Digital Video Processing - Color Correction

Blue Stretch Blue stretch increases the blue value of white and near-white colors in order to make whites appear brighter. When applying blue stretch, only colors only within a specified color range should be processed. Colors with a Y value of ~80% or more of the maximum, have a low saturation value, and fall within a white detection area in the CbCrplane, have their blue components increased by ~4% (the blue gain factor) and their red components decreased the same amount. For more complex designs, the white detection area and blue gain factor can be dependent on the color’s Y value and saturation level. A transition boundary can be used around the white detection area for gradually decreasing the blue gain factor as colors move away from the white detection area boundary. This can prevent hard transitions between areas that are blue stretched and areas that are not. If a color falls inside the transition boundary area, it is blue stretched using a fraction of the blue gain factor, with the fraction decreasing as the distance from the edge of the detection area boundary increases.

Green Enhancement Green enhancement creates a richer, more saturated green color when the level of green is low. Displays commonly use this technique to provide greener looking grass, plants, etc. When applying green enhancement, only colors only within a specified color range should be processed. Colors with a low green saturation value, and fall within a green detection area in the CbCr-plane, have their saturation increased. Rather then centering the green detection area about the green axis some designs use ~213° for the green detection axis so the same design can also easily be used to implement skin tone correction. Simple implementations have the maximum saturation gain (~1.2×) occurring on the green detection axis, with the saturation gain decreasing to 1× as the distance from the green detection axis increases. For more complex designs, the green detection area and maximum saturation gain can be dependent on the color’s Y value and saturation level Some displays also use this technique to implement blue enhancement, used to make the sky appear more blue.

Dynamic Contrast Using dynamic contrast (also called adaptive contrast enhancement), the differences between dark and light portions of the image are artificially enhanced based on the content in the image. Displays commonly use this technique to improve their contrast ratio. Bright colors in mostly dark images are enhanced by making them brighter (white stretch). This is typically done by using histogram information to modify the upper portion of the gamma curve. Dark colors in mostly light images are enhanced by making them darker (black stretch). This is typically done by using histogram information to modify the lower portion of the gamma curve. For a medium-bright image, both techniques may be applied. A minor gamma correction adjustment may also be applied to colors that are between dark and light, resulting in a more detailed and contrasting picture.

Color Correction The RGB chromaticities are usually slightly different between the source video and what the display uses. This results in red, green and blue colors that are not completely accurate. Color correction can be done on the source video to compensate for the display characteristics, enabling more accurate red, green and blue colors to be displayed. An alternate type of color correction is to perform color expansion, taking advantage of the greater color reproduction capabilities of modern displays. This can result in greener greens, bluer blues, etc. One common technique of implementing color expansion is to use independent hue and saturation controls for each primary and complementary color, plus the skin color.

Color Temperature Correction In an uncalibrated television, the color temperature (white color) varies based on the brightness level. The color temperature of D65, the white point specified by most video standards, is 6500 °K. Color temperatures above 6500 °K. are more bluish (cool); color temperatures below 6500 °K. are more reddish (warm). Many televisions ship from the factory with a very high average color temperature (7000–8000 °K.) to emphasize the brightness of the set. Viewers can select from two or three factory presets (warm, cool, etc.) or viewing modes (movies, sports, etc.) which are a reference to the color temperature. A “cool” setting is brighter (like what you see in midday light) and is better for daylight viewing, such as sporting events, because of the enhanced brightness. A “warm” setting is softer (like what you see in a softly lit indoor environment) and is better for viewing movies, or in darkened environments. The color temperature may be finely adjusted by using a 3 × 3 matrix multiplier to process the YCbCr or R´G´B´ data. 10 registers (one for every 10 IRE step from 10–100 IRE) provide the nine coefficients for the 3 × 3 matrix multiplier. The values of the registers are determined by a calibrating process. YCbCr or R´G´B´ values for intermediate IRE levels may be determined using interpolation.

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