Patent Application
20100156956
The present invention relates generally to display devices. More particularly, the present invention relates to non-CRT display devices. Still more particularly, the present invention relates to a method and system for producing a grayscale characteristic for non-CRT display devices.
In the broadcast video system, original motion image sequences are captured using an image-capture means, such as a broadcast video camera, which produces image-bearing signals. The image-bearing signals produced by the camera typically are encoded according to the ITU-R BT.709 broadcast-video encoding specification (hereinafter Rec. 709), and the encoded image-bearing signals are used to produce a displayed motion image sequence on a television receiver or other image display device such as, for example, a digital projector, digital picture frame, or a portable video playback device. In the foregoing discussion, references made to a scene, image, or images are understood to apply also to motion image sequences.
Rec. 709 is an image-capture and encoding specification based on a set of reference color primaries and an optoelectronic transfer characteristic. These attributes define the relationship between the color stimuli in original motion image sequences and the corresponding encoded signal values that a Rec. 709 compliant video camera or video signal source would deliver for captures of those stimuli.
For L<0.018, V=4.5L
For L≧0.018, V=1.099L0.45−0.099
where L is the luminance-factor value corresponding to an original color stimulus (with L=1.0 corresponding to that of a diffuse scene white), and V is the corresponding camera output voltage or signal value, normalized so that a scene white produces an output voltage or signal value of 1.0. In order for the broadcast video system to capture and appropriately reproduce images comprised of both neutral (i.e. achromatic or gray) and non-neutral (i.e. non-achromatic or colored) stimuli, the system must function on a trichromatic basis, i.e. the image capture, signal processing and image formation functions of the broadcast video system must utilize at least three imaging channels, typically corresponding to red, green, and blue color-image information. The Rec. 709 optoelectronic transfer then applies to each of the system's red, green, and blue image-capture signals. The CIE Standard Illuminant D65 defines the reference white chromaticity for the Rec. 709 encoding specification. When original-scene colors are spectrally nonselective neutrals illuminated by a light source approximating CIE Standard Illuminant D65, the corresponding Rec. 709 red, green, and blue image-bearing signal values produced by a video camera or video signal source are numerically equal in all three channels. Such trichromatic video signal values are commonly denoted as R′G′B′, the prime denoting the nonlinear relationship between original-scene luminance-factor values and the resulting signal values, as specified by the Rec. 709 optoelectronic transfer. So for neutral scene objects, the values R′, G′, B′, and V all are numerically equal. In the foregoing description, the single quantity V is used for convenience, recognizing that the invention also pertains to the red, green, and blue color-image channels.
Further steps in the video-signal encoding process can and typically are performed, such as, for example, conversions from R′G′B′ to lama and chroma components and conversions to digital realizations. By way of example only, in an 8-bit per color-channel digital camera, the image-capture signals may be scaled and quantized so that the resulting digital signal values for a captured white stimulus would be equal to 255 in all three color channels, and the resulting digital signal values for captured achromatic gray colors of luminance-factor values lower than that of a diffuse white, to correspond to numerically discrete integer values between 0 and 255. Other digital scaling may also be employed.
Providers of encoded motion image content for video broadcast or distribution are motivated to adhere to the Rec. 709 encoding specification in order that the displayed images are compatible in overall visual appearance with displayed images generated by encoded signals from other video signal providers. In turn, receivers of broadcast-compatible video signals are motivated to interpret Rec. 709 signals according to the Rec. 709 specification. In this way, the same output color-signal processing, embodied typically in the television or other display device, can be applied to broadcast video image signals from all sources and providers with predictable and consistent results across signal sources, as long as the Rec. 709 specification applies to the generation of the video signals.
The ITU-R BT.709 specification defines only the image-capture and encoding attributes of a broadcast-compatible video signal in terms of the relationship between the colorimetry of original-scene color stimuli and the corresponding R′G′B′ encoded signal values resulting from the capture, signal generation, and numerical encoding of those stimuli using an image-capturing means such as a broadcast video camera. The specification does not describe how the encoded signal values are to be used to create reproduced images. The specification does not specify the relationship between encoded signal values and the colorimetry of reproduced images produced from the encoded signals, and under what conditions those reproduced images should be viewed by an observer. So, the reproduced colorimetry and resulting color appearance of images displayed from broadcast-compatible video signals are open to interpretation. Specifically, the reproduced colorimetry will depend on the calorimetric characteristics of the image-reproducing means (e.g. a CRT, LCD, or other type of display technology), determined principally by its color primaries, reference white point, grayscale characteristic and other internal color-signal processing. The resulting color appearance will be influenced by the conditions under which the displayed image's reproduced colorimetry will be viewed, which affects the relationship between the displayed colorimetry and its color appearance.
