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Allow alpha_composite to use LA images #9066

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Merged
hugovk merged 3 commits into from
Aug 27, 2025
Merged

Allow alpha_composite to use LA images #9066

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5554e77
Removed unnecessary checks
radarhere Jul 5, 2025
3152da4
Allow alpha_composite to use LA images
radarhere Jul 5, 2025
7ec13fe
Merge branch 'main' into alpha_composite
radarhere Jul 7, 2025
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31 changes: 31 additions & 0 deletions Tests/test_image.py
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Original file line number Diff line number Diff line change
Expand Up @@ -398,6 +398,37 @@ def test_alpha_composite(self) -> None:
assert img_colors is not None
assert sorted(img_colors) == expected_colors

def test_alpha_composite_la(self) -> None:
# Arrange
expected_colors = sorted(
[
(3300, (255, 255)),
(1156, (170, 192)),
(1122, (128, 255)),
(1089, (0, 0)),
(1122, (255, 128)),
(1122, (0, 128)),
(1089, (0, 255)),
]
)

dst = Image.new("LA", size=(100, 100), color=(0, 255))
draw = ImageDraw.Draw(dst)
draw.rectangle((0, 33, 100, 66), fill=(0, 128))
draw.rectangle((0, 67, 100, 100), fill=(0, 0))
src = Image.new("LA", size=(100, 100), color=(255, 255))
draw = ImageDraw.Draw(src)
draw.rectangle((33, 0, 66, 100), fill=(255, 128))
draw.rectangle((67, 0, 100, 100), fill=(255, 0))

# Act
img = Image.alpha_composite(dst, src)

# Assert
img_colors = img.getcolors()
assert img_colors is not None
assert sorted(img_colors) == expected_colors

def test_alpha_inplace(self) -> None:
src = Image.new("RGBA", (128, 128), "blue")

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5 changes: 2 additions & 3 deletions src/PIL/Image.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3588,9 +3588,8 @@ def alpha_composite(im1: Image, im2: Image) -> Image:
"""
Alpha composite im2 over im1.

:param im1: The first image. Must have mode RGBA.
:param im2: The second image. Must have mode RGBA, and the same size as
the first image.
:param im1: The first image. Must have mode RGBA or LA.
:param im2: The second image. Must have the same mode and size as the first image.
:returns: An :py:class:`~PIL.Image.Image` object.
"""

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7 changes: 3 additions & 4 deletions src/libImaging/AlphaComposite.c
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Expand Up @@ -25,13 +25,12 @@ ImagingAlphaComposite(Imaging imDst, Imaging imSrc) {
int x, y;

/* Check arguments */
if (!imDst || !imSrc || strcmp(imDst->mode, "RGBA") ||
imDst->type != IMAGING_TYPE_UINT8 || imDst->bands != 4) {
if (!imDst || !imSrc ||
(strcmp(imDst->mode, "RGBA") && strcmp(imDst->mode, "LA"))) {
return ImagingError_ModeError();
}

if (strcmp(imDst->mode, imSrc->mode) || imDst->type != imSrc->type ||
imDst->bands != imSrc->bands || imDst->xsize != imSrc->xsize ||
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@radarhere radarhere Jul 5, 2025

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If the images are the same mode, then they will naturally have the same type and number of bands.

if (strcmp(imDst->mode, imSrc->mode) || imDst->xsize != imSrc->xsize ||
imDst->ysize != imSrc->ysize) {
return ImagingError_Mismatch();
}
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