Patent Application
20060291032
The invention relates generally to electronic reading devices such as electronic books and electronic newspapers and, more particularly, to a method and apparatus for updating an image with improved greyscale accuracy by compensating for image instability.
Recent technological advances have provided “user friendly” electronic reading devices such as e-books that open up many opportunities. For example, electrophoretic displays hold much promise. Such displays have an intrinsic memory behavior and are able to hold an image for a relatively long time without power consumption. Power is consumed only when the display needs to be refreshed or updated with new information. So, the power consumption in such displays is very low, suitable for applications for portable e-reading devices like e-books and e-newspaper. Electrophoresis refers to movement of charged particles in an applied electric field. When electrophoresis occurs in a liquid, the particles move with a velocity determined primarily by the viscous drag experienced by the particles, their charge (either permanent or induced), the dielectric properties of the liquid, and the magnitude of the applied field. An electrophoretic display is a type of bi-stable display, which is a display that substantially holds an image without consuming power after an image update.
For example, international patent application WO 99/53373, published Apr. 9, 1999, by E Ink Corporation, Cambridge, Mass., U.S., and entitled Full Color Reflective Display With Multichromatic Sub-Pixels, describes such a display device. WO 99/53373 discusses an electronic ink display having two substrates. One is transparent, and the other is provided with electrodes arranged in rows and columns. A display element or pixel is associated with an intersection of a row electrode and column electrode. The display element is coupled to the column electrode using a thin film transistor (TFT), the gate of which is coupled to the row electrode. This arrangement of display elements, TFT transistors, and row and column electrodes together forms an active matrix. Furthermore, the display element comprises a pixel electrode. A row driver selects a row of display elements, and a column or source driver supplies a data signal to the selected row of display elements via the column electrodes and the TFT transistors. The data signals correspond to graphic data to be displayed, such as text or figures.
The electronic ink is provided between the pixel electrode and a common electrode on the transparent substrate. The electronic ink comprises multiple microcapsules of about 10 to 50 microns in diameter. In one approach, each microcapsule has positively charged white particles and negatively charged black particles suspended in a liquid carrier medium or fluid. When a positive voltage is applied to the pixel electrode, the white particles move to a side of the microcapsule directed to the transparent substrate and a viewer will see a white display element. At the same time, the black particles move to the pixel electrode at the opposite side of the microcapsule where they are hidden from the viewer. By applying a negative voltage to the pixel electrode, the black particles move to the common electrode at the side of the microcapsule directed to the transparent substrate and the display element appears dark to the viewer. At the same time, the white particles move to the pixel electrode at the opposite side of the microcapsule where they are hidden from the viewer. When the voltage is removed, the display device remains in the acquired state and thus exhibits a bi-stable character. In another approach, particles are provided in a dyed liquid. For example, black particles may be provided in a white liquid, or white particles may be provided in a black liquid. Or, other colored particles may be provided in different colored liquids, e.g., white particles in blue liquid.
Other fluids such as air may also be used in the medium in which the charged black and white particles move around in an electric field (e.g., Bridgestone SID2003-Symposium on Information Displays. May 18-23, 2003, - digest 20.3). Colored particles may also be used.
To form an electronic display, the electronic ink may be printed onto a sheet of plastic film that is laminated to a layer of circuitry. The circuitry forms a pattern of pixels that can then be controlled by a display driver. Since the microcapsules are suspended in a liquid carrier medium, they can be printed using existing screen-printing processes onto virtually any surface, including glass, plastic, fabric and even paper. Moreover, the use of flexible sheets allows the design of electronic reading devices that approximate the appearance of a conventional book.
However, the greyscale accuracy needs to be further improved, in particular, in the region of relatively short image holding times. For example, during a scrolling mode, where the image on the screen is scrolled up or down, or left or right, by the user, image retention is observed because of the increased greyscale error due to strong image instability.
