Add image diffs against previous build to CI results

Author: QuLogic

Diff for: .github/workflows/main.yaml

@@ -62,6 +62,18 @@ jobs:
             cheatsheets.pdf
             handout-*.pdf
             ./docs/_build/html/
+      - uses: actions/upload-artifact@v4
+        id: diffs-artifact-upload
+        if: ${{ always() }}
+        with:
+          name: diffs
+          path: |
+            diffs/
+      - name: Output artifacts URL
+        run: |
+          echo 'Artifact URL:' \
+            '${{ steps.diffs-artifact-upload.outputs.artifact-url }}' \
+            >> $GITHUB_STEP_SUMMARY
       - name: Publish cheatsheets and handouts
         if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' }}
         uses: peaceiris/actions-gh-pages@v3

Diff for: Makefile

@@ -51,6 +51,7 @@ check:
 	./check-num-pages.sh handout-beginner.pdf 1
 	./check-num-pages.sh handout-intermediate.pdf 1
 	./check-links.py cheatsheets.pdf
+	./check-diffs.py
 
 .PHONY: docs
 docs:

Diff for: check-diffs.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python
+
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+
+ROOT_DIR = Path(__file__).parent
+
+if os.environ.get('GITHUB_ACTIONS', '') == '':
+    print('Not running when not in GitHub Actions.')
+    sys.exit()
+summary_file = os.environ.get('GITHUB_STEP_SUMMARY')
+if summary_file is None:
+    sys.exit('$GITHUB_STEP_SUMMARY is not set')
+
+gh_pages = ROOT_DIR.parent / 'pages'
+subprocess.run(['git', 'fetch', 'https://github.com/matplotlib/cheatsheets.git',
+                'gh-pages:upstream-gh-pages'], check=True)
+subprocess.run(['git', 'worktree', 'add', gh_pages, 'upstream-gh-pages'],
+               check=True)
+
+diff_dir = ROOT_DIR / 'diffs'
+diff_dir.mkdir(exist_ok=True)
+
+hashes = {}
+for original in gh_pages.glob('*.png'):
+    result = subprocess.run(
+        ['compare', '-metric', 'PHASH',
+         original,
+         ROOT_DIR / 'docs/_build/html' / original.name,
+         diff_dir / f'{original.stem}-diff.png'],
+        text=True, stderr=subprocess.PIPE)
+    if result.returncode == 2:  # Some kind of IO or similar error.
+        hashes[original] = (float('nan'), result.stderr)
+    elif result.stderr:  # Images were different.
+        hashes[original] = (float(result.stderr), '')
+    else:  # No differences.
+        hashes[original] = (0.0, '')
+
+with open(summary_file, 'w+') as summary:
+    print('# Cheatsheet image comparison', file=summary)
+    print('| Filename | Perceptual Hash Difference | Error message |', file=summary)
+    print('| -------- | -------------------------- | ------------- |', file=summary)
+    for filename, (hash, message) in hashes.items():
+        message = message.replace('\n', ' ').replace('|', '\\|')
+        print(f'| {filename.name} | {hash:.05f} | {message}', file=summary)
+    print(file=summary)
+
+subprocess.run(['git', 'worktree', 'remove', gh_pages])