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Fix alpha compositing in ft2font's draw_bitmap. #30043

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Fix alpha compositing in ft2font's draw_bitmap. #30043

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13 changes: 12 additions & 1 deletion src/ft2font.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -104,6 +104,16 @@ void FT2Image::resize(long width, long height)
}
}

static std::array<uint8_t, 0x10000> const alpha_cov_merge_table{[]() {
auto table = std::array<uint8_t, 0x10000>{};
for (auto dst = 0; dst < 0x100; ++dst) {
for (auto src = 0; src < 0x100; ++src) {
table[(dst << 8) + src] = dst + src - (dst * src + 0x7f) / 0xff;
}
}
return table;
} ()};

void FT2Image::draw_bitmap(FT_Bitmap *bitmap, FT_Int x, FT_Int y)
{
FT_Int image_width = (FT_Int)m_width;
Expand All @@ -124,7 +134,8 @@ void FT2Image::draw_bitmap(FT_Bitmap *bitmap, FT_Int x, FT_Int y)
unsigned char *dst = m_buffer + (i * image_width + x1);
unsigned char *src = bitmap->buffer + (((i - y_offset) * bitmap->pitch) + x_start);
for (FT_Int j = x1; j < x2; ++j, ++dst, ++src)
*dst |= *src;
// Provide a fast-path for the common case of black background.
*dst = *dst ? alpha_cov_merge_table[(*dst << 8) + *src] : *src;
}
} else if (bitmap->pixel_mode == FT_PIXEL_MODE_MONO) {
for (FT_Int i = y1; i < y2; ++i) {
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