The invention addresses the above and other issues by providing a method and apparatus for compensating image instability and improving greyscale accuracy for a bi-stable device such as an active matrix electrophoretic display. In particular, the time interval between two subsequent image updates on a pixel or on every pixel is considered. This time interval is defined as the image-holding time during which time period the pixel is not addressed or the power on the pixel is substantially zero. Drive waveforms for various optical transitions are made directly based on image holding times. This may be realized by pre-determining the waveforms for various image-holding times and, during an image update period, loading the correct waveform according the holding time of the present image on the pixel. Alternatively, the waveforms for a fixed (usually short) image holding time are pre-determined and a correction function (or table) is used for correcting the effect of brightness drift during the image-holding period on the greyscale accuracy. The correcting impulse may be determined by a curve of brightness variation versus image-holding time, which is usually a function of the characteristic of the ink material. In this way, the greyscale error induced by image instability is significantly reduced and the requirement for the image stability of the ink material becomes less critical. Thus, the invention accommodates material variations, which are unavoidable in the manufacturing process, to improve the image quality seen by the user.
In a particular aspect of the invention, a method for updating an image on a bi-stable display includes determining an image holding time for at least one pixel in the display, determining an energy for a compensating impulse according to the image holding time, and applying a drive waveform including the compensating impulse to the at least one pixel to update the at least one pixel. The energy for the compensating impulse is the integration of the voltage over the pulse duration, e.g., time×voltage level when the voltage is fixed. For simplicity, a pulse-width modulated (PWM) driving scheme is used in the following for describing this invention. In a PWM driving scheme, the energy variation in an impulse is realized by varying the pulse length while the voltage level is substantially constant.
A related electronic reading device and program storage device are also provided.
In the drawings:
In all the Figures, corresponding parts are referenced by the same reference numerals.
As an example, the electrophoretic medium 5 may contain negatively charged black particles 6 in a white fluid. When the charged particles 6 are near the first electrode 3 due to a potential difference of, e.g., +15 Volts, the appearance of the picture elements 2 is white. When the charged particles 6 are near the second electrode 4 due to a potential difference of opposite polarity, e.g., −15 Volts, the appearance of the picture elements 2 is black. When the charged particles 6 are between the electrodes 3 and 4, the picture element has an intermediate appearance such as a grey level between black and white. A drive control 100 controls the potential difference of each picture element 2 to create a desired picture, e.g., images and/or text, in a full display screen. The full display screen is made up of numerous picture elements that correspond to pixels in a display.
The control 100 may execute any type of computer code devices, such as software, firmware, micro code or the like, to achieve the functionality described herein. Moreover, the memory 120 may be a program storage device that tangibly embodies a program of instructions executable by a machine such as the control 100 or a computer to perform a method that achieves the functionality described herein. Such a program storage device may be provided in a manner apparent to those skilled in the art. A computer program product comprising such computer code devices may also be provided in a manner apparent to those skilled in the art.
The control 100 may have logic for periodically providing a forced reset of a display region of an electronic book, e.g., after every x pages are displayed, after every y minutes, e.g., ten minutes, when the electronic reading device is first turned on, and/or when the brightness deviation is larger than a value such as 3% reflection. For automatic resets, an acceptable frequency can be determined empirically based on the lowest frequency that results in acceptable image quality. Also, the reset can be initiated manually by the user via a function button or other interface device, e.g., when the user starts to read the electronic reading device, or when the image quality drops to an unacceptable level.
The invention may be used with any type of electronic reading device.
Various user interface devices may be provided to allow the user to initiate page forward, page backward commands and the like. For example, the first region 442 may include on-screen buttons 424 that can be activated using a mouse or other pointing device, a touch activation, PDA pen, or other known technique, to navigate among the pages of the electronic reading device. In addition to page forward and page backward commands, a capability may be provided to scroll up or down in the same page. Hardware buttons 422 may be provided alternatively, or additionally, to allow the user to provide page forward and page backward commands. The second region 452 may also include on- screen buttons 414 and/or hardware buttons 412. Note that the frame 405 around the first and second display regions 442, 452 is not required as the display regions may be frameless. Other interfaces, such as a voice command interface, may be used as well. Note that the buttons 412, 414; 422, 424 are not required for both display regions. That is, a single set of page forward and page backward buttons may be provided. Or, a single button or other device, such as a rocker switch, may be actuated to provide both page forward and page backward commands. A function button or other interface device can also be provided to allow the user to manually initiate a reset.
