compute the difference between two portable anymaps
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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
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
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].
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.