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LibGfx: Consider component interleaving when reading a scan

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Scan with only one component are by definition not interleaved, meaning
that each value is linearly ordered in the stream. Grayscale images
were supported thanks to a hack, by forcing the subsampling to 1.

Now we properly support grayscale image with other subsampling (even if
it doesn't make sense) and more generally scans with only one component
and any sampling factors.

While this solution is more general than the last one it also feels a
bit hackish. We should probably refactor the way we iterate over
components and macroblocks. But that's work for latter, especially when
we will add support for other subsampling than 4-2-2.
This commit is contained in:
Lucas CHOLLET 2023-02-25 17:15:54 -05:00 • committed by Andreas Kling
parent 893659c6aa
commit 2c98eff558
Notes: sideshowbarker 2024-07-17 03:35:16 +09:00
1 changed files with 10 additions and 6 deletions
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16
Userland/Libraries/LibGfx/JPEGLoader.cpp
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@ -202,6 +202,12 @@ struct Scan {
u8 successive_approximation {};
HuffmanStreamState huffman_stream;
// See the note on Figure B.4 - Scan header syntax
bool are_components_interleaved() const
{
return components.size() != 1;
}
};
struct JPEGLoadingContext {
@ -399,7 +405,11 @@ static ErrorOr<void> build_macroblocks(JPEGLoadingContext& context, Vector<Macro
for (u8 vfactor_i = 0; vfactor_i < scan_component.component.vsample_factor; vfactor_i++) {
for (u8 hfactor_i = 0; hfactor_i < scan_component.component.hsample_factor; hfactor_i++) {
// A.2.3 - Interleaved order
u32 mb_index = (vcursor + vfactor_i) * context.mblock_meta.hpadded_count + (hfactor_i + hcursor);
if (!context.current_scan.are_components_interleaved())
mb_index = vcursor * context.mblock_meta.hpadded_count + (hfactor_i + (hcursor * scan_component.component.vsample_factor) + (vfactor_i * scan_component.component.hsample_factor));
Macroblock& block = macroblocks[mb_index];
if (context.current_scan.spectral_selection_start == 0)
@ -844,12 +854,6 @@ static ErrorOr<void> read_start_of_frame(AK::SeekableStream& stream, JPEGLoading
component.vsample_factor = subsample_factors & 0x0F;
if (i == 0) {
// If there is only a single component, i.e. grayscale, the macroblocks will not be interleaved, even if
// the horizontal or vertical sample factor is larger than 1.
if (component_count == 1) {
component.hsample_factor = 1;
component.vsample_factor = 1;
}
// By convention, downsampling is applied only on chroma components. So we should
// hope to see the maximum sampling factor in the luma component.
if (!validate_luma_and_modify_context(component, context)) {