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LibGfx/BMP: Clear alpha in palette entries

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The semantics of BGRx8888 aren't super clear and it means different
things for different parts of the codebase. In particular, the PNG
writer still writes the x channel to the alpha channel of its output.

In BMPs, the 4th palette byte is usually 0, which means after #21412 we
started writing all .bmp files with <= 8bpp as completely transparent
to PNGs.

This works around that.

(See also #19464 for previous similar workarounds.)

The added `bitmap.bmp` is a 1bpp file I drew in Photoshop and saved
using its "Save as..." saving path.
This commit is contained in:
Nico Weber 2024-03-04 19:56:13 -05:00 • committed by Sam Atkins
parent b9a5442b59
commit ab143e9b0e
3 changed files with 12 additions and 2 deletions
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10
Tests/LibGfx/TestImageDecoder.cpp
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@ -71,6 +71,16 @@ TEST_CASE(test_bmp_top_down)
TRY_OR_FAIL(expect_single_frame(*plugin_decoder));
}
TEST_CASE(test_bmp_1bpp)
{
auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("bmp/bitmap.bmp"sv)));
EXPECT(Gfx::BMPImageDecoderPlugin::sniff(file->bytes()));
auto plugin_decoder = TRY_OR_FAIL(Gfx::BMPImageDecoderPlugin::create(file->bytes()));
auto frame = TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 399, 400 }));
EXPECT_EQ(frame.image->begin()[0], 0xff'ff'ff'ff);
}
TEST_CASE(test_ico_malformed_frame)
{
Array test_inputs = {

BIN
Tests/LibGfx/test-inputs/bmp/bitmap.bmp Normal file
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4
Userland/Libraries/LibGfx/ImageFormats/BMPLoader.cpp
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@ -979,11 +979,11 @@ static ErrorOr<void> decode_bmp_color_table(BMPLoadingContext& context)
if (bytes_per_color == 4) {
if (!streamer.has_u32())
return Error::from_string_literal("Cannot read 32 bits");
context.color_table.append(streamer.read_u32());
context.color_table.append(streamer.read_u32() | 0xff'00'00'00);
} else {
if (!streamer.has_u24())
return Error::from_string_literal("Cannot read 24 bits");
context.color_table.append(streamer.read_u24());
context.color_table.append(streamer.read_u24() | 0xff'00'00'00);
}
}