 However, there are only one fourth as many U and V values.The U and V values correspond to each 2 by 2 block of the image, meaning each U and V entry applies to four pixels.After the Y′ values, the next X/4 indices are the U values for each 2 by 2 block, and the next X/4 indices after that are the V values that also apply to each 2 by 2 block.