Using ImageMagick to visually diff images

ImageMagick is a well-known CLI tool that provides image processing capabilities. It can handle hundreds of different image formats and gives you a programmatic interface to create, edit, compose, and convert images. I personally use the convert command very often to resize an image (e.g., convert -resize 300x300 -quality 100 source.png out.png) or convert between different formats. ImageMagick also has the ability to produce a visual diff between two images, by using its compare & montage commands.

Looking at the man pages of the commands, we find these descriptions:

• compare(1) - mathematically and visually annotate the difference between an image and its reconstruction.
• convert(1) - convert between image formats as well as resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more.
• montage(1) - create a composite image by combining several separate images. The images are tiled on the composite image optionally adorned with a border, frame, image name, and more.

To illustrate a simple use case, we'll use compare to produce a visual representation of the differences between the following images. We'll then use montage to create a single image containing both the original images and the visual difference.

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Comparing images

We'll use the compare command to visually annotate differences between two input images. This command takes a -highlight-color option which controls the color to use for annotating the differences (I prefer seagreen). We'll use the defaults for the rest (do check the man page, there are tons of configuration options). Let's compare the two images and save the output to a png file called compare.png:

compare 2.png 1.png -highlight-color seagreen png:compare.png

Combining it all

We now have three separate image files, the two original images and the visual diff. Let's combine them into one for convenience! The montage command allows us to do this - we'll use it to lay out the three images next to each other in the order the images are provided:

montage -geometry +4+4 2.png compare.png 1.png png:montage.png

Leveraging stdout & stdin

ImageMagick provides great support for pipes. We can use this to reduce the above commands into a one-liner, and in doing so also avoid having to create intermediary files (- as a filename represents stdout/stdin, per common conventions):

compare 2.png 1.png -highlight-color seagreen png:- | montage -geometry +4+4 2.png - 1.png png:-

Conclusion

ImageMagick is a powerful tool with capabilities that exceed most people's expectations. Use it to automate image processing tasks! The following is a script that I use for simple comparisons (to use it: name it something cool, chmod +x it, and put it somewhere in your PATH).

#!/usr/bin/env bash

if [ -t 1 ]; then
    >&2 echo "Refusing to diff image in terminal without redirecting stdout."
    exit 1
fi

if [[ $1 == "-grayscale" ]]; then
    shift 1
    convert '(' "$2" -flatten -grayscale Rec709Luminance ')' \
        '(' "$1" -flatten -grayscale Rec709Luminance ')' \
        '(' -clone 0-1 -compose darken -composite ')' \
        -channel RGB -combine png:-;
else
    compare "$2" "$1" -highlight-color seagreen png:- | montage -geometry +4+4 "$2" - "$1" png:-;
fi

I keep this script in a user-local bin directory:

$ ls -l ~/.local/bin
imgdiff

$ imgdiff 1.png 2.png > diff.png