in equation 46 as follows:

$\begin{array}{cc}1fL=3.42\mathrm{Nits}& \mathrm{Eq}.\left(46\right)\end{array}$

Therefore to convert equation 45 to Imperial fL units, equation 47 may be written as follows:

$\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{fL}}\left(\frac{3.42\mathrm{Nits}}{1\mathrm{fL}}\right)={B_{\mathrm{ONit}}\left(L_{\mathrm{BGfL}}\left[\frac{3.42\mathrm{Nits}}{1\mathrm{fL}}\right]\right)}^C& \mathrm{Eq}.\left(47\right)\end{array}$

Therefore equation 47 may be changed to equation 48 and simplified to equation 49 as follows:

$\begin{array}{cc}ES{L}_{\mathrm{fL}}=B_{\mathrm{ONit}}\left[\frac{3.42^C}{3.42}\right]{\left(L_{\mathrm{BGfL}}\right)}^C& \mathrm{Eq}.\left(48\right)\end{array}$ $\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{fL}}=B_{\mathrm{ONit}}{3.42}^{\left(C-1\right)}{\left(L_{\mathrm{BGfL}}\right)}^C& \mathrm{Eq}.\left(49\right)\end{array}$

Therefore to change from Nits to fL, the offset constant B_ONit is multiplied by the factor 3.42^(C-1).

Conversely, to change from fL to Nits, equation 50 may be written as follows:

$\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{fL}}={B_{\mathrm{OfL}}\left(L_{\mathrm{BGfL}}\right)}^C& \mathrm{Eq}.\left(50\right)\end{array}$

Equation 50 may be rewritten as equation 51 using the conversion factor per equation 46 as follows:

$\begin{array}{cc}ES{L}_{\mathrm{Nit}}\left(\frac{1\mathrm{fL}}{3.42\mathrm{Nits}}\right)=B_{\mathrm{OfL}}{\left(L_{\mathrm{BGNit}}\left[\frac{1\mathrm{fL}}{3.42\mathrm{Nits}}\right]\right)}^C& \mathrm{Eq}.\left(51\right)\end{array}$

Therefore equation 51 may be rearranged per equation 52 and simplified to equation 53 as follows:

$\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{Nit}}=B_{\mathrm{OfL}}\left[\frac{3.42}{3.42^C}\right]{\left(L_{\mathrm{BGNit}}\right)}^C& \mathrm{Eq}.\left(52\right)\end{array}$ $\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{Nit}}=B_{\mathrm{OfL}}{3.42}^{\left(1-C\right)}{\left(L_{\mathrm{BGNit}}\right)}^C& \mathrm{Eq}.\left(53\right)\end{array}$

Therefore, to change from the footlambert equation to the nit equation, the offset constant B_OfL is multiplied by the factor 3.43^(1-C).

Using the conversion factor per equation 52, equations 42, 43, and 44 may be converted from Imperial to SI units per the following equations 54, 55, 56, 57, 58, and 59.

Minimum Threshold Legibility:

$\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{Nit}}=0.0126\left[\frac{3.42}{3.42^{0.799}}\right]{\left(L_{\mathrm{BGNit}}\right)}^{0.799}& \mathrm{Eq}.\left(54\right)\end{array}$ $\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{Nit}}=0.016133{\left(L_{\mathrm{BGNit}}\right)}^{0.799}& \mathrm{Eq}.\left(55\right)\end{array}$

50% Threshold Legibility:

$\begin{array}{cc}ES{L}_{\mathrm{Nit}}=0.05\left[\frac{3.42}{3.42^{0.8}}\right]{\left(L_{\mathrm{BGNit}}\right)}^{0.8}& \mathrm{Eq}.\left(56\right)\end{array}$ $\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{Nit}}=0.06394{\left(L_{\mathrm{BGNit}}\right)}^{0.8}& \mathrm{Eq}.\left(57\right)\end{array}$

Bargraph 99% Legibility:

$\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{Nit}}=0.159\left[\frac{3.42}{3.42^{\text{.799}}}\right]{\left(L_{\mathrm{BGNits}}\right)}^{0.799}& \mathrm{Eq}.\left(58\right)\end{array}$ $\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{Nit}}=0.203581{\left(L_{\mathrm{BGNits}}\right)}^{0.799}& \mathrm{Eq}.\left(59\right)\end{array}$

Note that the offset and slope constants are different than the values used earlier where c=0.273 and B_O=11 (nits), 4.5 (fL) and as shown in equations 60, 61 and 62 as follows:

$\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{Nit}}=11{\left(L_{\mathrm{BGNits}}\right)}^{0.273}& \mathrm{Eq}.\left(60\right)\end{array}$ $\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{fL}}=11\left(3.42^{\left(0.273-1\right)}\right){\left(L_{\mathrm{BGfL}}\right)}^{0.273}& \mathrm{Eq}.\left(61\right)\end{array}$ $\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{fL}}=4.5{\left(L_{\mathrm{BGfL}}\right)}^{0.273}& \mathrm{Eq}.\left(62\right)\end{array}$

If the various equations are analyzed in terms of contrast ratio, FIG. 8 and Table 3 result.

FIG. 8 illustrates a graph 180 of a contrast ratio as a function of background luminance in accordance with one or more exemplary embodiments. Table 3 provides contrast ratio versus background luminance.

TABLE 3
Contrast Ratio versus Background Luminance (fL)
LBGfL	Min	50%	Bargraph	BO = 4.5, c = 0.273
1	1.0126	1.05	1.159	5.5
10	1.007932	1.031548	1.100091	1.843748
100	1.004993	1.019905	1.063008	1.158202
1000	1.003143	1.012559	1.039664	1.029663

Line 182 illustrates the Min column. Line 184 illustrates the 50% column. Line 186 illustrates the Bargraph column, Line 188 illustrates the B_O/c column. From Table 3, although Burnette predicts visibility at lower emitted symbol luminance values, the contrast ratios are away from ISO 15008 recommendations of 5:1 for night conditions, 3:1 for twilight conditions, 3:1 for day conditions, and 2:1 for direct sunlight conditions. To determine the various luminance categories, a model is used that converts lux illuminance levels to background luminance that the user sees on the display.

FIG. 9 illustrates a graph 190 of an example illuminance inside an integrating hemisphere in accordance with one or more exemplary embodiments. The inside of a cockpit may be thought of as an integrating hemisphere with uniform wall luminance L as shown in FIG. 9. Although many different actual reflection components exist in an automotive cockpit, many of such sources that are not at the specular or within the haze angles may be thought of as being in the diffuse Lambertian reflection component region. Consequently, the exact angles are not consequential of all sources that are not in the haze/specular region. The amount of sources in the haze region generally subtends a relatively small solid angle and, therefore generally does not need to be subtracted to obtain the Lambertian diffuse component seen by the user. Note that once that illuminance sources are at angles greater than the haze angle, the diffuse reflectance, ρ_d, is constant (e.g., flat) and may not be dependent on the angle from the specular reference angle. Therefore, all sources greater than the haze angle may be combined into one hemispherical reflection luminance that is the total illuminance, E lux, times the reflectance, ρ_d divided by π.

The amount of illuminance on a small area, A, at a center of the integrating hemisphere is given by equation 63 as follows:

$\begin{array}{cc}E=\int dE=L{\int }_0^{2\pi }d\phi {\int }_0^{\pi /2}\cos \theta \sin \theta d\theta =2\pi L{\int }_0^{1}udu=\pi L& \mathrm{Eq}.\left(63\right)\end{array}$

The meaning of equation 63 is that if the cockpit interior is of uniform luminance, L (measured with a photometer in cd/m² SI units), then the amount of illuminance, E (lux in SI units), measured at the display is πL. As long as the total illuminance, E, and the diffuse Lambertian reflectance factor (coefficient), R (ρ_d), are known, then the amount of light seen by the user is given per equation 64 as follows:

$\begin{array}{cc}{L}_{\mathrm{User}}={\rho }_d\frac{E}{\pi }& \mathrm{Eq}.\left(64\right)\end{array}$

Generally, the amount of illuminance, E, in automotive cockpit interiors is estimated to be about 5000 lux per standard J1757-1 (16, section 4.1). Furthermore, measuring the maximum illuminance E at the display with a cosine corrected illuminance meter when the sun is illuminating the interior such as through the front windscreen is desirable. Light colored interiors (e.g., more towards white) may demonstrate a higher illuminance, E, than darker interiors (e.g., black).

Therefore, assuming a reflection rate of 1.5% for a typical high performance anti-reflective coated display, and a 5000 lux interior illuminance (e.g., sunlight condition), the background luminance may be determined per equation 65 as follows:

$\begin{array}{cc}{L}_{BG}=0.015\frac{5000\mathrm{lux}}{\pi }=24\mathrm{nits}& \mathrm{Eq}.\left(65\right)\end{array}$

Therefore, to meet the ISO 15008 contrast ratio recommendation of 3:1, the display luminance may be 48 nits.

