Fix alpha compositing in ft2font's draw_bitmap. #30043
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QuLogic
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May 12, 2025
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Actually, #30059 (direct rendering into the Agg buffer) would be even more general than this, as we'd just rely on Agg's compositing. |
The old formula (`*dst |= *src`) works fine when either dst or src is
full transparent or fully opaque, but not for compositing intermediate
values. Fix that (while keeping a fast-path for the common case of
writing on an empty buffer).
Example (note the more uniform gray zone between the two letters):
```
from matplotlib import pyplot as plt, ft2font as f, cbook
import numpy as np
font = 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()
```
|
Closing in favour of #30059. |
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The old formula (
*dst |= *src) works fine when either dst or src is full transparent or fully opaque, but not for compositing intermediate values. Fix that (while keeping a fast-path for the common case of writing on an empty buffer).Example (note the more uniform gray zone between the two letters):
old/new:
Somewhat unsurprisingly, this also breaks a bunch of baseline images; maybe this could be squashed into the FreeType update (#29816)?
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