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LibGfx/JPEG: Bring IDCT and YCbCr conversion closer to specification

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As mentioned in F.2.1.5 - Inverse DCT (IDCT), the decoder needs to
perform a level shift by adding 128. This used to be done in
`ycbcr_to_rgb` after the conversion. Now, we do it in `inverse_dct` in
order to ensure that the task is done unconditionally.

Consequences of this are that we are no longer required to explicitly
do it for RGB images and also, the `ycbcr_to_rgb` function is exactly
like the specification.
This commit is contained in:
Lucas CHOLLET 2023-03-11 19:51:54 -05:00 • committed by Linus Groh
parent 62290d57f7
commit df12e70541
1 changed files with 30 additions and 27 deletions
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57
Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp
View file

@ -1294,10 +1294,33 @@ static void inverse_dct(JPEGLoadingContext const& context, Vector<Macroblock>& m
}
}
}
// F.2.1.5 - Inverse DCT (IDCT)
for (u32 vcursor = 0; vcursor < context.mblock_meta.vcount; vcursor += context.vsample_factor) {
for (u32 hcursor = 0; hcursor < context.mblock_meta.hcount; hcursor += context.hsample_factor) {
for (u8 vfactor_i = 0; vfactor_i < context.vsample_factor; ++vfactor_i) {
for (u8 hfactor_i = 0; hfactor_i < context.hsample_factor; ++hfactor_i) {
u32 mb_index = (vcursor + vfactor_i) * context.mblock_meta.hpadded_count + (hcursor + hfactor_i);
for (u8 i = 0; i < 8; ++i) {
for (u8 j = 0; j < 8; ++j) {
macroblocks[mb_index].r[i * 8 + j] = clamp(macroblocks[mb_index].r[i * 8 + j] + 128, 0, 255);
macroblocks[mb_index].g[i * 8 + j] = clamp(macroblocks[mb_index].g[i * 8 + j] + 128, 0, 255);
macroblocks[mb_index].b[i * 8 + j] = clamp(macroblocks[mb_index].b[i * 8 + j] + 128, 0, 255);
macroblocks[mb_index].k[i * 8 + j] = clamp(macroblocks[mb_index].b[i * 8 + j] + 128, 0, 255);
}
}
}
}
}
}
}
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:
// See: https://www.itu.int/rec/dologin_pub.asp?lang=f&id=T-REC-T.871-201105-I!!PDF-E&type=items
// 7 - Conversion to and from RGB
for (u32 vcursor = 0; vcursor < context.mblock_meta.vcount; vcursor += context.vsample_factor) {
for (u32 hcursor = 0; hcursor < context.mblock_meta.hcount; hcursor += context.hsample_factor) {
const u32 chroma_block_index = vcursor * context.mblock_meta.hpadded_count + hcursor;
@ -1315,32 +1338,12 @@ static void ycbcr_to_rgb(JPEGLoadingContext const& context, Vector<Macroblock>&
const u32 chroma_pxrow = (i / context.vsample_factor) + 4 * vfactor_i;
const u32 chroma_pxcol = (j / context.hsample_factor) + 4 * hfactor_i;
const u32 chroma_pixel = chroma_pxrow * 8 + chroma_pxcol;
int r = y[pixel] + 1.402f * chroma.cr[chroma_pixel] + 128;
int g = y[pixel] - 0.344f * chroma.cb[chroma_pixel] - 0.714f * chroma.cr[chroma_pixel] + 128;
int b = y[pixel] + 1.772f * chroma.cb[chroma_pixel] + 128;
y[pixel] = r < 0 ? 0 : (r > 255 ? 255 : r);
cb[pixel] = g < 0 ? 0 : (g > 255 ? 255 : g);
cr[pixel] = b < 0 ? 0 : (b > 255 ? 255 : b);
}
}
}
}
}
}
}
static void signed_rgb_to_unsigned(JPEGLoadingContext const& context, Vector<Macroblock>& macroblocks)
{
for (u32 vcursor = 0; vcursor < context.mblock_meta.vcount; vcursor += context.vsample_factor) {
for (u32 hcursor = 0; hcursor < context.mblock_meta.hcount; hcursor += context.hsample_factor) {
for (u8 vfactor_i = 0; vfactor_i < context.vsample_factor; ++vfactor_i) {
for (u8 hfactor_i = 0; hfactor_i < context.hsample_factor; ++hfactor_i) {
u32 mb_index = (vcursor + vfactor_i) * context.mblock_meta.hpadded_count + (hcursor + hfactor_i);
for (u8 i = 0; i < 8; ++i) {
for (u8 j = 0; j < 8; ++j) {
macroblocks[mb_index].r[i * 8 + j] = clamp(macroblocks[mb_index].r[i * 8 + j] + 128, 0, 255);
macroblocks[mb_index].g[i * 8 + j] = clamp(macroblocks[mb_index].g[i * 8 + j] + 128, 0, 255);
macroblocks[mb_index].b[i * 8 + j] = clamp(macroblocks[mb_index].b[i * 8 + j] + 128, 0, 255);
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);
y[pixel] = clamp(r, 0, 255);
cb[pixel] = clamp(g, 0, 255);
cr[pixel] = clamp(b, 0, 255);
}
}
}
@ -1361,7 +1364,7 @@ static ErrorOr<void> handle_color_transform(JPEGLoadingContext const& context, V
// FIXME: implement CMYK
dbgln("CMYK isn't supported yet");
} else if (context.components.size() == 3) {
signed_rgb_to_unsigned(context, macroblocks);
// Note: components.size() == 3 means that we have an RGB image, so no color transformation is needed.
} else {
return Error::from_string_literal("Wrong number of components for CMYK or RGB, aborting.");
}