compare.im6

compare.im6 [options...] reference-image changed-image [difference-image]

-channel type
    analyze specific channel(s), e.g., Red, RGB, Alpha

-colorspace type
    perform metric in given colorspace, e.g., RGB, Gray

-compose operator
    set compose method for difference, e.g., difference

-decipher filename
    convert cipher pixels to plain using key

-define key=value
    define compare-specific attributes

-density geometry
    horizontal/vertical resolution in pixels/inch

-depth value
    image depth in bits-per-pixel

-dissimilarity-threshold value
    maximum tolerated distortion for match (0-1)

-encipher filename
    convert plain pixels to cipher pixels

-extract geometry
    extract pixels by area/offset

-fuzz distance
    colors within this distance considered equal (0-100%)

-highlight-color color
    highlight dissimilar pixels with this color

-lowlight-color color
    lowlight similar pixels with this color

-metric type
    error metric: AE, MAE, MSE, NCC, PSNR, PAE, RMSE

-quality value
    JPEG/MIFF/PNG compression level (0-100)

-read-mask filename
    grayscale mask channel

-regard-warnings
    continue on warnings

-repage geometry
    adjust image page offsets

-resize geometry
    resize images before comparison

-respect-parentheses
    settings honored per image

-similarity-threshold value
    minimum distortion for match (0-1)

-size geometry
    widthxheight of raw image data

-synchronize
    sync pixels to scanline size

compare.im6 is a command-line tool from the ImageMagick 6 suite designed to mathematically and visually annotate differences between two similar images. It computes an error metric for each pixel and generates a difference image where dissimilar pixels are highlighted. This is useful for quality assurance, regression testing, or detecting subtle changes in images like screenshots or photographs.

The tool supports several metrics: AE (Absolute Error Count), MAE (Mean Absolute Error), MSE (Mean Squared Error), NCC (Normalized Cross Correlation), PSNR (Peak Signal to Noise Ratio), PAE (Peak Absolute Error), and RMSE (Root Mean Squared Error). By default, it uses MAE and creates a visualization with red for high differences and blue for low ones.

Users specify a reference image and a changed image; an optional output file receives the annotated difference image. Numerical results are printed to stdout, including the metric value and percentage similarity. Options allow channel-specific comparisons, fuzz tolerance for near-matches, custom highlight colors, and subimage matching with similarity/dissimilarity thresholds. It handles various formats via ImageMagick's delegate libraries.

Invocation requires both input images; omitting the output sends visualized differences to stdout as a PPM stream. Ideal for automation in CI/CD pipelines or batch processing.

Legacy ImageMagick 6 tool; prefer magick compare in v7+. Requires both input images; large images may be memory-intensive. Outputs numerical metrics always to stdout.

AE: count of differing pixels
MAE: average difference magnitude
MSE: average squared differences
PSNR: signal-to-noise ratio in dB
RMSE: square root of MSE

compare.im6 -metric PSNR image1.png image2.png diff.png
Outputs PSNR value and creates visualized diff.png

Developed as part of ImageMagick 6.x (circa 2006-2018); .im6 suffix distinguishes from v7 binaries. Widely used for image diffing before v7 unified 'magick' command.

convert.im6(1), identify.im6(1), montage.im6(1), magick(1), display.im6(1)