In other possible designs, an electronic book has a single display screen with a single display region that displays one page at a time. Or, a single display screen may be partitioned into two or more display regions arranged, e.g., horizontally or vertically. In any case, the invention can be used with each display region to reduce image retention effects and to improve the smoothness of the image update.
Furthermore, when multiple display regions are used, successive pages can be displayed in any desired order. For example, in
Additionally, note that the entire page need not be displayed on the display region. A portion of the page may be displayed and a scrolling capability provided to allow the user to scroll up, down, left or right to read other portions of the page. A magnification and reduction capability may be provided to allow the user to change the size of the text or images. This may be desirable for users with reduced vision, for example.
Image Drift
The bi-stable display, such as the electrophoretic display, had advantages compared to other displays such as LCDs in terms of its high brightness, high contrast ratio, wide view angle and stable image. Additionally, the average power consumption is more than a factor of one hundred lower than with LCDs due to a lower refresh rate enabled by its bi-stability. That is, after completion of an image update, the image substantially holds on the pixel without supplying any voltage pulse. The voltage pulse is only needed during the next image update. It would also be possible to not update/refresh the pixels on which the optical state does not change during next image update, such as in a white-to-white transition, resulting in still lower power consumption. However, in practical electrophoretic displays, it is observed that the optical state drifts away during an image holding period, in particular, in the first 100 seconds directly after the image update.
For example,
The present invention provides a driving technique that compensates for image instability and improves greyscale accuracy for a bi-stable display by considering the image holding time on an individual pixel, a group of pixels, or every pixel. Drive waveforms for various optical transitions are made directly coupled with image-holding times. This may be realized by pre-determining the waveforms for various image-holding times and, during an image update period, loading the correct waveform according the holding time of the present image on the pixel. Alternatively, the waveforms for a fixed (short) image holding time are pre-determined and a correction function or table is used for correcting the effect of brightness drift during the image-holding period on the greyscale accuracy. The correcting impulse may be determined by a curve of brightness variation versus image holding time, which is usually a function of the characteristic of the ink material. In this way, the greyscale error induced by image instability is significantly reduced and/or the requirement for the image-stability of ink material becomes less critical. Image quality is therefore improved while manufacturing costs can be reduced.
In a first embodiment, an image-instability compensating impulse versus image holding time curve is used for recovering or correcting the optical state in the next image transition. The impulse is obtained by measuring the brightness as a function of impulse energy, which pulse tries to bring the present brightness, e.g., white, at the present image holding time to the original/initial level at a substantially zero image holding time, i.e., the level that is obtained directly after the image updating. The minimal impulse to fully restore the brightness is defined as the compensating impulse at this image holding time. The same procedure is repeated for other image-holding times. From these data, a curve of compensating impulse time versus image holding time is generated as schematically plotted in
In rail-stabilized driving schemes (e.g., as discussed in the above-identified European patent application 03100133.2), an over-reset pulse is sometimes used to achieve accurate greyscale with a reduced image update time and reduced optical flicker. In such driving schemes, the drive waveforms include reset pulses and greyscale driving pulses. The reset pulse is defined as a voltage pulse that moves particles from their present positions to one of the two extreme positions close to one of the two electrodes, and the greyscale driving pulse is the voltage pulse that sends the display or pixel to the desired final optical state. In such driving schemes, the above measured curve may also be used for compensating for the image instability effect. In this case, the reset pulse may include three parts: standard-reset, over-reset and image-instability correcting reset, as illustrated in
To implement the first embodiment, a memory, e.g., memory 120, may store standard drive waveforms at a fixed image-holding time for example in the greyscale update (GU) mode, which waveforms are used for updating greyscale images. The standard drive waveform refers to a drive waveform that is optimized at a fixed image holding time, which holding time is preferably short, e.g., close to zero or a few seconds. The standard drive waveform does not use a compensating impulse according to the invention, and may include, for instance, shaking pulses, a reset pulse and a drive pulse, as discussed further in connection with
In fact, the compensating impulse is largely determined by the material property, and is essentially not sensitive to the usage modes. It is therefore further advantageous to store the data for functions/curves of the pre-determined compensating impulse for various optical transitions (e.g., such as shown in
A further advantage of this method is to allow one to scale the compensating impulse according to the image holding time. Assuming a basic pulse length for compensating image stability is introduced in a drive waveform, one can obtain a scaling factor vs. image holding time curve according to the measured image holding time curve and the brightness correcting/restoring curve. The scaling factor curve may be stored together with the pre-determined image holding time and can be loaded according to the image holding time on the pixel during an image update. The “basic” or standard compensating impulse is a part of various drive waveforms with a variable pulse length or energy determined by the scaling factor according to the image holding time on the pixel. A reduced memory requirement together with an increased image update efficiency is realized because it is not necessary to separately load the standard drive waveforms and the compensating waveform and the scaling factor is read out when the image holding time is read.
