Compare/Diff Image Files

Some notes on comparing/contrasting/diffing image files.

Notes
See Also
- My Other Related Deepdive Gist's and Projects

Notes

ImageMagick – pixel-by-pixel image comparison

https://imagemagick.org
Install:
- macOS: brew install imagemagick
Basic comparison:
- ```
magick compare original.jpg altered.jpg -metric RMSE diff.png
```
- -metric RMSE → Root Mean Square Error (prints numerical difference)
- diff.png highlights pixel differences (black = no change; bright colors = differences)

Normalizing first avoids false positives from orientation/colorspace:

magick original.jpg  -auto-orient -colorspace sRGB -alpha off PNG24:orig_norm.png
magick altered.jpg   -auto-orient -colorspace sRGB -alpha off PNG24:alt_norm.png
magick compare -metric RMSE orig_norm.png alt_norm.png diff.png

Other useful metrics:

magick compare -metric PSNR  orig_norm.png alt_norm.png null:   # Peak Signal-to-Noise Ratio
magick compare -metric DSSIM orig_norm.png alt_norm.png null:   # Structural dissimilarity

RMSE: 0 = identical; 0.001–0.01 ≈ tiny difference
PSNR: >40 dB ≈ visually indistinguishable
DSSIM: 0 = identical; ≤0.01 ≈ negligible difference

Change highlight colors for easier viewing:

magick compare orig_norm.png alt_norm.png -metric RMSE \
  -highlight-color yellow -lowlight-color black diff.png

Visual toggling (flicker test)
- Side-by-side montage:
  - ```
  magick montage orig_norm.png alt_norm.png -tile 2x1 -geometry +0+0 side_by_side.png
```
- Or use an image viewer to rapidly flip between the two.

Perceptual metrics
- SSIMULACRA2 and Butteraugli are perceptual quality metrics from Google/Cloudinary, designed to align more with human vision than raw RMSE/PSNR.
- https://github.com/myint/perceptualdiff
  - perceptualdiff
  - A program that compares two images using a perceptually based image metric.
  - Install:
    - macOS: brew install perceptualdiff
  - Basic comparison:
    - perceptualdiff orig_norm.png alt_norm.png
    - Exit code 0 if perceptually identical, 1 if differences exceed threshold.
  - With visual diff output and verbose stats:
    - perceptualdiff orig_norm.png alt_norm.png -verbose -output diff.png
  - Adjusting sensitivity (higher threshold = less sensitive to small differences):
    - perceptualdiff orig_norm.png alt_norm.png -threshold 100 -verbose -output diff.png
GUI tools
- Beyond Compare – has an image compare mode with overlay/difference views.

0xdevalias/compare-diff-image-files.md

Compare/Diff Image Files

Table of Contents

Notes

See Also

My Other Related Deepdive Gist's and Projects