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WebP/Lossy: Clamp negative quantization indices to zero

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The spec doesn't talk about this happening in the text, but
`dequant_init()` in 20.4 stores `q` in an int and clamps that
to 0 later.
This commit is contained in:
Nico Weber 2023-06-01 09:09:24 -04:00 • committed by Andreas Kling
parent c2ec97dd79
commit 661b2d394d
3 changed files with 24 additions and 3 deletions
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21
Tests/LibGfx/TestImageDecoder.cpp
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@ -404,6 +404,27 @@ TEST_CASE(test_webp_extended_lossy_uncompressed_alpha)
EXPECT_EQ(frame.image->get_pixel(355, 106), Gfx::Color(0, 0, 0, 0));
}
TEST_CASE(test_webp_extended_lossy_negative_quantization_offset)
{
auto file = MUST(Core::MappedFile::map(TEST_INPUT("smolkling.webp"sv)));
EXPECT(Gfx::WebPImageDecoderPlugin::sniff(file->bytes()));
auto plugin_decoder = MUST(Gfx::WebPImageDecoderPlugin::create(file->bytes()));
MUST(plugin_decoder->initialize());
EXPECT_EQ(plugin_decoder->frame_count(), 1u);
EXPECT(!plugin_decoder->is_animated());
EXPECT(!plugin_decoder->loop_count());
EXPECT_EQ(plugin_decoder->size(), Gfx::IntSize(264, 264));
auto frame = MUST(plugin_decoder->frame(0));
EXPECT_EQ(frame.image->size(), Gfx::IntSize(264, 264));
// While VP8 YUV contents are defined bit-exact, the YUV->RGB conversion isn't.
// So pixels changing by 1 or so below is fine if you change code.
EXPECT_EQ(frame.image->get_pixel(16, 16), Gfx::Color(0x3c, 0x24, 0x1a, 255));
}
TEST_CASE(test_webp_lossy_4)
{
// This is https://commons.wikimedia.org/wiki/File:Fr%C3%BChling_bl%C3%BChender_Kirschenbaum.jpg,

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Tests/LibGfx/test-inputs/smolkling.webp Normal file
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6
Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp
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@ -621,7 +621,7 @@ i16 dequantize_value(i16 value, bool is_dc, QuantizationIndices const& quantizat
y_ac_base = segmentation.quantizer_update_value[segment_id];
}
u8 dequantization_index;
int dequantization_index;
if (index.is_y2())
dequantization_index = y_ac_base + (is_dc ? quantization_indices.y2_dc_delta : quantization_indices.y2_ac_delta);
else if (index.is_u() || index.is_v())
@ -631,9 +631,9 @@ i16 dequantize_value(i16 value, bool is_dc, QuantizationIndices const& quantizat
// clamp index
if ((index.is_u() || index.is_v()) && is_dc)
dequantization_index = min(dequantization_index, 117);
dequantization_index = clamp(dequantization_index, 0, 117);
else
dequantization_index = min(dequantization_index, 127);
dequantization_index = clamp(dequantization_index, 0, 127);
// "the multiplies are computed and stored using 16-bit signed integers."
i16 dequantization_factor;