# compare — Compare Images and Generate a Difference Image > Practical guide to ImageMagick compare: diff two images, generate a difference image and measure metrics like AE, RMSE or SSIM for visual regression tests. Source: https://www.jpkc.com/db/en/cheatsheets/images-media/compare/ ImageMagick's compare puts two images side by side and surfaces every difference – both as a colour-highlighted diff image and as a measurable number. Use it to check whether a rendering, an export or a screenshot has drifted from a reference. Metrics such as AE, RMSE or SSIM together with the exit code make the comparison scriptable – ideal for visual regression tests in CI/CD pipelines. On ImageMagick 7, prefer calling it as `magick compare`. ## Basic Comparison `compare ` — Create a visual diff image highlighting differences in red. ```bash compare original.png modified.png diff.png ``` `compare -metric AE null:` — Count the number of different pixels (Absolute Error). ```bash compare -metric AE original.png modified.png null: ``` `compare -metric RMSE null:` — Calculate Root Mean Square Error between images. ```bash compare -metric RMSE original.png modified.png null: ``` `compare -metric SSIM null:` — Calculate Structural Similarity Index (1.0 = identical). ```bash compare -metric SSIM original.png modified.png null: ``` ## Comparison Metrics `compare -metric AE null:` — Absolute Error: total count of different pixels. ```bash compare -metric AE baseline.png current.png null: ``` `compare -metric MAE null:` — Mean Absolute Error: average difference per pixel. ```bash compare -metric MAE baseline.png current.png null: ``` `compare -metric MSE null:` — Mean Squared Error: average of squared differences. ```bash compare -metric MSE baseline.png current.png null: ``` `compare -metric PSNR null:` — Peak Signal-to-Noise Ratio (higher = more similar, inf = identical). ```bash compare -metric PSNR baseline.png current.png null: ``` `compare -metric PHASH null:` — Perceptual Hash: perceptual difference (0 = identical). ```bash compare -metric PHASH baseline.png current.png null: ``` `compare -metric NCC null:` — Normalized Cross-Correlation (1.0 = identical). ```bash compare -metric NCC baseline.png current.png null: ``` ## Visual Diff Styles `compare -highlight-color ` — Set the color used to highlight differences. ```bash compare -highlight-color blue original.png modified.png diff.png ``` `compare -lowlight-color ` — Set the color for unchanged areas. ```bash compare -lowlight-color white original.png modified.png diff.png ``` `compare -compose src ` — Show only the different pixels (transparent elsewhere). ```bash compare -compose src original.png modified.png diff.png ``` `compare -fuzz % ` — Allow tolerance for near-matching pixels. ```bash compare -fuzz 5% original.png modified.png diff.png ``` `compare -fuzz % -metric AE null:` — Count differences with color tolerance. ```bash compare -fuzz 2% -metric AE original.png modified.png null: ``` ## Subimage Search `compare -subimage-search ` — Find a smaller image within a larger image. ```bash compare -subimage-search icon.png screenshot.png match.png ``` `compare -subimage-search -metric RMSE null:` — Find subimage and report similarity score. ```bash compare -subimage-search -metric RMSE button.png screenshot.png null: ``` `compare -subimage-search -dissimilarity-threshold ` — Set threshold for subimage search (0.0 = exact, 1.0 = any match). ```bash compare -subimage-search -dissimilarity-threshold 0.2 icon.png page.png match.png ``` ## CI/CD & Automation `compare -metric AE -fuzz 1% diff.png && echo 'PASS' || echo 'FAIL'` — Compare with exit code (0 = identical, 1 = different). ```bash compare -metric AE -fuzz 1% baseline.png screenshot.png diff.png && echo 'PASS' || echo 'FAIL' ``` `DIFF=$(compare -metric AE null: 2>&1); [ "$DIFF" -lt ]` — Check if pixel difference is below a threshold. ```bash DIFF=$(compare -metric AE baseline.png current.png null: 2>&1); [ "$DIFF" -lt 100 ] && echo 'OK' || echo 'Too many changes' ``` `compare -metric SSIM null: 2>&1` — Capture SSIM score (outputs to stderr, redirect to capture). ```bash SSIM=$(compare -metric SSIM baseline.png current.png null: 2>&1); echo "Similarity: $SSIM" ``` `for f in baseline/*.png; do compare -metric AE "$f" "current/$(basename $f)" "diff/$(basename $f)" 2>&1; done` — Batch compare all baseline images against current versions. ```bash for f in baseline/*.png; do echo "$(basename $f): $(compare -metric AE "$f" "current/$(basename $f)" null: 2>&1)"; done ``` ## Conclusion compare is the tool to reach for when you need to not just see image differences but quantify them. A quick visual check is covered by the diff image; for automation you lean on `-metric AE` (counted pixels) or `-metric RMSE`/`SSIM` (similarity scores) and evaluate the exit code and stderr output. Budget some `-fuzz` to tolerate noise and lossy compression, otherwise every JPEG artefact trips a failure. Note that compare overwrites the named output file without asking, and on ImageMagick 7 prefer `magick compare`. ## Further Reading - [ImageMagick: compare](https://imagemagick.org/script/compare.php) – official reference for all compare options and metrics - [ImageMagick Usage: Comparing Images](https://usage.imagemagick.org/compare/) – in-depth, example-driven guide to image comparison ## Related Commands - [identify](https://www.jpkc.com/db/en/cheatsheets/images-media/identify/) – read the format, dimensions and metadata of an image - [composite](https://www.jpkc.com/db/en/cheatsheets/images-media/composite/) – overlay and combine images - [convert](https://www.jpkc.com/db/en/cheatsheets/images-media/convert/) – convert, resize and edit images