Even though the Rec. 709 specification does not describe the reproduced colorimetry and color appearance of displayed video images, several references make assumptions regarding the reproduced colorimetry and color appearance of displayed video images. In an article entitled “Colorimetry for HDTV” by LeRoy DeMarsh, the author states the opto-electronic transfer function for broadcast video, i.e. that specified by Rec. 709, “is the sort of function currently implemented in many high quality broadcast color TV cameras and is known to produce high quality pictures on current TV monitors.” (page 2). At the time the article was published, virtually all television receivers and monitors used CRT display technology. The grayscale characteristic of typical CRT displays is well known in the art, and the relationship between the R′G′B′ input signals and the resulting output luminances on the display generally obeys a straightforward power-law relationship, given by the value of an exponent, gamma. Charles Poynton, in “Digital Video and HDTV: Algorithms and Interfaces” states on p. 264 that “Rec. 709 encoding assumes that encoded R′G′B′ signals will be converted to tristimulus values at a CRT (or some other display device) with a 2.5-power function.”
A power-law equation relating the input signals R′=G′=B′ (or V as discussed earlier) and the output R=G=B relative luminous intensity values (I) for a typical CRT-type display with a luminance dynamic range of 1000:1 intended for broadcast-video applications and having a gamma of 2.5 is defined by the equation/=0.999V2.5+0.001. This relationship is shown graphically in
The resulting system tone reproduction characteristic shown in
Also shown in
With the recent advent of alternate display technologies, such as, for example, LCD, plasma, Organic Light Emitting Diode (OLED), and digital projection, manufacturers of television display devices employing such technologies typically include means for causing the alternate display technology to more-or-less function according to a typical CRT-based power-law relationship in order to maintain compatibility with the existing broadcast-video system and to produce displayed images more or less compatible with those that would be displayed on a CRT for which the encoded signal is intended. Not doing so would run the risk of introducing a television display device employing alternate display technology to reproduce motion image sequences in a fashion that would appear quite different and quite possibly visually objectionable when compared to the same broadcast video signal displayed using a high-quality CRT-based television display. While it is recognized that alternate display technologies may exhibit and employ colorimetric features unachievable by existing CRT-based technologies (e.g. wider color gamut), the foregoing discussion applies to the overall tone reproduction characteristic.
A particular display device grayscale characteristic can vary considerably from this example depending on the basic display technology employed (LCD, plasma, OLED), the sub-technologies within a given basic technology (twisted nematic vs. in-plane switching LCD technologies, for example), the differences caused by manufacturing variation in individual units of the same type, as well as the differences caused by the particular digital or analog electronics and software or firmware integrated with the display panel in the device. These differences can produce significantly different visual results.
Since alternate non-CRT display technologies may or may not adhere to a power-law grayscale characteristic comparable to that of a CRT-based display for which the Rec. 709 encoded signal is presumably optimized, television display device manufactures typically employ lookup tables as part of the television's video signal processing. The combination of the lookup table and the particular display panel's grayscale characteristic generally corresponds substantially to that of a high-quality CRT-based display, which can be used together with Rec. 709 encoded signals with the expectation that the displayed image will exhibit a tonal relationship to the original scene colors corresponding essentially to that of the system tone reproduction characteristic of
Next, the reference RGB relative intensity triads may be converted to colorimetrically equivalent RGB relative intensity triads expressed in terms of the actual device RGB color primaries, again using color primary conversion techniques described in the prior art (S2). The actual device RGB relative intensity values are then converted to corresponding RGB device input signal values (S3). This conversion is accomplished by a mapping or lookup table process using the device grayscale characteristic expressed in terms of device RGB relative intensity values and corresponding device input signal values. Finally, a compensation lookup table or other compensation function is derived by forming a relationship between Rec. 709 input signal values and corresponding actual device input signal values (S4).
Use of such a compensation relationship or lookup table in a display device will cause the device's grayscale characteristic to emulate that of a reference display device, as illustrated in
While the incorporation of such compensation lookup tables have enabled the rapid conversion of the television display market to non-CRT based technology, the range of tonal variation that can be effected using various power-law relationships is quite limited, with only a few degrees of freedom available to the system integrator. Furthermore, the use of power-law relationship grayscales has done little to take advantage of the possibilities afforded using alternate display technologies and the many degrees of freedom made available by the incorporation of digital lookup table processing capability.
