Do not load transparent pixels from subsequent GIF frames

Tests/test_file_gif.py

@@ -468,12 +468,25 @@ def test_dispose2_background(tmp_path):
         assert im.getpixel((0, 0)) == 0
 
 
-def test_iss634():
+def test_transparency_in_second_frame():
+    with Image.open("Tests/images/different_transparency.gif") as im:
+        assert im.info["transparency"] == 0
+
+        # Seek to the second frame
+        im.seek(im.tell() + 1)
+        assert im.info["transparency"] == 0
+
+        assert_image_equal_tofile(im, "Tests/images/different_transparency_merged.gif")
+
+
+def test_no_transparency_in_second_frame():
     with Image.open("Tests/images/iss634.gif") as img:
         # Seek to the second frame
         img.seek(img.tell() + 1)
+        assert "transparency" not in img.info
+
         # All transparent pixels should be replaced with the color from the first frame
-        assert img.histogram()[img.info["transparency"]] == 0
+        assert img.histogram()[255] == 0
 
 
 def test_duration(tmp_path):

Tests/test_file_webp_animated.py

@@ -45,12 +45,12 @@ def test_write_animation_L(tmp_path):
             # Compare first and last frames to the original animated GIF
             orig.load()
             im.load()
-            assert_image_similar(im, orig.convert("RGBA"), 25.0)
+            assert_image_similar(im, orig.convert("RGBA"), 32.9)
             orig.seek(orig.n_frames - 1)
             im.seek(im.n_frames - 1)
             orig.load()
             im.load()
-            assert_image_similar(im, orig.convert("RGBA"), 25.0)
+            assert_image_similar(im, orig.convert("RGBA"), 32.9)
 
 
 @pytest.mark.xfail(is_big_endian(), reason="Fails on big-endian")

src/PIL/GifImagePlugin.py

@@ -145,7 +145,6 @@ def _seek(self, frame):
             self.dispose_extent = [0, 0, 0, 0]  # x0, y0, x1, y1
             self.__frame = -1
             self.__fp.seek(self.__rewind)
-            self._prev_im = None
             self.disposal_method = 0
         else:
             # ensure that the previous frame was loaded
@@ -174,6 +173,8 @@ def _seek(self, frame):
         self.palette = copy(self.global_palette)
 
         info = {}
+        frame_transparency = None
+        interlace = None
         while True:
 
             s = self.fp.read(1)
@@ -192,7 +193,7 @@ def _seek(self, frame):
                     #
                     flags = block[0]
                     if flags & 1:
-                        info["transparency"] = block[3]
+                        frame_transparency = block[3]
                     info["duration"] = i16(block, 1) * 10
 
                     # disposal method - find the value of bits 4 - 6
@@ -250,9 +251,6 @@ def _seek(self, frame):
                 # image data
                 bits = self.fp.read(1)[0]
                 self.__offset = self.fp.tell()
-                self.tile = [
-                    ("gif", (x0, y0, x1, y1), self.__offset, (bits, interlace))
-                ]
                 break
 
             else:
@@ -278,11 +276,26 @@ def _seek(self, frame):
         except (AttributeError, KeyError):
             pass
 
-        if not self.tile:
+        if interlace is not None:
+            transparency = -1
+            if frame_transparency is not None:
+                if frame == 0:
+                    self.info["transparency"] = frame_transparency
+                else:
+                    transparency = frame_transparency
+            self.tile = [
+                (
+                    "gif",
+                    (x0, y0, x1, y1),
+                    self.__offset,
+                    (bits, interlace, transparency),
+                )
+            ]
+        else:
             # self.__fp = None
             raise EOFError
 
-        for k in ["transparency", "duration", "comment", "extension", "loop"]:
+        for k in ["duration", "comment", "extension", "loop"]:
             if k in info:
                 self.info[k] = info[k]
             elif k in self.info:
@@ -295,20 +308,6 @@ def _seek(self, frame):
     def tell(self):
         return self.__frame
 
-    def load_end(self):
-        ImageFile.ImageFile.load_end(self)
-
-        # if the disposal method is 'do not dispose', transparent
-        # pixels should show the content of the previous frame
-        if self._prev_im and self._prev_disposal_method == 1:
-            # we do this by pasting the updated area onto the previous
-            # frame which we then use as the current image content
-            updated = self._crop(self.im, self.dispose_extent)
-            self._prev_im.paste(updated, self.dispose_extent, updated.convert("RGBA"))
-            self.im = self._prev_im
-        self._prev_im = self.im.copy()
-        self._prev_disposal_method = self.disposal_method
-
     def _close__fp(self):
         try:
             if self.__fp != self.fp:

src/decode.c

@@ -430,7 +430,8 @@ PyImaging_GifDecoderNew(PyObject *self, PyObject *args) {
     char *mode;
     int bits = 8;
     int interlace = 0;
-    if (!PyArg_ParseTuple(args, "s|ii", &mode, &bits, &interlace)) {
+    int transparency = -1;
+    if (!PyArg_ParseTuple(args, "s|iii", &mode, &bits, &interlace, &transparency)) {
         return NULL;
     }
 
@@ -448,6 +449,7 @@ PyImaging_GifDecoderNew(PyObject *self, PyObject *args) {
 
     ((GIFDECODERSTATE *)decoder->state.context)->bits = bits;
     ((GIFDECODERSTATE *)decoder->state.context)->interlace = interlace;
+    ((GIFDECODERSTATE *)decoder->state.context)->transparency = transparency;
 
     return (PyObject *)decoder;
 }

src/libImaging/Gif.h

@@ -30,6 +30,9 @@ typedef struct {
      */
     int interlace;
 
+    /* The transparent palette index, or -1 for no transparency. */
+    int transparency;
+
     /* PRIVATE CONTEXT (set by decoder) */
 
     /* Interlace parameters */

src/libImaging/GifDecode.c

@@ -248,29 +248,33 @@ ImagingGifDecode(Imaging im, ImagingCodecState state, UINT8 *buffer, Py_ssize_t
         /* To squeeze some extra pixels out of this loop, we test for
            some common cases and handle them separately. */
 
-        /* FIXME: should we handle the transparency index in here??? */
-
-        if (i == 1) {
-            if (state->x < state->xsize - 1) {
-                /* Single pixel, not at the end of the line. */
-                *out++ = p[0];
-                state->x++;
+        /* If we have transparency, we need to use the regular loop. */
+        if (context->transparency == -1) {
+            if (i == 1) {
+                if (state->x < state->xsize - 1) {
+                    /* Single pixel, not at the end of the line. */
+                    *out++ = p[0];
+                    state->x++;
+                    continue;
+                }
+            } else if (state->x + i <= state->xsize) {
+                /* This string fits into current line. */
+                memcpy(out, p, i);
+                out += i;
+                state->x += i;
+                if (state->x == state->xsize) {
+                    NEWLINE(state, context);
+                }
                 continue;
             }
-        } else if (state->x + i <= state->xsize) {
-            /* This string fits into current line. */
-            memcpy(out, p, i);
-            out += i;
-            state->x += i;
-            if (state->x == state->xsize) {
-                NEWLINE(state, context);
-            }
-            continue;
         }
 
         /* No shortcut, copy pixel by pixel */
         for (c = 0; c < i; c++) {
-            *out++ = p[c];
+            if (p[c] != context->transparency) {
+                *out = p[c];
+            }
+            out++;
             if (++state->x >= state->xsize) {
                 NEWLINE(state, context);
             }