For nighttime illuminance of about 10 lux, the background luminance may be determined per equation 66 as follows:

$\begin{array}{cc}{L}_{BG}=0.015\frac{10\mathrm{lux}}{\pi }=0.05\mathrm{nits}& \mathrm{Eq}.\left(66\right)\end{array}$

Therefore, to meet the ISO 15008 contrast ratio recommendation of 5:1, the display luminance may be 0.2 nits.

Therefore, the equations 65 and 66 may be written as equations 67 and 68 as follows:

$\begin{array}{cc}48=B_O{\left(24\right)}^C& \mathrm{Eq}.\left(67\right)\end{array}$ $\begin{array}{cc}0.2=B_O{\left(0.05\right)}^C& \mathrm{Eq}.\left(68\right)\end{array}$

Solving the equations 67 and 68 yields equations 69 and 70 as follows:

$\begin{array}{cc}\mathrm{ES}{L}_{\mathrm{Nit}}=2.86{\left(L_{\mathrm{BGNits}}\right)}^{0.887}& \mathrm{Eq}.\left(69\right)\end{array}$ $\begin{array}{cc}{\mathrm{ESL}}_{\mathrm{fL}}=0.87{\left(L_{\mathrm{BGfL}}\right)}^{0.887}& \mathrm{Eq}.\left(70\right)\end{array}$

Due to the aggressive nature of the ISO 15008 results, the constants per equation 70 meet the ISO 15008, but may only be used on a display with a low reflectance rate because the lower gray shade luminance increases dramatically for background luminances greater than 100 nits and may consume the available AIE stretching range. A lower slope constant may be considered for displays with higher reflectance rates greater than 1.5%, because the lower gray shades may still be visible according to the Burnette data.

AIE End Point Enhancement:

The AIE stretching function may have some luminance overhead such that pixels that exceed the threshold limit may be differentiated to some degree, instead of limiting such pixels to the same peak display luminance value. The overhead generally subtracts from the dynamic range of the stretched portion. In various embodiments, the feasibility of specifying the overhead makes a modification to equation 27 where the maximum display luminance used for L_Max is less than the display maximum luminance of L_DMax. Therefore, if a % End Point Overhead, %_EPOH, is employed, a new display limit luminance, L_DLimit, may be specified per equation 71 as follows:

$\begin{array}{cc}{L}_{\mathrm{DLimit}}=\left(100\%-{\%}_{\mathrm{EPOH}}\right)\times L_{D\mathrm{Max}}& \mathrm{Eq}.\left(71\right)\end{array}$

However, the amount of overhead may be adjusted automatically so that as N_Max approaches 255, the amount of overhead is reduced per Equation 72 as follows:

$\begin{array}{cc}{L}_{\mathrm{DLimit}}=\left(100\%-{\%}_{\mathrm{EPOH}}\left[\frac{255-N_{\mathrm{Max}}}{255}\right]\right)\times L_{D\mathrm{Max}}& \mathrm{Eq}.\left(72\right)\end{array}$

So, for example, if an overhead is desired for all gray levels over the threshold, the stretching may be limited according to Equation 72. The method to accomplish the limiting is to modify Equation 27 to become Equation 73 as follows:

$\begin{array}{cc}G\text{⁠}{S}_N=\text{⁠}\left(2^{N_{New}}-1\right)\text{⁠}{L_{D\mathrm{Max}}^{\left(-1/{\gamma }_D\right)}\left[\left[L_{\mathrm{Max}}\right]{\left(\frac{N-1}{N_{\mathrm{Max}}-1}\right)}_{\left[L_{\mathrm{Min}}\right]}\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)-L_{BG}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(73\right)\end{array}$ $\mathrm{Where}:$ $L_{\mathrm{Max}}=L_{\mathrm{DLimit}}+L_{BG};\mathrm{and}$ $L_{\mathrm{Min}}={B_O\left(L_{BG}\right)}^C+L_{BG}.$

Note that L_DMax may be used in the Equation 73. Therefore, the Equation 73 sets the maximum display luminance to be L_DLimit where N=N_Max.

There are several way to use the overhead luminance for the video pixels that are above the threshold: 1 set all pixels with luminances above the N_Max threshold to the maximum display luminance; 2 do a linear interpolation from the L_DLimit, N_Max point to the L_DMax, 255 point; and 3 use a function to match the slope of the L_DLimit, N_Max point and try to hide the demarcation of the L_DLimit, N_Max point.

A function that may be used for the third method is the gamma function. The gamma function, as shown in Equation 74, has the desirable attributes of:

• • GS_ΔyN=0 where GS_ΔxN=0
  • GS_ΔyN=GS_Δy where GS_ΔxN=GS_Δx

The γ_EP may be used to change the initial slope of the function

$\begin{array}{cc}G{S}_{\Delta yN}=G{S}_{\Delta y}{\left(\frac{G{S}_{\Delta xN}}{G{S}_{\Delta x}}\right)}^{{\gamma }_{EP}}& \mathrm{Eq}.\left(74\right)\end{array}$

FIG. 10 illustrates a graph 200 of an example end point slope gamma matching function at an end point 202 in accordance with one or more exemplary embodiments. In the example, the luminance overhead is set at 90% and there is a 100 nit background luminance level. In addition, the input gray scale threshold is determined by the histogram data to be set at 150. So although the gray shades above 150 are compressed (e.g., line 204), the higher gray shade pixels may be differentiated among each other instead of being clipped to the same maximum luminance value.

FIG. 11 illustrates a graph 210 of an example of a display luminance with end point slope gamma matching function in accordance with one or more exemplary embodiments. The graph 210 generally illustrates conversion results of the target display luminance values shown in FIG. 10.

The sequence of calculations used to implement the end point slope gamma matching function may be as follows:

Step 1: Calculate the end point output gray shade number.

Equation 75 may be utilized to determine the end point output gray shade number, GS_NEP, as follows:

$\begin{array}{cc}\begin{array}{c}{L}_{\mathrm{DLimit}}=\left(100\%-{\%}_{\mathrm{EPOH}}\left[\frac{255-N_{\mathrm{Max}}}{255}\right]\right)\times L_{D\mathrm{Max}}\\ =L_{D\mathrm{Max}}{\left(\frac{G{S}_{\mathrm{NEP}}}{2^{10}-1}\right)}^{\gamma }\end{array}& \mathrm{Eq}.\left(75\right)\end{array}$

Solving for GS_N yields Equation 76 as follows:

$\begin{array}{cc}G{S}_{\mathrm{NEP}}={\left(2^{10}-1\right)\left[100\%-{\%}_{\mathrm{EPOH}}\left(\frac{255-N_{\mathrm{Max}}}{255}\right)\right]}^{\left(\frac{1}{\gamma }\right)}& \mathrm{Eq}.\left(76\right)\end{array}$

For the example above per FIG. 11, with an overhead luminance 212 defined as 90% and N_Max=150, the result is shown in Equation 77 as follows:

$\begin{array}{cc}{\mathrm{GS}}_{\mathrm{NEP}}={\left(2^{10}-1\right)\left[100\%-{\%}_{\mathrm{EPOH}}\left(\frac{255-N_{\mathrm{Max}}}{255}\right)\right]}^{\left(\frac{1}{\gamma }\right)}& \mathrm{Eq}.\left(77\right)\end{array}$ ${\mathrm{GS}}_{\mathrm{NEP}}=\left(1023\right){\left(100\%-37.06\%\right)}^{\left(\frac{1}{2.2}\right)}=828.862$

Therefore, a 10-bit output end point is at y=GS_NEP=828.862. This value may be available in the assignment table generated per Equation 73 because N=N_Max. Note that non-rounded values are used to determine the slope.

Step 2: Determine the slope of the end point, S_E.

