matplotlib · anntzer · May 12, 2025
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_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;
@@ -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) {