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LibGfx/TIFF: Apply the HorizontalDifferencing on the alpha channel

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When present, the alpha channel is also affected by the horizontal
differencing predictor.

The test case was generated with GIMP with the following steps:
 - Open an RGB image
 - Add a transparency layer
 - Export as TIFF with the LZW compression scheme
This commit is contained in:
Lucas CHOLLET 2024-01-22 21:19:42 -05:00 committed by Andrew Kaster
parent 0e6d87fe83
commit 1faf9bb44f
Notes: sideshowbarker 2024-07-16 18:03:21 +09:00 
Author: https://github.com/LucasChollet
Commit: https://github.com/SerenityOS/serenity/commit/1faf9bb44f
Pull-request: https://github.com/SerenityOS/serenity/pull/22908
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/nico ✅
3 changed files with 14 additions and 0 deletions
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12
Tests/LibGfx/TestImageDecoder.cpp
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@ -676,6 +676,18 @@ TEST_CASE(test_tiff_palette_alpha)
EXPECT_EQ(frame.image->get_pixel(60, 75), Gfx::Color::NamedColor::Red);
}
TEST_CASE(test_tiff_alpha_predictor)
{
auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("tiff/alpha_predictor.tiff"sv)));
EXPECT(Gfx::TIFFImageDecoderPlugin::sniff(file->bytes()));
auto plugin_decoder = TRY_OR_FAIL(Gfx::TIFFImageDecoderPlugin::create(file->bytes()));
auto frame = TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 400, 300 }));
EXPECT_EQ(frame.image->get_pixel(0, 0).alpha(), 255);
EXPECT_EQ(frame.image->get_pixel(60, 75), Gfx::Color::NamedColor::Red);
}
TEST_CASE(test_tiff_16_bits)
{
auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("tiff/16_bits.tiff"sv)));

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Tests/LibGfx/test-inputs/tiff/alpha_predictor.tiff Normal file
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2
Userland/Libraries/LibGfx/ImageFormats/TIFFLoader.cpp
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@ -267,6 +267,8 @@ private:
color.set_red(last_color->red() + color.red());
color.set_green(last_color->green() + color.green());
color.set_blue(last_color->blue() + color.blue());
if (alpha_channel_index().has_value())
color.set_alpha(last_color->alpha() + color.alpha());
}
last_color = color;