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LibGfx/JPEG: Move subsample-undoing to a separate function

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Previously, we handled sampling factors as part of ycbcr_to_rgb().
That meant it worked ok for code paths that used YCbCr ("normal"
jpegs, and the YCC part of YCCK jpegs), but it didn't work for
example for the K channel in YCCK jpegs, nor for CMYK.

By making this a separate pass, it should now work for all cases.
It also makes it easier to support more subsampling arrangements
in the future, and to use something better than nearest neighbor
for upsampling subsampled blocks.
This commit is contained in:
Nico Weber 2024-01-14 18:35:51 -05:00 • committed by Andrew Kaster
parent 398f34bb81
commit d99d086da3
Notes: sideshowbarker 2024-07-18 03:35:30 +09:00
1 changed files with 53 additions and 13 deletions
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66
Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp
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@ -1586,6 +1586,51 @@ static void inverse_dct(JPEGLoadingContext const& context, Vector<Macroblock>& m
}
}
static void undo_subsampling(JPEGLoadingContext const& context, Vector<Macroblock>& macroblocks)
{
// The first component has sampling factors of context.sampling_factors, while the others
// are either 1x1 or for the 4th component also context.sampling_factors. See
// read_start_of_frame() which currently enforces these restrictions.
// This function undoes the subsampling by duplicating the values of the smaller components.
// See https://www.w3.org/Graphics/JPEG/itu-t81.pdf, A.2 Order of source image data encoding.
//
// FIXME: Allow more combinations of sampling factors.
// See https://calendar.perfplanet.com/2015/why-arent-your-images-using-chroma-subsampling/ for
// subsampling factors visble on the web. In PDF files, YCCK 2111 and 2112 and CMYK 2111 and 2112 are also present.
for (u32 component_i = 0; component_i < context.components.size(); component_i++) {
auto& component = context.components[component_i];
if (component.sampling_factors == context.sampling_factors)
continue;
for (u32 vcursor = 0; vcursor < context.mblock_meta.vcount; vcursor += context.sampling_factors.vertical) {
for (u32 hcursor = 0; hcursor < context.mblock_meta.hcount; hcursor += context.sampling_factors.horizontal) {
u32 const component_block_index = vcursor * context.mblock_meta.hpadded_count + hcursor;
Macroblock& component_block = macroblocks[component_block_index];
auto* block_component_source = get_component(component_block, component_i);
// Overflows are intentional.
for (u8 vfactor_i = context.sampling_factors.vertical - 1; vfactor_i < context.sampling_factors.vertical; --vfactor_i) {
for (u8 hfactor_i = context.sampling_factors.horizontal - 1; hfactor_i < context.sampling_factors.horizontal; --hfactor_i) {
u32 macroblock_index = (vcursor + vfactor_i) * context.mblock_meta.hpadded_count + (hfactor_i + hcursor);
Macroblock& block = macroblocks[macroblock_index];
auto* block_component_destination = get_component(block, component_i);
for (u8 i = 7; i < 8; --i) {
for (u8 j = 7; j < 8; --j) {
u8 const pixel = i * 8 + j;
// The component is 8x8 subsampled 2x2. Upsample its 2x2 4x4 tiles.
u32 const component_pxrow = (i / context.sampling_factors.vertical) + 4 * vfactor_i;
u32 const component_pxcol = (j / context.sampling_factors.horizontal) + 4 * hfactor_i;
u32 const component_pixel = component_pxrow * 8 + component_pxcol;
block_component_destination[pixel] = block_component_source[component_pixel];
}
}
}
}
}
}
}
}
static void ycbcr_to_rgb(JPEGLoadingContext const& context, Vector<Macroblock>& macroblocks)
{
// Conversion from YCbCr to RGB isn't specified in the first JPEG specification but in the JFIF extension:
@ -1593,24 +1638,18 @@ static void ycbcr_to_rgb(JPEGLoadingContext const& context, Vector<Macroblock>&
// 7 - Conversion to and from RGB
for (u32 vcursor = 0; vcursor < context.mblock_meta.vcount; vcursor += context.sampling_factors.vertical) {
for (u32 hcursor = 0; hcursor < context.mblock_meta.hcount; hcursor += context.sampling_factors.horizontal) {
u32 const chroma_block_index = vcursor * context.mblock_meta.hpadded_count + hcursor;
Macroblock const& chroma = macroblocks[chroma_block_index];
// Overflows are intentional.
for (u8 vfactor_i = context.sampling_factors.vertical - 1; vfactor_i < context.sampling_factors.vertical; --vfactor_i) {
for (u8 hfactor_i = context.sampling_factors.horizontal - 1; hfactor_i < context.sampling_factors.horizontal; --hfactor_i) {
for (u8 vfactor_i = 0; vfactor_i < context.sampling_factors.vertical; ++vfactor_i) {
for (u8 hfactor_i = 0; hfactor_i < context.sampling_factors.horizontal; ++hfactor_i) {
u32 macroblock_index = (vcursor + vfactor_i) * context.mblock_meta.hpadded_count + (hcursor + hfactor_i);
auto* y = macroblocks[macroblock_index].y;
auto* cb = macroblocks[macroblock_index].cb;
auto* cr = macroblocks[macroblock_index].cr;
for (u8 i = 7; i < 8; --i) {
for (u8 j = 7; j < 8; --j) {
for (u8 i = 0; i < 8; ++i) {
for (u8 j = 0; j < 8; ++j) {
u8 const pixel = i * 8 + j;
u32 const chroma_pxrow = (i / context.sampling_factors.vertical) + 4 * vfactor_i;
u32 const chroma_pxcol = (j / context.sampling_factors.horizontal) + 4 * hfactor_i;
u32 const chroma_pixel = chroma_pxrow * 8 + chroma_pxcol;
int r = y[pixel] + 1.402f * (chroma.cr[chroma_pixel] - 128);
int g = y[pixel] - 0.3441f * (chroma.cb[chroma_pixel] - 128) - 0.7141f * (chroma.cr[chroma_pixel] - 128);
int b = y[pixel] + 1.772f * (chroma.cb[chroma_pixel] - 128);
int r = y[pixel] + 1.402f * (cr[pixel] - 128);
int g = y[pixel] - 0.3441f * (cb[pixel] - 128) - 0.7141f * (cr[pixel] - 128);
int b = y[pixel] + 1.772f * (cb[pixel] - 128);
y[pixel] = clamp(r, 0, 255);
cb[pixel] = clamp(g, 0, 255);
cr[pixel] = clamp(b, 0, 255);
@ -1934,6 +1973,7 @@ static ErrorOr<void> decode_jpeg(JPEGLoadingContext& context)
auto macroblocks = TRY(construct_macroblocks(context));
TRY(dequantize(context, macroblocks));
inverse_dct(context, macroblocks);
undo_subsampling(context, macroblocks);
TRY(handle_color_transform(context, macroblocks));
if (context.components.size() == 4)
TRY(compose_cmyk_bitmap(context, macroblocks));