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⚡ TLDR: pnmpsnr (tldr-pages)

Compute the difference between two images.

Compute the difference, i.e. the peak signal-to-noise ratio (PSNR) between two images
pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}}
Compare the color components rather than the luminance and chrominance components of the images
pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -rgb
Run in comparison mode, i.e. only output `nomatch` or `match` depending on whether the computing PSNR exceeds `n` or not
pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -target {{n}}
Run in comparison mode and compare the individual image components, i.e. Y, Cb, and Cr, to the corresponding thresholds
pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -target1 {{threshold_Y}} -target2 {{threshold_Cb}} -target3 {{threshold_Cr}}
Run in comparison mode and compare the individual image components, i.e. red, green, and blue to the corresponding thresholds
pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -rgb -target1 {{threshold_red}} -target2 {{threshold_green}} -target3 {{threshold_blue}}
Produce machine-readable output
pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -machine

pnmpsnr(1) General Commands Manual pnmpsnr(1)

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.