The slope of the end point of the gray scale curve as shown in FIG. 10 may be determined by starting with Equation 78 as follows:

$\begin{array}{cc}{S}_E=\frac{\Delta y}{\Delta x}=\frac{G{S}_{\mathrm{NEP}}-G{S}_{\mathrm{NEP}-1}}{N_{\mathrm{Max}}-\left(N_{\mathrm{Max}}-1\right)}=G{S}_{\mathrm{NEP}}-G{S}_{\mathrm{NEP}-1}& \mathrm{Eq}.\left(78\right)\end{array}$

• • Where GS_NEP-1 is the output gray shade associated with input gray shade N_Max-1. Therefore Equation 73 may be modified as Equation 79 to solve for GS_NEP-1 as follows:

$\begin{array}{cc}{\mathrm{GS}}_{\mathrm{NEP}-1}=\left(2^{N_{\mathrm{New}}}-1\right){L_{\mathrm{DMax}}^{\left(-1/{\gamma }_D\right)}\left[{{\left[L_{\mathrm{Max}}\right]}^{\left(\frac{N_{\mathrm{Max}}-1-1}{N_{\mathrm{Max}}-1}\right)}\left[L_{\mathrm{Min}}\right]}^{\left(\frac{N_{\mathrm{Max}}-\left(N_{\mathrm{Max}}-1\right)}{N_{\mathrm{Max}}-1}\right)}-L_{\mathrm{BG}}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(79\right)\end{array}$

Using the result from Equation 79, the slope may be determined per Equation 78. Note that GS_NEP-1 is the next lowest from N_Max non-round value in the assignment table. Using the following values for the example shown in FIG. 10, the value for GS_NEP-1 may be determined per Equation 80 as follows:

$\begin{array}{cc}{\mathrm{GS}}_{\mathrm{NEP}-1}=824.008& \mathrm{Eq}.\left(80\right)\end{array}$

• • Where:
  • L_BG=100 nits.
  • L_DMax=1000 nits.
  • c=0.273.
  • B_O=11 nits.
  • N_Max=15.

Therefore the slope, S_E, may be calculated per Equation 81 as follows:

$\begin{array}{cc}{S}_E=828.862-824.008=4.855& \mathrm{Eq}.\left(81\right)\end{array}$

Step 3: Calculate γ_EP:

Using Equation 74, the slope for the first step in the gamma curve is determined per Equation 82 as follows:

$\begin{array}{cc}{S}_{\gamma }=\frac{{{\mathrm{GS}}_{\Delta y}\left(\frac{1}{{\mathrm{GS}}_{\Delta x}}\right)}^{{\gamma }_{\mathrm{EP}}}-{{\mathrm{GS}}_{\Delta y}\left(\frac{0}{{\mathrm{GS}}_{\Delta x}}\right)}^{{\gamma }_{\mathrm{EP}}}}{1-0}& \mathrm{Eq}.\left(82\right)\end{array}$ $S_{\gamma }={{\mathrm{GS}}_{\Delta y}\left(\frac{1}{{\mathrm{GS}}_{\Delta x}}\right)}^{{\lambda }_{\mathrm{EP}}}$

Setting S_γ=S_E yields Equation 83 as follows:

$\begin{array}{cc}{S}_E={{\mathrm{GS}}_{\Delta y}\left(\frac{1}{{\mathrm{GS}}_{\Delta x}}\right)}^{{\gamma }_{\mathrm{EP}}}& \mathrm{Eq}.\left(83\right)\end{array}$

Equation 83 may be rearranged to provide Equation 84 as follows:

$\begin{array}{cc}{\left(\frac{1}{{\mathrm{GS}}_{\Delta x}}\right)}^{{\gamma }_{\mathrm{EP}}}=\frac{S_E}{{\mathrm{GS}}_{\Delta y}}& \mathrm{Eq}.\left(84\right)\end{array}$

Taking the logarithm of both sides of Equation 84 yields Equation 85 as follows:

$\begin{array}{cc}{\gamma }_{\mathrm{EP}}\log \left(\frac{1}{{\mathrm{GS}}_{\Delta x}}\right)=\log \left(\frac{S_E}{{\mathrm{GS}}_{\Delta y}}\right)& \mathrm{Eq}.\left(85\right)\end{array}$

Solving for γ_EP gives Equation 86 as follows:

$\begin{array}{cc}{\gamma }_{\mathrm{EP}}=\frac{\log \left(\frac{S_E}{{\mathrm{GS}}_{\Delta y}}\right)}{\log \left(\frac{1}{{\mathrm{GS}}_{\Delta x}}\right)}& \mathrm{Eq}.\left(86\right)\end{array}$

Manipulating Equation 86, yields Equation 87 as follows:

$\begin{array}{cc}{\gamma }_{\mathrm{EP}}=\frac{\log \left(\frac{S_E}{{\mathrm{GS}}_{\Delta y}}\right)}{\log \left(1\right)-\log \left({\mathrm{GS}}_{\Delta x}\right)}& \mathrm{Eq}.\left(87\right)\end{array}$

Since the log(1)=0, Equation 87 may be simplified to Equation 88 as follows:

$\begin{array}{cc}{\gamma }_{\mathrm{EP}}=\frac{-\log \left(\frac{S_E}{{\mathrm{GS}}_{\Delta y}}\right)}{\log \left({\mathrm{GS}}_{\Delta x}\right)}& \mathrm{Eq}.\left(88\right)\end{array}$

For the example provided in FIG. 10, the T_EP may be determined per Equation 89 as follows:

$\begin{array}{cc}{\gamma }_{\mathrm{EP}}=\frac{-\log \left(\frac{4.855}{1023-828.862}\right)}{\log \left(255-150\right)}=0.792576& \mathrm{Eq}.\left(89\right)\end{array}$

Therefore, the final gamma function that is added to the end point is given per Equation 90 as follows:

$\begin{array}{cc}{F}_{{\gamma }_{\mathrm{EP}}}={{\mathrm{GS}}_{\Delta y}\left(\frac{{\mathrm{GS}}_{\Delta \mathrm{xN}}}{{\mathrm{GS}}_{\Delta x}}\right)}^{{\gamma }_{\mathrm{EP}}}& \mathrm{Eq}.\left(90\right)\end{array}$

For GS_ΔxN=0:(255−N_Max), Equation 90 becomes Equation 91 as follows:

$\begin{array}{cc}{F}_{{\gamma }_{\mathrm{EP}}}=\left(1023-828.862\right){\left(\frac{N}{255-150}\right)}^{0.792576}& \mathrm{Eq}.\left(91\right)\end{array}$

For N=0:150, another way to write Equation 91 is per Equation 92 as follows:

$\begin{array}{cc}{F}_{{\gamma }_{\mathrm{EP}}}=\left(1023-828.862\right){\left(\frac{N-150}{255-150}\right)}^{0.792576}& \mathrm{Eq}.\left(92\right)\end{array}$

A more general form of Equation 92 is presented shown in Equation 93 as follows:

$\begin{array}{cc}{F}_{{\gamma }_{\mathrm{EP}}}=\left(2^{N_{\mathrm{New}}}-1-{\mathrm{GS}}_{\mathrm{NEP}}\right){\left(\frac{N-N_{\mathrm{Max}}}{255-N_{\mathrm{Max}}}\right)}^{{\gamma }_{\mathrm{EP}}}& \mathrm{Eq}.\left(93\right)\end{array}$

The Fγ_EP function may be add to the GS_NEP value from GS_N=N_Max to GS_N=255. Therefore for GS_N>GN_EP, the Equation 94 may be utilized.

$\begin{array}{cc}G{S}_N={\mathrm{GS}}_{\mathrm{NEP}}+\left(2^{N_{New}}-1-{\mathrm{GS}}_{\mathrm{NEP}}\right){\left(\frac{N-N_{\mathrm{Max}}}{255-N_{\mathrm{Max}}}\right)}^{{\gamma }_{EP}}& \mathrm{Eq}.\left(94\right)\end{array}$

• • Where N=N_Max:255.

AIE-LVE Inflection Point Modifier (IPM):

A method to implement the Inflection Point Modifier may be to use a sine wave concept with one phase before the inflection point gray shade number, N_I, and a second phase after the inflection point. The basic concept is to multiply the original assignment table output gray scale numbers by the sine wave and then add the result to the original assignment table output gray shade numbers. The Equations for the two phases are shown in Equations 95 and 96 as follows:

$\begin{array}{cc}{F}_{\mathrm{IPMPhase}1}=A_1\sin \left(\frac{\pi \times {\mathrm{GS}}_N}{N_I}\right)& \mathrm{Eq}.\left(95\right)\end{array}$ $\begin{array}{cc}{F}_{\mathrm{IPMPhase}2}=A_2\sin \left(\frac{\pi \times \left({\mathrm{GS}}_N-N_I\right)}{N_{\mathrm{Max}}-N_I}\right)& \mathrm{Eq}.\left(96\right)\end{array}$

Therefore, where the input gray shade number, GS_N, is less than or equal to the inflection gray shade number, N_I, Equation 95 is utilized to multiply each of the original output gray shade values and the result is added to the original output gray shade values. Where the input gray shade number, GS_N, is greater than the infection gray shade number, N_I, Equation 96 is utilized to multiply each of the original output gray shade values and the result is added to the original output gray shade numbers. Usually one of the two amplitudes is negative. In addition the inflection gray shade number may be structured to be a percentage of N_Max so that as N_Max changes dynamically for AIE the inflection point remains relatively in the correct location. Another structure that may be implemented is controlling the relative gains per Equation 97 such that the inflection magnitude for both phases remains relatively constant as follows:

$\begin{array}{cc}{A}_2=-\left(\frac{N_{\mathrm{Max}}-N_I}{N_I}\right)A_1& \mathrm{Eq}.\left(97\right)\end{array}$

