NAME¶
pnmpsnr - compute the difference between two portable anymaps
SYNOPSIS¶
pnmpsnr [
pnmfile1] [
pnmfile2]
DESCRIPTION¶
Reads two PBM, PGM, or PPM files, or PAM equivalents, as input. Prints the peak
signal-to-noise ratio (PSNR) difference between the two images. This metric is
typically used in image compression papers to rate the distortion between
original and decoded image.
If the inputs are PBM or PGM,
pnmpsnr prints the PSNR of the luminance
only. Otherwise, it prints the separate PSNRs of the luminance, and
chrominance (Cb and Cr) components of the colors.
The PSNR of a given component is the ratio of the mean square difference of the
component for the two images to the maximum mean square difference that can
exist betwee any two images. It is expressed as a decibel value.
The mean square difference of a component for two images is the mean square
difference of the component value, comparing each pixel with the pixel in the
same position of the other image. For the purposes of this computation,
components are normalized to the scale [0..1].
The maximum mean square difference is identically 1.
So the higher the PSNR, the closer the images are. A luminance PSNR of 20 means
the mean square difference of the luminances of the pixels is 100 times less
than the maximum possible difference, i.e. 0.01.
SEE ALSO¶
pnm(5)