Merge pull request #4726 from hugovk/fix-png-disposal-comparison-bug

APNG: Fix setting disposal

Tests/test_file_apng.py

@@ -494,6 +494,26 @@
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
 
+def test_apng_save_disposal_previous(tmp_path):
+    test_file = str(tmp_path / "temp.png")
+    size = (128, 64)
+    transparent = Image.new("RGBA", size, (0, 0, 0, 0))
+    red = Image.new("RGBA", size, (255, 0, 0, 255))
+    green = Image.new("RGBA", size, (0, 255, 0, 255))
+
+    # test APNG_DISPOSE_OP_NONE
+    transparent.save(
+        test_file,
+        save_all=True,
+        append_images=[red, green],
+        disposal=PngImagePlugin.APNG_DISPOSE_OP_PREVIOUS,
+    )
+    with Image.open(test_file) as im:
+        im.seek(2)
+        assert im.getpixel((0, 0)) == (0, 255, 0, 255)
+        assert im.getpixel((64, 32)) == (0, 255, 0, 255)
+
+
 def test_apng_save_blend(tmp_path):
     test_file = str(tmp_path / "temp.png")
     size = (128, 64)

src/PIL/PngImagePlugin.py

@@ -1038,7 +1038,7 @@
                 prev_disposal = previous["encoderinfo"].get("disposal")
                 prev_blend = previous["encoderinfo"].get("blend")
                 if prev_disposal == APNG_DISPOSE_OP_PREVIOUS and len(im_frames) < 2:
-                    prev_disposal == APNG_DISPOSE_OP_BACKGROUND
+                    prev_disposal = APNG_DISPOSE_OP_BACKGROUND
 
                 if prev_disposal == APNG_DISPOSE_OP_BACKGROUND:
                     base_im = previous["im"]