AIE-LVE Gamma Modifier:

The gamma modifier function generally consists of multiplying the delta of the output gray shades by the gamma function and adding back to the original output gray scale 0 output value. Assume that the nomenclature is that any subscript with an “O” is the output gray shade value. The value that follows the “O” is the corresponding input gray scale number in the assignment table. Therefore, as an example, GS_O0 is the output gray scale value corresponding to the input gray shade number of 0 in the assignment table structure. Equation 98 is a form of a gamma modifier function as follows:

$\begin{array}{cc}G{S}_{ON}=G{S}_{O0}+({\mathrm{GS}}_{\mathrm{ON}}-{\mathrm{GS}}_{O0}){\left(\frac{G{S}_N}{N_{\mathrm{Max}}}\right)}^{{\gamma }_M}& \mathrm{Eq}.\left(98\right)\end{array}$

Light Sensor Filter Factors:

FIG. 12 illustrates a graph 220 of an example pupillary light reflex (e.g., curve 224) to a blue light stimulus in accordance with one or more exemplary embodiments. The curve 222 is a control curve. The light sensor filter is based on Equation 99 where the F_Up and F_Down are used according to the following logic:

• • If L_BGNew>L_BGOld, F=F_Up; and
  • If L_BGNew<L_BGOld, F=F_Down.

$\begin{array}{cc}{L}_{BG}=\frac{L_{BGNew}+\left(F-1\right)L_{\mathrm{BGOld}}}{F}& \mathrm{Eq}.\left(99\right)\end{array}$

As such, the response time for an increasing luminance may be controlled for about 1 second with an F=32 value and a loop time of 16.67 ms. For a decreasing luminance, a value of F=2048 may be used for a 90% to 10% fall time of about 75 seconds (e.g., approximately 60 seconds). The approach is used because the eye has about a 1 second rise time response and about a 60 second fall time response.

FIG. 13 illustrates an example intermediate functional block diagram of a system in accordance with one or more exemplary embodiments. In various embodiments, the diagram 230 may be implemented with a field programmable gate array (FPGA) or application specific integrated circuit (ASIC) 232 connected to a vehicle interface processor (VIP) 234 or other processor. Each of the blocks in the diagram are described as follows:

Pixel Luminance Converter Block 236:

The pixel luminance converter block 236 converts every RGB pixel into a monochrome gray shade value based on Equation 100 where the RGB coefficients are (0.19, 0.68, 0.13).

$\begin{array}{cc}\begin{array}{c}{\left(\frac{G{S}_M}{G{S}_{\mathrm{Max}}}\right)}^{\gamma }=0.19{\left(\frac{G{S}_R}{G{S}_{\mathrm{Max}}}\right)}^{\gamma }\\ +0.68{\left(\frac{G{S}_G}{G{S}_{\mathrm{Max}}}\right)}^{\gamma }\\ +0.13{\left(\frac{G{S}_B}{G{S}_{\mathrm{Max}}}\right)}^{\gamma }\end{array}& \mathrm{Eq}.\left(100\right)\end{array}$

Cancelling terms and solving for the equivalent monochrome gray shade results in Equation 101.

$\begin{array}{cc}G{S}_M={\left[0.19{\left(G{S}_R\right)}^{\gamma }+0.68{\left(G{S}_G\right)}^{\gamma }+0.13{\left(G{S}_B\right)}^{\gamma }\right]}^{\left(\frac{1}{\gamma }\right)}& \mathrm{Eq}.\left(101\right)\end{array}$

Gray Scale Data Accumulators Block 238:

Each pixel is converted into a monochrome gray scale value per Equation 101 and a value of 1 is added for each pixel to each corresponding gray scale accumulator consisting of GS_Max accumulators for each frame. For 8-bit video, this would use 256 accumulators. Histogram data is sent to the vehicle interface processor 234 at the end of each video frame. In addition, a Frame End Trigger (FET) is sent to the vehicle interface processor 234 indicating that the accumulators are ready to be down loaded to the vehicle interface processor 234. In addition, the frame end trigger is used to trigger the uploading of the AIE-LVE assignment table in preparation for the next frame of video conversion. Finally, the frame end trigger is used to signal that the new light sensor data is to be sampled. The light sensor sampling generally occurs once per frame cycle such that all functions used the same filtered data L_BG light sensor data for each frame.

Metadata AIE-LVE Bypass Selector Block 240:

The function of bypass selector block 240 is to use the metadata embedded in the video stream to determine safety related or other video symbols to which the AIE-LVE enhancement is not to be applied. In this case, the AIE-LVE assignment table is not utilized and the video for the areas are sent directly to the display.

Video Gray Shade Assignment Table Block 242:

The gray shade assignment table block 242 utilizes the AIE-LVE assignment table to convert the incoming video gray shade values into the enhanced video gray shade values. Table 1 is an example of a video look up table that is uploaded by the FPGA or ASIC 232 during the video vertical blanking time in preparation for the next active video time period. The output from the gray scale assignment table may be either 10-bit video or 11-bit video.

10-Bit or 11-Bit Frame Rate Control Block 244:

The frame rate control block 244 applies frame rate control (FRC) methods to convert the 10-bit (or 11-bit) video data to dithered 8-bit video.

Gray Shade Detection Threshold Determination Block 246:

The gray shade detection threshold determination block 246 determines the maximum gray scale number such that a specific percentage of the pixels are above a threshold percentage according to Equation 102. Therefore, the method starts at a highest accumulator and keeps adding the next lower accumulator value until the threshold percentage is determined for the corresponding gray shade value. This is the GS_Max N value used for stretching the dynamic video range after being filtered into variable N_Max by the rate limiting filter per Block 7.

$\begin{array}{cc}\mathrm{If}\left[\frac{\mathrm{SUM}\left(N:N_{\mathrm{Max}}\right)}{\mathrm{SUM}\left(0:N_{\mathrm{Max}}\right)}>\frac{T_{\%}}{100},{\mathrm{GS}}_{\mathrm{Max}}=N\right]& \mathrm{Eq}.\left(102\right)\end{array}$

Max Gray Shade Filter Block 248:

The filter block 248 is a rate limiter whereby the N_Max value in incremented or decremented by one gray shade value (8 bit) towards the GS_Max value each TL time increment. The rate of the filter is set by the TL time increment input.

Backlight Level Block 250:

A commanded backlight pulse width modulation (PWM) level is obtained and used to determine the display luminance, L_DMax, per Equation 103 as follows, where LD is the display luminance at 100% backlight drive, % BL.

$\begin{array}{cc}{L}_{D\mathrm{Max}}={\%}_{BL}\times L_D& \mathrm{Eq}.\left(103\right)\end{array}$

Light Sensor Pre-Filter Block 252:

In response to the Frame End Trigger (FET), N samples of the light sensor are successively measured and averaged resulting in the variable L_BGNew per Equation 104 as follows:

$\begin{array}{cc}{L}_{BGNew}=R_F\times \mathrm{AVG}\left(N_{\mathrm{Samples}}\right)& \mathrm{Eq}.\left(104\right)\end{array}$

Note that the reflectance factor, RF, is used to determine how much of the light sensor value is reflected to the user and also to do any conversion to user observed nits.

Light Sensor Filter Block 254:

The light sensor value obtained in block 252 is further filtered according to Equation 105 to obtain a faster exponential rise time of about 1 second than the fall time of about 60 seconds to mimic the eye adaptation times of the human visual system and to provide a peak detector function for the picket fence effects.

$\begin{array}{cc}{L}_{BG}=\frac{L_{BGNew}+\left(F-1\right)L_{\mathrm{BGOld}}}{F}& \mathrm{Eq}.\left(105\right)\end{array}$

The F values are selected to be F_Up when the new light sensor value, L_BGNew is larger than the previous filtered value, L_BGOld, and F_Down where the new light sensor value, L_BGNew is smaller than the previous filtered value, L_BGOld. The time constant for the filter is selected for the 1 second rise time and 60 second fall time goal.

L_Max Determination Block 256:

The user observed maximum luminance is determined per Equation 106, as follows, and is the summation of the end point display luminance (reference Block 270) and the reflected background luminance, L_BG, that the user sees.

$\begin{array}{cc}{L}_{\mathrm{Max}}=L_{DLimit}+L_{BG}& \mathrm{Eq}.\left(106\right)\end{array}$

L_Max is the higher end point for Equation 27 or Equation 34

Gray Shade 0 Calculator Block 258:

Gray shade 0 (GS₀) needs to be handled as a special case. If there is no active privacy function on the display, GS₀ is set to 0 nits to obtain the appearance of a black display where no information is present. If the display does have the privacy function active, the GS₀ background luminance may be raised to reduce the contrast ratio of the image. In various embodiments, the Weber Fraction level may be less than 25 for low black luminance values.

