Xilinx pg013 LogiCORE IP RGB to YCrCb Color-Space Converter v5 ...

Color-Space Conversion BackgroundYR – YB – Y=CA 1 – CA – CB CB1 – CA CA + CB – 1 – CB– CA CA + CB – 1 1 – CBRGBEquation 4-2Coefficients CA and CB are chosen between 0 and 1, which guarantees that the range of Yis constrained between the maximum and minimum RGB values permitted, RGB max andRGB min respectively.The minimum and maximum values of R-Y are:min R-Y = RGB min – (CA*RGB min + (1- CA- CB)*RGB max + CB*RGB max ) = (CA-1) * (RGB max -RGB min )max R-Y = RGB max – (CA*RGB max + (1- CA- CB)*RGB min + CB*RGB min ) = (1-CA) * (RGB max -RGB min )Thus, the range of R-Y is:2( CA – 1) ( RGB max – RGB min )Equation 4-3Similarly, the minimum and maximum values of B-Y are:min B-Y =RGB min -(CA*RGB max +(1-CA-CB)RGB max +CB*RGB min )=(CB-1)(RGB max -RGB min )maxB-Y=RGB max -(CA*RGB min +(1-CA-CB)RGB min +CB*RGB max )=(1-CB)(RGB max -RGB min )Thus, the range of B-Y is:2( CB – 1) ( RGB max – RGB min )Equation 4-4In most practical implementations, the range of the luminance and chrominancecomponents should be equal. There are two ways to accomplish this: chrominancecomponents (B-Y and R-Y) can be normalized (compressed and offset compensated), orvalues above and below the luminance range can be clipped.Both clipping and dynamic range compression result in loss of information; however, theintroduced artifacts are different. To leverage differences in the input (RGB) range, differentstandards choose different trade-offs between clipping and normalization.The RGB to YCrCb color space conversion core facilitates both range compression andoptional clipping and clamping. Range, offset, clipping and clamping levels areparameterizable. The core supports conversions that fit the following general form:Y CA 1 – CA – CB CBC R = CC( 1 – CA) CC( CA + CB – 1) CC( – CB)C B CD( – CA) CD( CA + CB – 1) CD( 1 – CB)RGB+O YO CrO CbEquation 4-5RGB to YCrCb Color-Space Converter www.xilinx.com 34PG013 April 24, 2012