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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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- ft2font.cpp

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+14

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`‎src/ft2font.cpp`

Lines changed: 14 additions & 4 deletions

Original file line number	Diff line number	Diff line change
`@@ -73,11 +73,20 @@ FT2Image::~FT2Image()`
`73`	`73`	`free(m_buffer);`
`74`	`74`	`}`
`75`	`75`
`76`		`-voiddraw_bitmap(`
`77`		`- py::array_t<uint8_t, py::array::c_style> im, FT_Bitmap *bitmap, FT_Int x, FT_Int y)`
	`76`	`+static std::array<uint8_t,0x10000>const alpha_cov_merge_table{[]() {`
	`77`	`+auto table = std::array<uint8_t,0x10000>{};`
	`78`	`+for (auto dst =0; dst <0x100; ++dst) {`
	`79`	`+for (auto src =0; src <0x100; ++src) {`
	`80`	`+ table[(dst <<8) + src] = dst + src - (dst * src +0x7f) /0xff;`
	`81`	`+ }`
	`82`	`+ }`
	`83`	`+return table;`
	`84`	`+} ()};`
	`85`	`+`
	`86`	`+void`
	`87`	`+draw_bitmap(py::array_t<uint8_t, py::array::c_style> im, FT_Bitmap *bitmap, FT_Int x, FT_Int y)`
`78`	`88`	`{`
`79`	`89`	`auto buf = im.mutable_data(0);`
`80`		`-`
`81`	`90`	`FT_Int image_width = (FT_Int)im.shape(1);`
`82`	`91`	`FT_Int image_height = (FT_Int)im.shape(0);`
`83`	`92`	`FT_Int char_width = bitmap->width;`
`@@ -96,7 +105,8 @@ void draw_bitmap(`
`96`	`105`	`unsignedchar dst = buf + (i image_width + x1);`
`97`	`106`	`unsignedchar src = bitmap->buffer + (((i - y_offset) bitmap->pitch) + x_start);`
`98`	`107`	`for (FT_Int j = x1; j < x2; ++j, ++dst, ++src)`
`99`		`- dst \|= src;`
	`108`	`+// Provide a fast-path for the common case of black background.`
	`109`	`+ dst = dst ? alpha_cov_merge_table[(dst <<8) + src] : *src;`
`100`	`110`	`}`
`101`	`111`	`}elseif (bitmap->pixel_mode == FT_PIXEL_MODE_MONO) {`
`102`	`112`	`for (FT_Int i =y1; i < y2; ++i) {`

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Commitaa1f4cb

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`‎src/ft2font.cpp`

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