The Weber Fraction is the amount of white crosstalk that the driver sees when observing the passenger display. The “Fraction” is the amount of white luminance seen divided by the black luminance that the driver sees as formulated per Equation 107 as follows:

$\begin{array}{cc}{W}_F=\frac{W_{L\mathrm{eakage}}}{L_{\mathrm{GS}0}+L_{BG}}<25& \mathrm{Eq}.\left(107\right)\end{array}$

• • Where:
  • W_Leakage=the amount of white leakage that the user sees;
  • L_GS0=the amount of display black luminance; and
  • L_BG=the amount of reflected background luminance that the user sees at night.

Equation 107 may be rearranged to yield Equation 108, as follows, that determines how much the display black luminance value may be increased to obtain a Weber Fraction of less than 25.

$\begin{array}{cc}{L}_{GS0}>\frac{W_{\mathrm{Leakage}}}{25}-L_{BG}& \mathrm{Eq}.\left(108\right)\end{array}$

For proper operation in the formula for L_Min, L_BG is added to the display luminance to obtain L_Min. Therefore, to meet the Weber Fraction criteria, L_Min may be determined per Equation 109 as follows:

$\begin{array}{cc}{L}_{\mathrm{Min}}>\frac{W_{\mathrm{Leakage}}}{25}& \mathrm{Eq}.\left(109\right)\end{array}$

Such a AIE-LVE implementation provides that the black level luminance, L_GS0, may be increased to meet the Weber Fraction requirement.

L_Min Determination Block 260:

A minimum luminance level for the assignment table calculation is based on the larger of the Weber Fraction black luminance level calculated per Equation 110 or the minimum luminance level calculated for display visibility per Equation 111 as follows:

$\begin{array}{cc}{L}_{\mathrm{Min}}=\frac{W_{Leakage}}{25}& \mathrm{Eq}.\left(110\right)\end{array}$ $\begin{array}{cc}{L}_{\mathrm{Min}}={B_O\left(L_{BG}\right)}^C+L_{BG}& \mathrm{Eq}.\left(111\right)\end{array}$

The Fechner offset B_O and slope c constants are used together with the background luminance, L_BG, to determine the minimum luminance, L_Min, that the user needs to barely perceive the lowest gray shade information values. L_Min is the lower end point for Equation 27 as depicted in FIG. 39. The minimum Fechner constant value may be calculated per Equation 112 as follows:

$\begin{array}{cc}{L}_{\mathrm{MinFechner}}={B_O\left(L_{BG}\right)}^C+L_{BG}& \mathrm{Eq}.\left(112\right)\end{array}$

Assignment Table Calculator Block 262:

The assignment table calculation may have two parts. A first part is the calculation for the “black” level G_S0 per Equation 113 as follows:

$\begin{array}{cc}G{S}_0=\left(2^{N_{\mathrm{New}}}-1\right){L_{D\mathrm{Max}}^{\left(-1/{\gamma }_D\right)}\left[\left[L_{GS0}\right]\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(113\right)\end{array}$

A second calculation involves first choosing which assignment table calculation method may be utilized based on the “Table Select” command.

• • 1. Constant Contrast Ratio Assignment Table 264; or
  • 2. Gamma Assignment Table 266.

Constant Contrast Ratio Assignment Table 264:

The process of calculating the intermediate input video gray shade values GS₁ through GS_NMax between L_Min and L_Max such that each successive gray shade has the same contrast ratio taking the reflected ambient into account the reflected ambient per Equation 114 as follows:

$\begin{array}{cc}{\mathrm{GS}}_N=\left(2^{N_{\mathrm{New}}}-1\right){L_{\mathrm{DMax}}^{\left(-1{\gamma }_D\right)}\left[\left[L_{\mathrm{Max}}\right]\left(\frac{N-1}{N_{\mathrm{Max}}-1}\right)\left[L_{\mathrm{Min}}\right]\left(\frac{N_{\mathrm{Max}}-N}{N_{\mathrm{Max}}-1}\right)-L_{\mathrm{BG}}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(114\right)\end{array}$

Gamma Assignment Table 266:

The process of calculating the intermediate input video gray shade values GS₁ through GS_NMax between L_Min and L_Max such that each of the gray shades follow an offset gamma function are determined according to Equation 115 as follows:

$\begin{array}{cc}{\mathrm{GS}}_N=\left(2^{N_{\mathrm{New}}}-1\right){L_{D\mathrm{Max}}^{\left(-1/{\gamma }_D\right)}\left[\left(L_{\mathrm{Max}}-L_{\mathrm{Min}}\right){\left(\frac{N}{N_{\mathrm{Max}}}\right)}^{{\gamma }_G}+\left(L_{\mathrm{Min}}-L_{\mathrm{BG}}\right)\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(115\right)\end{array}$

Assignment Table Modifiers Block 268:

Block 268 provides an assignment table developed per Block 262 and is modified by multiplying other shaping functions.

End Point Backlight Level Block 270:

Block 270 provides some overhead luminance that may be used in conjunction with AIE. If AIE is not used, or no overhead is desired, the input variable “% End Point Overhead” is set to zero. The end point modifier function in Block 268 may be activated if the “% End Point Overhead” is not zero.

Selector S1, Metadata Control:

The selector S1 block 272 has two functions. If the metadata control is activated, the metadata is used to either bypass the AIE-LVE video assignment table or use the assignment table as a function of the metadata. If the metadata control is deactivated, the assignment table is used for all video data.

Selector S2, FRC Bypass Control:

The selector S2 block 274 controls whether the Frame Rate Control function is utilized. If the display has a native 10-bit or 11-bit video input, the FRC function may be bypassed.

Selector S3, Assignment Table Upload Control:

The selector S3 block 276 is triggered to upload the video assignment table as a function of the Frame End Trigger (FET) emanating from the FPGA (or ASIC) when the last video pixel for the current frame has been received. The video assignment table is to be uploaded during the video vertical blanking time in preparation for the next frame.

Selector S4, AIE Activation Control:

The selector S4 block 278 control activates the AIE stretching function. If AIE stretching is not activated, N_Max is set to 255 and therefore all input gray shades (e.g., 0-255) are formulated to have a constant contrast ratio or gamma offset between L_Min and L_Max. If however the AIE stretching function is activated, N_Max is set according to the filtered video accumulator threshold per Blocks 246 and 248.

Selector S5, Active Privacy Control:

The selector S5 block 280 control is utilized to activate raising the black gray shade 0 luminance, L_GS0, to a level required to meet the Weber Fraction criteria. This has the effect of raising the black level for both the driver and the passenger. Since the black level luminance, L_GS0, is a function of the (ambient) light sensor determined background luminance, L_BG, per Equation 108, as the reflected background luminance increases towards daylight conditions, the black level luminance, L_GS0, is decreased such that under daytime conditions the black gray shade luminance is not increased.

VIP and FPGA Input List and Description:

Table 4 is a tabulation of the various inputs to the AIE-LVE software (or code). The variables may be stored in a Variable Control Register (VCR) that may be changed via a controller area network (CAN) bus or other communication methods.

