Not sure if it was explained to me elsewhere before https://sites.google.com/view/ananyamukherjeehome/image-processing/color-spaces
==== UV coefficients from Y coefficients In a 3x3 RGB to YUV matrix, it is enough to know the RGB to Y conversion coefficients. The coefficients for U and V can be derived from them. [..] Often, x and y of red green and blue and a white point (eg D65 <https://www.google.com/url?q=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FIlluminant_D65&sa=D&sntz=1&usg=AOvVaw1bJwIHCrhis7NKs6vy6VKk>) are specified in specs such as BT 709 <https://www.google.com/url?q=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FRec._709%23Primary_chromaticities&sa=D&sntz=1&usg=AOvVaw3wBydyI2YYVmqxo_GZ-eSM>. Given the 6 primary chromaticity numbers and the 2 white point numbers, you can plug them in here <http://www.google.com/url?q=http%3A%2F%2Fwww.russellcottrell.com%2Fphoto%2FmatrixCalculator.htm&sa=D&sntz=1&usg=AOvVaw3UhBjMsRqOR1VdSDooBpYP> and get your RGB->XYZ matrix. The mathematical details of the conversion can be found here <http://www.google.com/url?q=http%3A%2F%2Fwww.brucelindbloom.com%2Findex.html%3FEqn_RGB_XYZ_Matrix.html&sa=D&sntz=1&usg=AOvVaw05o46rC34j3SnJHd_Mh6no>. Now the middle row of this matrix converts from RGB to Y, and this Y is the Y of YUV color space. So you can get the coefficients of Y from here given primary chromaticities ==== but of course it up to reader to figure out how limited/full range difference should be used in those calculations .... from example in mjpegtools-2.2.1/lavtools/jpeg2yuv.c /** Rescales the YUV values from the range 0..255 to the range 16..235 @param yp: buffer for Y plane of decoded JPEG @param up: buffer for U plane of decoded JPEG @param vp: buffer for V plane of decoded JPEG */ static void rescale_color_vals(int width, int height, uint8_t *yp, uint8_t *up, uint8_t *vp) { int x,y; for (y = 0; y < height; y++) for (x = 0; x < width; x++) yp[x+y*width] = (float)(yp[x+y*width]) * ((235.0 - 16.0)/255.0) + 16.0; for (y = 0; y < height/2; y++) for (x = 0; x < width/2; x++) { up[x+y*width/2] = (float)(up[x+y*width/2]) * ((240.0 - 16.0)/255.0) + 16.0; vp[x+y*width/2] = (float)(vp[x+y*width/2]) * ((240.0 - 16.0)/255.0) + 16.0; } } for 4:2:0 subsampled yuv arrays as far as I understand. so, this can be simplified to multiplication of chroma and luma values by their constants? Also matrix, but 2*1 ? And what to do with two differently sized matrixes? I am cornered myself ..... _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-user To unsubscribe, visit link above, or email ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe".
