Fix alpha compositing in ft2font's draw_bitmap.

anntzer · anntzer · commit 0aaab8c97b15 · 2025-05-12T12:49:54.000+02:00
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()
```
diff --git a/src/ft2font.cpp b/src/ft2font.cpp
@@ -104,6 +104,16 @@ void FT2Image::resize(long width, long height)
     }
 }
 
+static std::array<uint8_t, 0x10000> const alpha_composite_table{[]() {
+    auto table = std::array<uint8_t, 0x10000>{};
+    for (auto a = 0; a < 0x100; ++a) {
+        for (auto b = 0; b < 0x100; ++b) {
+            table[(a << 8) + b] = a + b - (a * b + 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;
@@ -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_composite_table[(*dst << 8) + *src] : *src;
         }
     } else if (bitmap->pixel_mode == FT_PIXEL_MODE_MONO) {
         for (FT_Int i = y1; i < y2; ++i) {
