Apple patent involves color correction of electronic displays
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Apple patent involves color correction of electronic displays

Apple is eyeing ways to beef up the color correction on its electronic displays. A patent (number 20110032275) for color correction of electronic displays utilizing gain controls has appeared at the US Patent & Trademark Office. It generally relates to display correction and, more specifically, to correcting the displayed color by reducing its dependency on various variables, such as temperature.

A video-rendering chip performs gain correction on received display input, based on a display temperature, to produce output values that are shown on the display. The video-rendering chip includes multipliers, a microprocessor, and a memory. The microprocessor receives a display temperature from a sensor, determines gain correction coefficients that correspond to the display temperature, and provides the correction coefficients to the multipliers. The multipliers then multiply the display input by the correction coefficients to produce the output values. The microprocessor may determine the correction coefficients utilizing a lookup table or a correction coefficient formula stored in the memory.

The microprocessor may receive an updated display temperature periodically and may determine new correction coefficients that correspond to the updated display temperature. The microprocessor may receive updated display temperatures at fixed periods or at varying periods based on the previous display temperature. The inventors Gabriel G. Marcu, David Lum and Wei Chen.

Here's Apple's background and summary of the invention: "Many computing devices use an electronic display to present information to a user. Such displays may be, for example, liquid crystal displays ('LCDs'), cathode ray tubes ('CRTs'), organic light emitting diode displays ("OLED displays") and so on. Most such displays can show color images. However, the color response of a display may change as the display operates.

"In particular, the display's white point may shift along a blackbody curve as the physical temperature of the display reaches a steady operating temperature. For example, when a display is turned on, the display may be cold and the temperature of the display may increase as the display warms up over time. The changing temperature of the display may cause the display colors to shift. For example, some displays depict white as somewhat yellowish when initially powered on and cold.

"As the display warms, the white point of the display shifts toward a more neutral white, such as defined by the standard illuminant, D65. The same is true for any colors shown on the display; they too shift within a color space as the temperature of the display increases. This is true even if, for example, the display only outputs grayscale colors (e.g., is a black and white display). Similarly, other parameters of the display may shift as a function of temperature such as luminance, black level, contrast, or electro-optical transfer function, which may be referred to as the "native gamma" of the display. This set of parameters may be referred to as the color profile of the display.

"The shift in the color profile due to temperature increase of the display generally causes each pixel of the display to change color until a stable operating temperature is achieved, at which point the pixel colors are likewise stable. That is, although a pixel may be instructed to display the same color at an initial temperature and a stable operating temperature, the actual color displayed, as objectively measured by its chrominance and luminance, may vary.

"It should be noted that, in many electronic systems, individual pixels of a display receive a red, green and blue value that together define the color to be created by the pixel. These red, green and blue values are referred to herein in the aggregate as an 'RGB value,' as understood to those of ordinary skill in the art.

"Thus, a method of adjusting the display colors over a range of display temperatures is desirable. Accordingly, there is a need in the art for an improved method of providing consistent display colors over a range of parameters including temperature.

"In an embodiment, a display device receives video input and utilizes a video-rendering chip to perform gain correction on the video input, based on a display temperature, to produce output values. The output values are provided to a display driver which controls the hardware of the display to show the output values on the display.

"The video-rendering chip includes a videorendering engine, a microprocessor, and a memory. The microprocessor receives a sampled display temperature from a temperature sensor, determines correction coefficients that correspond to the sampled display temperature, and provides the correction coefficients to the video-rendering engine. The video-rendering engine then utilizes multipliers to multiply the display input by the correction coefficients to produce the output values. The video-rendering engine may utilize a dithering component to dither the output values before providing the output values to the display driver.

"In some embodiments, the microprocessor may determine the correction coefficients by retrieving the correction coefficients that correspond to the sampled display temperature from a lookup table stored in the memory. The lookup table stored in the memory may include correction coefficients that correspond to the sampled display temperature. Alternatively, the correction coefficients may be interpolated from correction coefficients that correspond to other display temperatures included in the lookup table. In other embodiments, the microprocessor may determine the correction coefficients by retrieving a correction coefficient formula stored in the memory and applying the correction coefficient formula to the sampled display temperature to produce the correction coefficients.

"The microprocessor may receive a sampled display temperature periodically. After the microprocessor receives the display temperature at a first time and determines correction coefficients that corresponded to the sampled display temperature at the first time, the video-rendering engine may apply the correction coefficients that corresponded to the sampled display temperature at the first time to received display input until the microprocessor received a sampled display temperature at a second time. After the microprocessor received the display temperature sampled at the second time, the microprocessor determines correction coefficients that correspond to the display temperature sampled at the second time and the video-rendering engine applies the correction coefficients that correspond to the display temperature sampled at the second time to the display input received after the second time.

"In some embodiments, the video-rendering chip may sample the display temperature at fixed periods, such as every second. In other embodiments, the video-rendering chip may vary the periods at which the microprocessor receives the sampled display temperature based on the previously sampled display temperature."

-- Dennis Sellers
dsellers@applecentral.com

 
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