C# Class BitMiracle.LibJpeg.Classic.Internal.jpeg_color_converter

Convert some rows of samples to the JPEG colorspace. This version handles Adobe-style CMYK->YCCK conversion, where we convert R=1-C, G=1-M, and B=1-Y to YCbCr using the same conversion as above, while passing K (black) unchanged. We assume rgb_ycc_start has been called.

Convert some rows of samples to the JPEG colorspace. This version handles grayscale output with no conversion. The source can be either plain grayscale or YCC (since Y == gray).

Convert some rows of samples to the JPEG colorspace. This version handles multi-component colorspaces without conversion. We assume input_components == num_components.

Convert some rows of samples to the JPEG colorspace. This version handles RGB->grayscale conversion, which is the same as the RGB->Y portion of RGB->YCbCr. We assume rgb_ycc_start has been called (we only use the Y tables).

RGB -> YCbCr conversion: most common case YCbCr is defined per CCIR 601-1, except that Cb and Cr are normalized to the range 0..MAXJSAMPLE rather than -0.5 .. 0.5. The conversion equations to be implemented are therefore Y = 0.29900 * R + 0.58700 * G + 0.11400 * B Cb = -0.16874 * R - 0.33126 * G + 0.50000 * B + CENTERJSAMPLE Cr = 0.50000 * R - 0.41869 * G - 0.08131 * B + CENTERJSAMPLE (These numbers are derived from TIFF 6.0 section 21, dated 3-June-92.) To avoid floating-point arithmetic, we represent the fractional constants as integers scaled up by 2^16 (about 4 digits precision); we have to divide the products by 2^16, with appropriate rounding, to get the correct answer. For even more speed, we avoid doing any multiplications in the inner loop by precalculating the constants times R,G,B for all possible values. For 8-bit JSAMPLEs this is very reasonable (only 256 entries per table); for 12-bit samples it is still acceptable. It's not very reasonable for 16-bit samples, but if you want lossless storage you shouldn't be changing colorspace anyway. The CENTERJSAMPLE offsets and the rounding fudge-factor of 0.5 are included in the tables to save adding them separately in the inner loop.

Initialize for RGB->YCC colorspace conversion.