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Commitda6647b

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Fix alpha compositing in ft2font's draw_bitmap.
The old formula (`*dst |= *src`) works fine when either dst or src isfull transparent or fully opaque, but not for compositing intermediatevalues. Fix that (while keeping a fast-path for the common case ofwriting on an empty buffer).Example (note the more uniform gray zone between the two letters):```from matplotlib import pyplot as plt, ft2font as f, cbookimport numpy as npfont = f.FT2Font(str(cbook._get_data_path("fonts/ttf/DejaVuSans.ttf")))font.set_size(24, 72)im = f.FT2Image(30, 30)ga = font.load_char(ord("A"))gv = font.load_char(ord("V"))font.draw_glyph_to_bitmap(im, 2, 2, ga)font.draw_glyph_to_bitmap(im, 12, 2, gv)(plt.figure(layout="constrained", figsize=(3, 3)) .add_subplot(xticks=[], yticks=[]) .imshow(np.asarray(im), cmap="gray"))plt.show()```
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‎src/ft2font.cpp

Lines changed: 12 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -104,6 +104,16 @@ void FT2Image::resize(long width, long height)
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}
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}
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107+
static std::array<uint8_t,0x10000>const alpha_cov_merge_table{[]() {
108+
auto table = std::array<uint8_t,0x10000>{};
109+
for (auto dst =0; dst <0x100; ++dst) {
110+
for (auto src =0; src <0x100; ++src) {
111+
table[(dst <<8) + src] = dst + src - (dst * src +0x7f) /0xff;
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}
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}
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return table;
115+
} ()};
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voidFT2Image::draw_bitmap(FT_Bitmap *bitmap, FT_Int x, FT_Int y)
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{
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FT_Int image_width = (FT_Int)m_width;
@@ -124,7 +134,8 @@ void FT2Image::draw_bitmap(FT_Bitmap *bitmap, FT_Int x, FT_Int y)
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unsignedchar *dst = m_buffer + (i * image_width + x1);
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unsignedchar *src = bitmap->buffer + (((i - y_offset) * bitmap->pitch) + x_start);
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for (FT_Int j = x1; j < x2; ++j, ++dst, ++src)
127-
*dst |= *src;
137+
// Provide a fast-path for the common case of black background.
138+
*dst = *dst ? alpha_cov_merge_table[(*dst <<8) + *src] : *src;
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}
129140
}elseif (bitmap->pixel_mode == FT_PIXEL_MODE_MONO) {
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for (FT_Int i =y1; i < y2; ++i) {

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