- pix-compare-gray-or-RGBprocedure
- in - PPIX1, PPIX2 - image data to compare
- in - CMPTYPE: CMP-SUBTRACT or CMP-ABS-DIFF
- in - PLOT - plot type, use 0 for no plot.
- out - PSAME PDIFF PRMSDIFF PPIXPTR - these are optional, use #f to skip.
- PSAME: ptr to int32 => 1 if pixel values identical
- PDIFF: ptr to float => avg diff
- PRMSDIFF: ptr to float => rms of diff
- PPIXDIFF: ptr to ppix => 8bpp image - pixels set to PPIX1/2 diff
- returns: 0 if “OK”, 1 if error.