A method for generating a target display characteristic for a non-CRT display device includes establishing a sequence of luminance-factor values corresponding to original-scene neutrals. The luminance-factor values are converted to corresponding Rec. 709 signal values. The luminance-factor values are then mapped according to a desired system tone reproduction characteristic to corresponding luminous intensity values to be reproduced by the non-CRT television display device. The target display characteristic is generated by relating the corresponding Rec. 709 signal values to corresponding luminous intensity values.
Embodiments of the invention are better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other.
Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” The term “connected” means either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means either a single component or a multiplicity of components, either active or passive, that are connected together to provide a desired function. The term “signal” means at least one current, voltage, or data signal.
Referring to the drawings, like numbers indicate like parts throughout the views.
Processor 806 is configured, for example, as a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a Field Programmable Gate Array (FPGA), a digital signal processor (DSP), or other processing device, or combinations of multiple such devices, in one or more embodiments in accordance with the invention. Processor 806 may store the one or more image files in memory 808. Memory 808 is implemented as any type of memory, such as, for example, random access memory (RAM), DRAM, SDRAM, flash memory, disk-based memory, removable memory, or other types of storage elements, in any combination, in an embodiment in accordance with the invention.
Communications port 810 is an input/output port for communicating with other devices and networks, such as, for example, various on-screen controls, buttons or other user interfaces, network interfaces, and remote or voice control interfaces. And finally, display 812 is used to display the one or more image files. Display 812 is configured as a liquid crystal display (LCD), an organic light emitting diode (OLED) display, a plasma display panel (PDP), a projection display, or other non-CRT display technology in one or more embodiments in accordance with the invention. When processor 806 processes the one or more image files using the target grayscale characteristic derived pursuant to the method shown in
Because today's alternate technology non-CRT television displays are much higher in their overall peak luminance, generally fill more of the observers' field of view, and typically operate at much higher overall correlated color temperatures than a CRT display and associated viewing conditions at the time the Rec. 709 standard was developed, the relationship between displayed television colorimetry and its color appearance most likely differs in today's typical viewing situations versus that of the broadcast television conditions during which CRT displays were prevalent. Therefore, a different target display characteristic is needed to take advantage of these new factors. The present invention provides method for generating the target display characteristic for a non-CRT display device. The target display grayscale characteristic is produced by the combination of grayscale compensation lookup tables and the display panel's native or actual grayscale characteristic. Use of the target display characteristic results in visually preferred displayed images from Rec. 709 signals compared to that of typical non-CRT televisions where the relative luminance-factor values of their reproduced neutrals are related according to a gamma or power-law relationship, which may or may not include scalar and offset factors.
Referring now to
Achieving such an overall tonal characteristic requires a unique target display characteristic to be used in the lookup table generation process described with reference to
The log luminance-factor values are then mapped according to the desired system tone reproduction characteristic to corresponding log luminous intensity values that are to be reproduced by the non-CRT television display device (S4). The log luminous intensity values may be transformed to relative intensity values by taking their antilogarithms, if the system tone reproduction characteristic is again expressed in logarithmic terms (S5). Then a target display grayscale characteristic is derived relating Rec. 709 encoding values to corresponding relative intensity values (S6).
Referring now to
The nature of the improved display grayscale characteristic of the present invention is apparent in
A power-law grayscale characteristic comprised of a lower value of gamma, e.g. 1.7 as shown as plot 1206 in
A comparison showing the relative luminance differences between the target grayscale characteristic of the present invention and a typical gamma 2.5 power-law grayscale characteristic is shown in
The target grayscale characteristic of the present invention provides an improved television display device wherein scene neutrals, captured and encoded according to the ITU-R BT.709 standard are reproduced in a visually preferred manner according to the target grayscale characteristic. The improved system tone reproduction characteristic has the chromaticity coordinates of the reproduced neutrals lying substantially along the continuum of chromaticities corresponding to CIE standard daylight illuminants. The chromaticies are constant throughout a majority of the display luminance dynamic range. The reproduced neutrals appear achromatic to an observer adapted to the television viewing environment.
For example, an LCD native grayscale typically exhibits a tendency departing from that of an achromatic, as shown in the grayscale tracking plot of
Three one-dimensional compensation lookup tables are used to impart a desired overall shape and relative luminance level to the effective grayscale characteristic in an embodiment in accordance with the invention. And as described with reference to
In some embodiments, it is necessary to display some amount of light from one or two of the color primaries in order to produce an achromatic black color corresponding in chromaticity to that of the rest of the grayscale. When this is done for the black point, the light emitted by the one or two primaries required to produce the achromatic black have some luminance component associated with them, thus increasing the minimum luminance displayable by the display device and reducing the contrast ratio achievable. Observers may object to this reduction on contrast ratio more than they object to a grayscale that tracks through the majority of its characteristic, but tapers toward the panel black point. An improved grayscale characteristic chromaticity tracking is depicted in
Since the chromaticity of the panel black point can vary from display panel to display panel depending on numerous factors including the backlight which may be employed, the specific panel technology, and any light management films and materials that may be incorporated in the display panel, the nature of the final taper will depend on the chromaticity selected for the achromatic neutral and specific panel black point. Thus, other embodiments in accordance with the invention will have different tapers to the chromaticity of the panel black chromaticity at the lowest portion of the grayscale characteristic.
