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Fix issues with GIF transparency #3434

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Fix issues with GIF transparency #3434

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e44f567
Add a test for an image that works with other GIF decoders.
zewt Oct 24, 2018
35c6ebd
GIF: Support transparency in the native decoder.
zewt Oct 24, 2018
68b844d
GIF: Handle GIF transparency in the decoder.
zewt Oct 24, 2018
df10a72
GIF: Clear the buffer to the correct initial color before decoding fr…
zewt Oct 24, 2018
b5806c7
GIF: Clear to transparency rather than background if available.
zewt Oct 24, 2018
87e54b4
GIF: Fix the previous frame not always being decoded in _seek.
zewt Oct 24, 2018
2dcf6ca
Fix Flake8
hugovk Dec 29, 2018
4ff4770
Fix Flake8
hugovk Dec 29, 2018
644e7c0
Replaced tabs with spaces
radarhere Mar 15, 2019
60c068f
Fixed indentation
radarhere Mar 15, 2019
f96b405
Merge branch 'master' into gif-transparency
radarhere Apr 2, 2021
be24990
Merge branch 'master' into gif-transparency
radarhere Apr 16, 2021
8bfd763
Use context manager
radarhere Jun 25, 2021
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Binary file added Tests/images/transparent_dispose.gif
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10 changes: 10 additions & 0 deletions Tests/test_file_gif.py
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Original file line number Diff line number Diff line change
Expand Up @@ -331,6 +331,16 @@ def test_dispose_background():
pass


def test_transparent_dispose():
expected_colors = [(2, 1, 2), (0, 1, 0), (2, 1, 2)]
with Image.open("Tests/images/transparent_dispose.gif") as img:
for frame in range(3):
img.seek(frame)
for x in range(3):
color = img.getpixel((x, 0))
assert color == expected_colors[frame][x]


def test_dispose_previous():
with Image.open("Tests/images/dispose_prev.gif") as img:
try:
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23 changes: 19 additions & 4 deletions src/PIL/GifImagePlugin.py
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Expand Up @@ -262,16 +262,20 @@ def _seek(self, frame):
# do not dispose or none specified
self.dispose = None
elif self.disposal_method == 2:
# replace with background colour
# replace with transparency if there is one, otherwise
# background colour
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Why potentially use the transparency? I don't see this in the spec.

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I've found https://legacy.imagemagick.org/Usage/anim_basics/#dispose

is cleared to transparency
...
There is some thinking that rather than clearing the overlaid area to the transparent color, this disposal should clear it to the 'background' color meta-data setting stored in the GIF animation
...
However all modern web browsers clear just the area that was last overlaid to transparency, as such this is now accepted practice, and what IM now follows.


# only dispose the extent in this frame
x0, y0, x1, y1 = self.dispose_extent
dispose_size = (x1 - x0, y1 - y0)

Image._decompression_bomb_check(dispose_size)
self.dispose = Image.core.fill(
"P", dispose_size, self.info.get("background", 0)
)

if frame_transparency is not None:
bg = frame_transparency
else:
bg = self.info.get("background", 0)
self.dispose = Image.core.fill("P", dispose_size, bg)
else:
# replace with previous contents
if self.im:
Expand Down Expand Up @@ -309,6 +313,17 @@ def _seek(self, frame):
if self.palette:
self.mode = "P"

def load_prepare(self):
super(GifImageFile, self).load_prepare()

if self.__frame == 0:
# On the first frame, clear the buffer. Use the transparency index
# if we have one, otherwise use the background index.
default_color = self.info.get(
"transparency", self.info.get("background", 0)
)
self.im.paste(default_color, (0, 0, self.size[0], self.size[1]))
Comment on lines +317 to +325
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Suggested change
super(GifImageFile, self).load_prepare()
if self.__frame == 0:
# On the first frame, clear the buffer. Use the transparency index
# if we have one, otherwise use the background index.
default_color = self.info.get(
"transparency", self.info.get("background", 0)
)
self.im.paste(default_color, (0, 0, self.size[0], self.size[1]))
if not self.im:
# On the first frame, clear the buffer. Use the transparency index
# if we have one, otherwise use the background index.
default_color = self.info.get(
"transparency", self.info.get("background", 0)
)
self.im = Image.core.fill(self.mode, self.size, default_color)
super(GifImageFile, self).load_prepare()

This saves the parent load_prepare from creating an image only to have it overwritten here. Instead, this just creates the image and the parent does not.

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I've created #5557 with this functionality, minus the background color fallback.


def tell(self):
return self.__frame

Expand Down