Exclude Group 3 images from optimization

src/ocrmypdf/optimize.py

@@ -95,6 +95,10 @@ def extract_image_filter(
         log.debug(f"Skipping JPEG2000 iamge, xref {xref}")
         return None  # Don't do JPEG2000
 
+    if filtdp[0] == Name.CCITTFaxDecode and filtdp[1].get('/K', 0) >= 0:
+        log.debug(f"Skipping CCITT Group 3 image, xref {xref}")
+        return None  # pikepdf doesn't support Group 3 yet
+
     if Name.Decode in image:
         log.debug(f"Skipping image with Decode table, xref {xref}")
         return None  # Don't mess with custom Decode tables

tests/test_optimize.py

@@ -185,3 +185,16 @@ def test_optimize_off(resources, outpdf):
         '--plugin',
         'tests/plugins/tesseract_noop.py',
     )
+
+
+def test_group3(resources, outdir):
+    with pikepdf.open(resources / 'ccitt.pdf') as pdf:
+        im = pdf.pages[0].Resources.XObject['/Im1']
+        assert (
+            opt.extract_image_filter(pdf, outdir, im, im.objgen[0]) is not None
+        ), "Group 4 should be allowed"
+
+        im.DecodeParms['/K'] = 0
+        assert (
+            opt.extract_image_filter(pdf, outdir, im, im.objgen[0]) is None
+        ), "Group 3 should be disallowed"