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LibGfx: Fix BMP mask detection off-by-one

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Also, since the loops can be replaced by a little bit-twiddling,
call ctz() directly. This might be a bit faster, or it might not.
This commit is contained in:
Ben Wiederhake 2020-12-01 22:21:16 +01:00 committed by Andreas Kling
parent 6be9b6349d
commit 36daeee34f
Notes: sideshowbarker 2024-07-19 01:06:34 +09:00 
Author: https://github.com/BenWiederhake
Commit: https://github.com/SerenityOS/serenity/commit/36daeee34ff
Pull-request: https://github.com/SerenityOS/serenity/pull/4294
Issue: https://github.com/SerenityOS/serenity/issues/4206
1 changed files with 9 additions and 23 deletions
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32
Libraries/LibGfx/BMPLoader.cpp
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@ -343,10 +343,10 @@ static void populate_dib_mask_info(BMPLoadingContext& context)
return;
// Mask shift is the number of right shifts needed to align the MSb of the
// mask to the MSb of the LSB.
// mask to the MSb of the LSB. Note that this can be a negative number.
// Mask size is the number of set bits in the mask. This is required for
// color scaling (for example, ensuring that a 4-bit color value spans the
// entire 256 value color spectrum.
// color scaling (for example, ensuring that a 4-bit color value spans the
// entire 256 value color spectrum.
auto& masks = context.dib.info.masks;
auto& mask_shifts = context.dib.info.mask_shifts;
auto& mask_sizes = context.dib.info.mask_sizes;
@ -361,29 +361,15 @@ static void populate_dib_mask_info(BMPLoadingContext& context)
for (size_t i = 0; i < masks.size(); ++i) {
u32 mask = masks[i];
u8 shift = 0;
u8 size = 0;
bool found_set_bit = false;
while (shift <= 32) {
u8 bit = (mask >> shift) & 0x1;
if (found_set_bit)
size++;
if (!found_set_bit && bit) {
found_set_bit = true;
} else if (found_set_bit && !bit) {
break;
}
shift++;
}
if (shift > 32) {
if (!mask) {
mask_shifts.append(0);
mask_sizes.append(0);
} else {
mask_shifts.append(shift - 8);
mask_sizes.append(size);
continue;
}
int trailing_zeros = count_trailing_zeroes_32(mask);
int size = count_trailing_zeroes_32(~(mask >> trailing_zeros));
mask_shifts.append(trailing_zeros - 8);
mask_sizes.append(size);
}
}