TABLE 4
VIP Input List Description
I.D.	Name	Description
0	Threshold %	% of upper gray shade
		pixels that are not
		stretched by AIE. The
		range for this variable is
		0% to 30% with a typical
		value of 5%.
1	TL	GSMax rate limiter ΔT.
		This is the time between
		Incrementing or
		decrementing the NMax
		towards GSMax. This time
		will be in the range of
		10 ms to 100 ms. The
		default value should be
		10 ms
2	AE Sel	Enables or disables AIE
		stretching function.
		0 = Disable and NMax is set
		to 255. 1 = Enable. The
		default value should be 1.
3	γD	This is the gamma value
		for the display which is
		normally 2.2. The range
		for this gamma will be
		from 1 to 3 with two
		decimal points. The
		default value should be
		2.2.
4	NNew	This is the number of bits
		for the output of the
		assignment table
		calculator (normally 10 or
		11). The selection should
		be 8, 10, 11 or 12 bits.
		The default value should
		be 8 bits.
5	LDisplay	Specifies the target display
		luminance to the
		algorithm. The range of
		this variable will be 500 to
		1500 nits with 1000 nits as
		the default value.
6	Backlight %	Specifies the backlight
		PWM % commanded to
		the display. This variable
		will range from 0% to
		100% with 100% being
		the default value.
7	FUp	Filter Up weighting
		number. For a 16.67 ms
		frame loop time, the factor
		will be 32 for a 10% to
		90% time of 1.2 seconds.
		The range should be 16 to
		64 with increments of 16
		with the default value of
		32.
8	FDown	Filter Down weighting
		number. For a 16.67 ms
		frame loop time, the factor
		will be 2048 for a 90% to
		10% time of 75 seconds.
		The range should be 256
		to 2304 with increments of
		256 with the default value
		of 2048.
9	Reflectance	Display reflectance factor
	Factor	used with light sensor
		input to determine the
		reflected luminance that
		the user sees in nit units.
		The range of the
		reflectance should be
		0.005 to 0.100 with 3
		decimal places and a
		default value of 0.01.
10	Light Sensor	Light sensor input in Lux
		units. This input will
		range from 1 lux to 100K
		lux with a default value of
		5K lux.
11	NSamples	Number of samples to be
		averaged by the pre-filter.
		This number can range
		from 4 to 32 by
		increments of 4 and a
		default value of 16.
12	WLeakage	The amount of white
		leakage that the driver sees
		in privacy mode. The
		amount can range from 0.1
		to 10 nits with a default
		value of 0.75 nits.
13	Active	Conveys active privacy
	Privacy	mode to algorithm. 0 = No
		Privacy. 1 = Active Privacy.
		Default is 0.
14	Fechner	Offset BO constant in nits.
	Constants	Slope Constant c. These
		constants are used to
		determine the minimum
		viewing luminance as a
		function of the light sensor
		input and reflectance
		factor.
		The offset constant BO
		will range from 1 to 100
		with a default value of 11.
		Use 1 decimal place.
		The slope constant “c”
		will range from 0.1 to 0.5
		with a default value of
		0.273 (3 decimal places.
15	Table Mod	Selects which table
	Selection	modification functions are
		activated.
16	Table Mod	(not shown in Block
	Upload	diagram) Allows
		uploading of tables.
17	% End Point	This is the percentage of
	Overhead	the maximum display
		luminance that is specified
		to use for input gray shade
		levels that are above the
		threshold. The percentage
		should range from 0 to
		50%.
18	Table Select	Selects either between the
		Constant Contrast Ratio
		Assignment Table (Table
		Select = 1, default) or the
		Gamma Assignment Table
		(Table Select = 1)
19	γG	This is the gamma value
		for the gamma function
		assignment table which is
		normally 2.2. The range
		for this gamma will be
		from 0.5 to 3 with two
		decimal points. The
		default value should be 2.2.
20	Modifiers	This will be an 8 bit word
		that controls invocation of
		modifier functions.
		Bit 0: M0 = 0 (No
		Modifiers), 1 (Modifiers)
		Bit 1: M1 = 0 (No Mod), 1
		(End Modifier)
		Bit 2: M2 = 0 (No Mod), 1
		(IPM Modifier)
		Bit 3: M3 = 0 (No Mod), 1
		(Gamma Modifier)
		Bits 4-7: M4 = 0 (No Mod),
		1 (Reserved)
21	M1 Inputs	End Point Modifier
		Function:
		“% End Point Overhead”
		input is used to allocate
		how much luminance is
		allocated for the input gray
		shades that are greater
		than the threshold.
22	M2 Inputs	Inflection Point Modifier
		(IPM) Function:
		A1 = Gain of 1st phase
		which can range from −0.5
		to +0.5 with a default
		value of 0.
		A2 = Gain of 2nd phase
		which can range from −0.5
		to +0.5 with a default
		value of 0.
		NI = Input gray shade
		inflection number.
23	M3 Inputs	Gamma Modifier
		Function:
		γM = −0.9 to 3 with a
		default value of 0.5.

Architecture:

FIG. 14 illustrates an example functional block diagram 290 of an image enhancement architecture in accordance with one or more exemplary embodiments. The architecture generally implements an image enhancement solution (or technique) without a graphics processor and associated full frame memories. In addition, the architecture may minimize latency and provide a cost effective solution. As shown in the figure, an FPGA (or ASIC) 292 creates the histogram on a frame by frame basis (e.g., blocks 236 and 238). The histogram data is sent to the VIP (Vehicle Interface Processor) 294 and the VIP software 296 develops the gray shade look up table 298 based on the histogram and a variety of other inputs. The table is sent back to the FPGA 292 for in-line video processing. The VIP 294 includes a protected zones bypass data block 300 that feeds the bypass selector block 240. The FPGA 292 uses the look up table to convert gray scale levels (e.g., block 242). In addition, the FPGA 292 performs a FRC function (e.g., block 244) to mimic a 10-bit to 11-bit gray shade resolution. Commercial display products generally have a VIP to handle such things as luminance control, touch interface and temperature derating functions. Generally, the VIP 294 has additional bandwidth to handle and process the image enhancement technique and therefore does not add to the incremental cost of providing an image enhancement solution.

AIE Start Point Enhancement:

The start point enhancement function is operational to smooth the initial shades from the black level instead of immediately having gray shade 1 jump to the LVE level. The technique creates a “Start Function”. The start function may stitch into the main blue “GSN” function created in block 262 of FIG. 13.

An aspect of the Start Point Function is the use of the L_BG value to determine an intersection where the Start Point Function connects to the AIE-LVE function. Since L_BG is related to the reflected ambient, the use of L_BG automatically adjust the intersection between the two functions automatically. As an example, if L_BG is lowered from 100 nits to 10 nits, the Start function changes such that the Start Point function uses less of the starting gray shades to accomplish the smoothing function. Note that the end of the Start Point Function is reduced from input gray shade 56 to gray shade 32 in the example.

FIG. 15 illustrates a graph 310 for an example start point function definition in accordance with one or more exemplary embodiments. To determine the Start Point Function 312, the following definitions are used as shown in the figure.

N_S=Intersection gray shade number between the Start Point Function and the AIE-LVE GS_N function. N_S is also the end input gray shade for the Start Point Function.

N_S+1=Is the next integer gray shade number above input gray shade N_S. This is found by adding 1 to N_S.

N_S−1=Is the next integer gray shade number below input gray shade N_S. This is found by subtracting 1 from N_S.

N_S0=The floating point output gray shade associated with input gray shade number N_S for the transfer function used in block 262 of FIG. 13.

N_S0+1=The floating point output gray shade associated with input gray shade N_S+1 for the transfer function used in block 262 of FIG. 13.

Note that an objectives of the start point function is to match the slope of the transfer function used in block 262 at the intersection point defined by input gray shade number N_S.

In order to accomplish the objective of slope matching at the intersection point, a method may be to start by using a Start Point gamma function as shown in Equation 116 as follows:

$\begin{array}{cc}{F}_{\gamma \mathrm{SP}}={N_{\mathrm{SO}}\left[\frac{N}{N_S}\right]}^{{\gamma }_S}& \mathrm{Eq}.\left(116\right)\end{array}$

• • Where γ_S is a gamma slope.

The slope of the gamma function just before the intersection point is defined by Equation 117 as follows:

$\begin{array}{cc}{S}_{\gamma \mathrm{SP}}=\frac{{N_{\mathrm{SO}}\left[\frac{N_S}{N_S}\right]}^{{\gamma }_S}-{N_{\mathrm{SO}}\left[\frac{N_{S-1}}{N_S}\right]}^{{\gamma }_S}}{N_S-N_{S-1}}& \mathrm{Eq}.\left(117\right)\end{array}$

Equation 117 simplifies to Equation 118, as follows, because the denominator is equal to 1 and the N_S/N_S term also reduces to unity.

$\begin{array}{cc}{S}_{\gamma \mathrm{SP}}=N_{\mathrm{SO}}-{N_{\mathrm{SO}}\left[\frac{N_{S-1}}{N_S}\right]}^{{\gamma }_S}& \mathrm{Eq}.\left(118\right)\end{array}$

The idea is to match the slope of the gamma Start Point Function to the AIE-LVE GS_N transfer function curve 314. The slope of the AIE-LVE transfer function curve 314 at the intersection point may be defined per Equation 119 as follows:

$\begin{array}{cc}{S}_S=\frac{N_{\mathrm{SO}+1}-N_{\mathrm{SO}}}{N_{S+1}-N_S}& \mathrm{Eq}.\left(119\right)\end{array}$

Equation 119 simplifies to Equation 120, as follows, since the denominator is equal to unity:

$\begin{array}{cc}{S}_S=N_{\mathrm{SO}+1}-N_{SO}& \mathrm{Eq}.\left(120\right)\end{array}$

Equation 118 and Equation 120 may be combined by setting the slopes equal to each other per Equation 121 as follows:

$\begin{array}{cc}{N}_{\mathrm{SO}}-{N_{\mathrm{SO}}\left[\frac{N_{S-1}}{N_S}\right]}^{{\gamma }_S}=N_{\mathrm{SO}+1}-N_{\mathrm{SO}}& \mathrm{Eq}.\left(121\right)\end{array}$

Solving for the gamma slope γ_S yields Equation 122 as follows:

$\begin{array}{cc}{N}_{\mathrm{SO}}-N_{\mathrm{SO}+1}+N_{\mathrm{SO}}={N_{\mathrm{SO}}\left[\frac{N_{S-1}}{N_S}\right]}^{{\gamma }_S}& \mathrm{Eq}.\left(122\right)\end{array}$

