The output files are the subsampled raw YUV representation of the input PPM image, as required by the Stanford MPEG codec. The Y output file contains a byte for each pixel in the image, with the rows going from top to bottom and the columns within each row going left to right. The U and V output files are arranged similarly, except that each byte represents a square of 4 pixels of the image. The value is the arithmetic mean of the value for each of those 4 pixels. Hence, the Y file is 4 times the size of the U file or V file.
The YUV values are scaled according to CCIR.601, as assumed by MPEG.
Based on ppmtoyuv.c