The above description of grayscale chromaticity tracking with improved black point adjustment used a reference achromatic chromaticity corresponding to that of CIE Standard Illuminant D75. Other embodiments in accordance with the invention can use other reference achromatic chromaticities such as, for example, D65 and D90.
Referring now to
Since the chromaticity of the panel black point can vary from panel to panel, depending on numerous factors including, but limited to, the backlight which may be employed, the specific panel technology, and any light management films and materials that may be incorporated in the display panel, the nature of the final taper will depend on the chromaticity selected for the achromatic neutral and specific panel black point.
Today's television displays tend toward higher correlated color temperatures (9000K and higher) for their reproduced whites and neutral colors as compared to the D65 reference white defined for the Rec. 709 video signal encoding standard. Current LCD panel technology produces a native grayscale characteristic at higher correlated color temperatures, and so system integrators and manufacturers are motivated to design their television sets closer to this native color temperature than to the Rec. 709 reference white chromaticity to provide maximum luminance. In other words, designing an LCD TV around a D65 white point necessitates a reduction in luminance in one, or most likely two of the panel's constituent primary colors, which also would reduce the maximum luminance achievable. This change in reference white chromaticity can be handled most successfully if the higher operating reference-white color temperature is selected such that its CIE chromaticity coordinates lie substantially along the chromaticity continuum corresponding to the CIE Standard Daylight illuminant series. Doing so produces reproduced neutrals that are perceived to be achromatic using today's larger higher-luminance panels that more fill the observer's visual field than typically sized CRT displays viewed at more or less the same distance. By establishing the reproduced neutral along this continuum, the reproduction of other important scene colors becomes more straightforward and is accomplished in a visually preferred and natural fashion.
For example, TV manufacturers often quote correlated color temperatures for their white points. However, those skilled in the art will recognize a correlated color temperature of 9000K can relate to a wide variety of reference white chromaticities, many of which may deviate so far from the daylight continuum that it is difficult, if not impossible, for an observer to fully adapt to the color of the reproduced neutral. Instead, the observer perceives an overall color cast in the reproduced images. It is not uncommon for manufactures to use a reference white chromaticity that corresponds to a point along an iso-correlated color temperature line that is decidedly magenta or pink in overall appearance. This causes other problems in color reproduction, especially that of natural foliage and green colors. By shifting the neutral scale in the magenta direction, the greens concomitantly lose much of their chroma and appear devoid of the level of color expected in their reproduction. Again, by establishing the neutral along the daylight chromaticity continuum, such color reproduction problems are avoided.
It is recognized that some televisions in the market include algorithms for adaptively adjusting the effective device grayscale, and thereby the system tone reproduction, by making adjustments to compensation lookup tables, histograms, luminance signals, and the like, based on incoming image-bearing signal spatial, temporal, or tonal statistics. Such algorithms effectively apply modifications to the device grayscale characteristic to improve the visual quality of the final displayed image. Furthermore, display modes, such as those typically denoted as “Normal”, “Vivid”, and the like may include brightness, contrast, and other such modifications. Further yet, the user may, through the television user-interface make modifications to the overall brightness and contrast parameters of the television display device. The present invention is directed toward the static, or native television display grayscale characteristic in the absence of such adaptive algorithms or mode or user modifications. Similar novel device grayscale characteristics, and thereby system tone reproduction characteristics can be generated by adjusting parameters such as the luminance contrast of the device or system tone reproduction, deriving a corresponding device grayscale characteristic, and computing and implementing compensating lookup tables based on the derived characteristic and actual device characteristic, or on the difference between the new original characteristic and the new characteristic. All such modifications and adjustments are within the spirit and scope of the invention described.
Even though specific embodiments of the invention have been described herein, it should be noted that the application is not limited to these embodiments. In particular, any features described with respect to one embodiment may also be used in other embodiments, where compatible. And the features of the different embodiments may be exchanged, where compatible.
This application claims the benefit of U.S. Provisional Patent Application 61/138,976 filed on Dec. 19, 2008, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61138976 | Dec 2008 | US |