In a second possible embodiment, instead of reading a function/curve that describes curves such as those of
To illustrate, the curves of
The above eight points are provided as an example only. Fewer or more points can be used as desired. Data for additional tables can be obtained by interpolating other tables. Moreover, data for determining compensating impulse energies when pulse width modulation is not used may be obtained analogously. For instance, a curve similar to
In the waveforms of
This embodiment illustrates that the standard waveforms for the image transitions without substantial optical state changes on the pixel can be simplified to, e.g., a single polarity waveform. This will further reduce the optical flicker during an image update. Again, a compensating (C) pulse with variable energy according to the image holding time is part of the drive waveform and provided at various time moments in the waveform as discussed in
The polarity of a compensating impulse (C) is selected such that the particles in the display are able to move towards the direction, resulting in the initial/original optical state that is obtained during previous image update at a substantially zero image holding time, regardless of the polarity of the pulses in the subsequent standard drive waveform.
It is emphasized that the time intervals between any two subsequent pulses can be substantially equal to zero as an advantage of shorter total image update time. To measure the image holding time on a pixel, a timer may be introduced on the pixel. The timer automatically starts counting directly after the image update is complete and the elapsed time since last image update on the pixel is read, which is used during the subsequent image update for loading the correct compensating impulse. In the mean time, the timer can be reset to zero and start new counting after the next update. This process can be repeated. Although it is beneficial to count the image holding time for every individual pixel, it is, in practice, possible to count the image holding time for a single pixel on the display, and the timer information can be used for updating the entire display or a portion of the display. Note that, in the above examples, pulse-width modulated (PWM) driving is used for illustrating the invention, i.e., the pulse time is varied in each waveform while the voltage amplitude is kept constant. However, the invention is also applicable to other driving schemes, e.g., based on voltage modulated driving (VM), where the pulse voltage amplitude is varied in each waveform, or combined PWM and VM driving. When VM driving or combined VM and PWM driving is used, the compensating impulse is selected such that the energy involved in the compensating impulse is just enough to fully restore the brightness to the initial level obtained directly after update. This invention is also applicable to color bi-stable displays. Also, the electrode structure is not limited. For example, a top/bottom electrode structure, honeycomb structure or other combined in- plane-switching and vertical switching may be used. Moreover, the invention may be implemented in passive matrix as well as active matrix electrophoretic displays. In fact, the invention can be implemented in any bi-stable display that does not consume power while the image substantially remains on the display after an image update. Also, the invention is applicable to both single and multiple window displays, where, for example, a typewriter mode exists.
While there has been shown and described what are considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention not be limited to the exact forms described and illustrated, but should be construed to cover all modifications that may fall within the scope of the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB04/51532 | 8/23/2004 | WO | 2/22/2006 |
| Number | Date | Country | |
|---|---|---|---|
| 60497660 | Aug 2003 | US | |
| 60526186 | Dec 2003 | US |