Manipulating Equation 122 yields Equation 123 as follows:

$\begin{array}{cc}{\left[\frac{N_{S-1}}{N_S}\right]}^{{\gamma }_S}=\frac{N_{\mathrm{SO}}-N_{\mathrm{SO}+1}+N_{\mathrm{SO}}}{N_{\mathrm{SO}}}& \mathrm{Eq}.\left(123\right)\end{array}$

Taking the logarithm of both sides results in Equation 124 as follows:

$\begin{array}{cc}{\gamma }_S\log \left[\frac{N_{S-1}}{N_S}\right]=\log \left[\frac{2N_{\mathrm{SO}}-N_{\mathrm{SO}+1}}{N_{\mathrm{SO}}}\right]& \mathrm{Eq}.\left(124\right)\end{array}$

Finally solving for γ_S results in Equation 125 as follows:

$\begin{array}{cc}{\gamma }_S=\frac{\log \left[\frac{2N_{\mathrm{SO}}-N_{\mathrm{SO}+1}}{N_{\mathrm{SO}}}\right]}{\log \left[\frac{N_{S-1}}{N_S}\right]}& \mathrm{Eq}.\left(125\right)\end{array}$

The gamma value calculated from Equation 125 is then used in conjunction with Equation 116 to develop the Start Point Function 312.

An aspect of the calculations may be that the N_S point is developed based on the L_BG value that is the same as the output value for GS1. To add some additional control, a gain factor, K, may be applied to the GS1 value. Therefore, Equation 126 may be used to determine the rounded integer end point N_S value as follows:

$\begin{array}{cc}{N}_S=K\frac{G{S}_{O1}}{G{S}_{\mathrm{OMax}}}\times 255& \mathrm{Eq}.\left(126\right)\end{array}$

Alternatively, N_S may be determined by using Equation 127 as follows:

$\begin{array}{cc}{N}_S=K\frac{L_{\mathrm{BG}}}{L_{\mathrm{Max}}}\times 255& \mathrm{Eq}.\left(127\right)\end{array}$

Note that the GS_O1 is derived from the background luminance value according to the power function formula. There are two forms of the formula to determine the output gray shades: Constant Contrast Ratio; and Offset Gamma.

The constant contrast ratio formula may be calculate per Equation 128 as follows:

$\begin{array}{cc}{\mathrm{GS}}_N={\left(2^{N_{\mathrm{New}}}-1\right)\left[\frac{\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{\left[\frac{L_{\mathrm{DMax}}+L_{\mathrm{BG}}}{{B_O\left(L_{\mathrm{BG}}\right)}^C+L_{\mathrm{BG}}}\right]}^{\left(\frac{T-N}{T-1}\right)}}-L_{\mathrm{BG}}}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{\gamma }\right)}& \mathrm{Eq}.\left(128\right)\end{array}$

If the input gray shade is set to N=1, Equation 128 becomes Equation 129 as follows:

$\begin{array}{cc}G{S}_{N1}={\left(2^{N_{\mathrm{New}}}\right)\left[\frac{{B_O\left(L_{BG}\right)}^C}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{\gamma }\right)}& \mathrm{Eq}.\left(129\right)\end{array}$

For the offset gamma formula per Equation 130, as follows, if N=0, the result is the same as Equation 129.

$\begin{array}{cc}G{S}_N=\left(2^{N_{\mathrm{New}}}-1\right){L_{\mathrm{DMax}}^{\left(-1/{\gamma }_D\right)}\left[\left(L_{\mathrm{DLimit}}-{B_O\left(L_{BG}\right)}^C\right){\left(\frac{N}{N_{\mathrm{Max}}}\right)}^{{\gamma }_G}+{B_O\left(L_{BG}\right)}^C\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(130\right)\end{array}$

Therefore, with either method, the lowest gray shade is determined by Equation 129. From a luminance point of view, the display converts the lowest gray shade per Equation 131, as follows, which is just the luminance suitable according to the Silverstein power Equation 2-1.

$\begin{array}{cc}{L}_{N1}={L_{\mathrm{DMax}}\left(\frac{G{S}_{N1}}{\left(2^{N_{\mathrm{New}}}-1\right)}\right)}^{\gamma }={B_O\left(L_{BG}\right)}^C& \mathrm{Eq}.\left(131\right)\end{array}$

Image Enhancement Start Point Exposé:

An issue with automotive display imagery is that the lower video gray shade content becomes less visible under reflected ambient lighting conditions. For example, a white poster board (e.g., a white shirt) reflection of 387 nits from the passenger seat washes out the display as seen by the driver for a display with a 1.25% total SCI display reflection rate. Considering the Burnette and Silverstein human factor studies, a 387 nit reflected background luminance results in an approximately 300 nits display luminance to be visible. In various situations, 300 nits equates to about gray shade 150 for a 1000 nit display. Therefore, for a typical cluster display, most of the lower gray shade content may not be visible as shown by the histogram in FIG. 16. Increasing the display luminance does little for lower video content visibility due to the nature of the gamma function that is used in automotive displays.

FIG. 16 illustrates a graph 320 of an example gray shade histogram in accordance with one or more exemplary embodiments. To deal with the lower and intermediate gray shade visibility situations, display luminance criteria continue to increase to levels beyond what is suitable to see the peak white levels. Therefore, image enhancement methods are becoming more attractive to enhance display visibility while reducing power that is becoming more important for electric vehicles.

To have improved visibility of lower and intermediate video content, the lower and intermediate video levels to higher video levels may be dynamically adjusted as a function of ambient lighting conditions, referred to as the adaptive image enhancement. The adaptive image enhancement may be accomplished by measuring the lighting condition and dynamically adjusting the image content for image visibility.

Adaptive Image Enhancement (AIE) offers a method to be able to see lower gray shade content of a display image. Previously, automatic luminance control methods used light sensor information to increase or decrease the luminance level of a display. The previous methods only addressed the visibility of the upper gray shades in a video image. The lower gray shades however remained largely not visible due to the reflected luminance of the display overwhelming and washing out the lower gray shade luminance levels. With the advent of adaptive image enhancement methods, the luminance of the lower gray shades may be increased to the level of visibility with just increasing the overall luminance of the display. However, the control techniques to automatically control both the video path and the display luminance functions may be integrated to provide seamless operation.

The maximum values are controlled by setting the maximum display luminance while the minimum value is set using the gray scale function. As the reflected background luminance changes (x-axis), the minimum gray shade luminance and the maximum gray shade luminance should be automatically changed to the y axis display luminance values. Note that the double arrow red dashed lines indicate the range of the display luminance gray shades. As the reflected background increases, the double red arrow range moves to the right as indicated by the solid green arrow. Therefore, the reflected background luminance (L_BG) may be estimated with a light sensor, and the amount of display luminance (ESL) may be calculated to ensure visibility.

Until the advent of adaptive image enhancement, only the top maximum visibility curve may be realized by adjusting the backlight. However, by adjusting the video gray shades using adaptive image enhancement, the bottom minimum visibility curve may now be realized resulting in visibility of the lower gray shades that normally are “washed out” by the attending reflected background luminance.

FIG. 17 illustrates a graph 330 on an example offset gray shade remapping in accordance with one or more exemplary embodiments. To address the lower gray shade visibility problem under a reflected ambient lighting condition, the starting gray shade may be raised 332 to the level of visibility as shown in the FIG. 17. In the figure, the “GS Out” curve 334 is the normal mapping in a display. The “OS GS Out” curve 336 is the remapped gray shades for the offset gamma function.

Minimum Gray Shade Analysis:

A features of the image enhancement implementation is that the minimum gray shades that the user sees remains constant and is independent of the display maximum luminance. Therefore, the backlight luminance may be decreased to the level suitable for relevant symbology luminance levels, and the minimum gray shade luminance may remain constant. This concept may be explained by first understanding the overall approach of the Adaptive Image Enhancement System.

Between the start point and the end point, the gray shades are determined based on the selected function. Equation 130, above, expresses the mathematical relationship that automatically recalculates the lower gray shade levels as a function of the display luminance.

To analyze Equation 130, above, with respect to lower gray shade independence, the first step is to simplify Equation 130 by rearranging terms as shown in Equation 132 as follows:

$\begin{array}{cc}G{S}_N={\left(2^{N_{\mathrm{New}}}-1\right)\left[\frac{\left(L_{\mathrm{Max}}-L_{\mathrm{Min}}\right){\left(\frac{N}{N_{\mathrm{Max}}}\right)}^{{\gamma }_G}+{B_O\left(L_{BG}\right)}^C}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(132\right)\end{array}$

Substituting the equations for L_Max and L_Min into Equation 132 yields Equation 133, as follows, which may be utilized to determine the gray shade transfer function between the start point (GS=1) and the end point.

