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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
1 changed files with 9 additions and 23 deletions
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28
Libraries/LibGfx/BMPLoader.cpp
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@ -343,7 +343,7 @@ static void populate_dib_mask_info(BMPLoadingContext& context)
return; return;
// Mask shift is the number of right shifts needed to align the MSb of the // 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 // 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 // color scaling (for example, ensuring that a 4-bit color value spans the
// entire 256 value color spectrum. // entire 256 value color spectrum.
@ -361,29 +361,15 @@ static void populate_dib_mask_info(BMPLoadingContext& context)
for (size_t i = 0; i < masks.size(); ++i) { for (size_t i = 0; i < masks.size(); ++i) {
u32 mask = masks[i]; u32 mask = masks[i];
u8 shift = 0; if (!mask) {
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) {
mask_shifts.append(0); mask_shifts.append(0);
mask_sizes.append(0); mask_sizes.append(0);
} else { continue;
mask_shifts.append(shift - 8);
mask_sizes.append(size);
} }
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);
} }
} }