$\begin{array}{cc}G{S}_N=\left(2^{N_{New}}-1\right){\left[\frac{\left(L_{\mathrm{DMax}}-{B_O\left(L_{BG}\right)}^C\right){\left(\frac{N}{N_{\mathrm{Max}}}\right)}^{{\gamma }_G}+{B_O\left(L_{BG}\right)}^C}{L_{D\mathrm{Max}}}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(133\right)\end{array}$

For the purposed of illustration, the values used in the example are:

• • c=0.799;
  • L_BG=380 nits (this is the reflected background luminance for 10K Lux illuminance on the display);

$B_O=0.263;\mathrm{and}$ ${\gamma }_G={\gamma }_D=2.2.$

From these assumptions, the amount of display luminance suitable for minimum visibility may be determined according to Equation 134 as follows:

$\begin{array}{cc}{B_O\left(L_{BG}\right)}^C=0.263{\left(380\right)}^{0.799}=30.28\mathrm{nits}& \mathrm{Eq}.\left(134\right)\end{array}$

If a minimum value of N_Max=87 is used, the first term in the numerator for a display luminance of 1000 nits and 500 nits may be determined according to Equations 135 and 136, as follows, for the lowest input gray shade of N=1.

$\begin{array}{cc}\left(1000-30.28\right){\left(\frac{1}{87}\right)}^{2.2}=0.052\mathrm{nits}& \mathrm{Eq}.\left(135\right)\end{array}$ $\begin{array}{cc}\left(500-30.28\right){\left(\frac{1}{87}\right)}^{2.2}=0.025\mathrm{nits}& \mathrm{Eq}.\left(136\right)\end{array}$

The results observed in Equations 135 and 136 are smaller than the second term (30.28 nits) in the numerator of Equation 133 and may therefore be ignored for N=1. Therefore, for N=1, Equation 137 is realized as follows:

$\begin{array}{cc}G{S}_N={\left(2^{N_{New}}\right)\left[\frac{{B_O\left(L_{BG}\right)}^C}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}& \mathrm{Eq}.\left(137\right)\end{array}$

Conversion to actual luminance for input gray shade N=1 is accomplished per Equation 138 as follows:

$\begin{array}{cc}L={L_{\mathrm{DMax}}\left(\frac{G{S}_N}{2^{N_{\mathrm{New}}}-1}\right)}^{{\gamma }_D}& \mathrm{Eq}.\left(138\right)\end{array}$

Substituting Equation 137 into Equation 138, yields Equation 139 as follows:

$\begin{array}{cc}L={L_{\mathrm{DMax}}\left(\frac{{\left(2^{N_{\mathrm{New}}}-1\right)\left[\frac{{B_O\left(L_{BG}\right)}^C}{L_{\mathrm{DMax}}}\right]}^{\left(\frac{1}{{\gamma }_D}\right)}}{2^{N_{\mathrm{New}}}-1}\right)}^{{\gamma }_D}& \mathrm{Eq}.\left(139\right)\end{array}$

Cancelling terms and simplifying Equation 139 results in Equation 140, as follows, which shows that the correct lower luminance level is maintained independent of the display luminance L_DMax.

$\begin{array}{cc}L={B_O\left(L_{BG}\right)}^C& \mathrm{Eq}.\left(140\right)\end{array}$

The image enhancement system generally maintains the correct luminance level for the lower shades independent of the display backlight luminance. Therefore. the automatic luminance control system may change the upper gray shades luminance levels without affecting the lowest image enhanced gray shade luminance levels. This characteristic is due to the two-point method used to develop the image enhancement technique where the lower levels and upper luminance levels are independently determined and controlled rather than using a simplistic image enhancement method where only the overall gamma function is controlled.

Those having ordinary skill in the art will recognize that terms such as “above,” “below,” “front,” “back,” “upward,” “downward,” “top,” “bottom,” etc., may be used descriptively herein without representing limitations on the scope of the disclosure. Furthermore, the present teachings may be described in terms of functional and/or logical block components and/or various processing steps. Such block components may be comprised of various hardware components, software components executing on hardware, and/or firmware components executing on hardware.

The foregoing detailed description and the drawings are supportive and descriptive of the disclosure, but the scope of the disclosure is defined solely by the claims. As will be appreciated by those of ordinary skill in the art, various alternative designs and embodiments may exist for practicing the disclosure defined in the appended claims.

Claims

1. An image enhancement system comprising: an ambient light sensor operational to measure an ambient light level;a circuit operational to: generate a histogram based on an input video signal;export the histogram;receive a gray shade look up table; andgenerate an output video signal by converting a plurality of gray shades in the input video signal based on the gray shade look up table; anda processor operational to: receive the histogram from the circuit;develop the gray shade look up table based on the histogram and the ambient light level; andtransfer the gray shade look up table to the circuit.

2. The image enhancement system according to claim 1, wherein the development of the gray shade look up table involves three functions that are independent of each other, the three functions provide: a start point function that improves visibility of a plurality of low gray shades based on ambient lighting conditions;an intermediate function that ensures image gray shade visibility by stretching a plurality of intermediate gray shades; andan end point function that differentiates a plurality of image white shades based on white shade image content.

3. The image enhancement system according to claim 2, wherein the development of the gray shade look up table includes: calculate a starting output gray shade;calculate an associated start point slope; andchange the start point function based on the starting output gray shade and the associated start point slope to seamlessly stitch the start point function to the intermediate function.

4. The image enhancement system according to claim 3, wherein the development of the gray shade look up table includes: calculate the starting output gray shade with a Fechner function and a light sensor value to maintain low gray shade visibility.

5. The image enhancement system according to claim 4, wherein the development of the gray shade look up table includes: filter the light sensor value to mimic eye adaptation times.

6. The image enhancement system according to claim 2, wherein the intermediate function includes either a constant contrast ratio between gray shades or an offset gamma function.

7. The image enhancement system according to claim 2, wherein the development of the gray shade look up table includes: calculate an end point based on a gray shade percentage of the histogram.

8. The image enhancement system according to claim 7, wherein the development of the gray shade look up table includes: match a gamma slope with a gamma function at the end point.

9. The image enhancement system according to claim 1, wherein the generation of the output video signal includes: convert a 10-bit video signal or an 11-bit video signal to an 8-bit output video signal with a frame rate control function.

10. The image enhancement system according to claim 1, further comprising: a display panel operational to generate a visible image based on the video output signal.

11. A method for image enhancement comprising: measuring an ambient light level with an ambient light sensor;generating a histogram with a circuit based on an input video signal;exporting the histogram from the circuit to a processor;receive the histogram at the processor from the circuit;developing a gray shade look up table with the processor based on the histogram and the ambient light level;transferring the gray shade look up table from the processor to the circuit;receiving the gray shade look up table at the circuit from the processor;generating an output video signal with the circuit by converting a plurality of gray shades in the input video signal based on the gray shade look up table; andgenerating a visible image based on the video output signal.

12. The method according to claim 11, wherein the developing of the gray shade look up table involves three functions that are independent of each other, and the three functions provide: improving visibility of a plurality of low gray shades with a start point function based on ambient lighting conditions;ensuring image gray shade visibility by stretching a plurality of intermediate gray shades with an intermediate function; anddifferentiating a plurality of image white shades with an end point function based on white shade image content.

13. The method according to claim 12, wherein the developing of the gray shade look up table includes: calculating a starting output gray shade;calculating an associated start point slope; andchanging the start point function based on the calculated starting output gray shade and the associated start point slope to seamlessly stitch the start point function to the intermediate function.

14. The method according to claim 13, wherein the developing of the gray shade look up table includes: calculating the starting output gray shade using a Fechner function and a light sensor value to maintain low gray shade visibility.

15. The method according to claim 14, wherein the developing of the gray shade look up table includes: filtering the light sensor value to mimic eye adaptation times.

16. The method according to claim 12, wherein the intermediate function includes either a constant contrast ratio between gray shades or an offset gamma function.

17. The method according to claim 12, wherein the developing of the gray shade look up table includes: calculating an end point of the end point function based on a gray shade percentage of the histogram.

18. The method according to claim 17, wherein the developing of the gray shade look up table includes: matching a gamma slope with a gamma function at the end point.

19. The method according to claim 11, wherein the generating of the output video signal includes: converting a 10-bit video signal or an 11-bit video signal to an 8-bit output video signal with a frame rate control function.

20. A vehicle comprising: an ambient light sensor operational to measure an ambient light level;a control unit coupled to the ambient light sensor and operational to: generate an input video signal;generate a histogram based on the input video signal;develop a gray shade look up table based on the histogram and the ambient light level; andgenerate an output video signal by converting a plurality of gray shades in the input video signal based on the gray shade look up table; anda display panel coupled to the control unit and operational to generate a visible image based on the video output signal.