Fix incorrect color blending for overlapping glyphs

Tests/fonts/LICENSE.txt

@@ -2,7 +2,6 @@
 NotoNastaliqUrdu-Regular.ttf and NotoSansSymbols-Regular.ttf, from https://github.com/googlei18n/noto-fonts
 NotoSans-Regular.ttf, from https://www.google.com/get/noto/
 NotoSansJP-Thin.otf, from https://www.google.com/get/noto/help/cjk/
-NotoColorEmoji.ttf, from https://github.com/googlefonts/noto-emoji
 AdobeVFPrototype.ttf, from https://github.com/adobe-fonts/adobe-variable-font-prototype
 TINY5x3GX.ttf, from http://velvetyne.fr/fonts/tiny
 ArefRuqaa-Regular.ttf, from https://github.com/google/fonts/tree/master/ofl/arefruqaa
@@ -25,3 +24,5 @@ FreeMono.ttf is licensed under GPLv3.
 10x20-ISO8859-1.pcf, from https://packages.ubuntu.com/xenial/xfonts-base
 
 "Public domain font.  Share and enjoy."
+
+CBDTTestFont.ttf and EBDTTestFont.ttf from https://github.com/nulano/font-tests are public domain.

Tests/test_imagefont.py

@@ -856,6 +856,19 @@ def test_bitmap_font_stroke(layout_engine):
     assert_image_similar_tofile(im, target, 0.03)
 
 
+@pytest.mark.parametrize("embedded_color", (False, True))
+def test_bitmap_blend(layout_engine, embedded_color):
+    font = ImageFont.truetype(
+        "Tests/fonts/EBDTTestFont.ttf", size=64, layout_engine=layout_engine
+    )
+
+    im = Image.new("RGBA", (128, 96), "white")
+    d = ImageDraw.Draw(im)
+    d.text((16, 16), "AA", font=font, embedded_color=embedded_color, fill="#8E2F52")
+
+    assert_image_equal_tofile(im, "Tests/images/bitmap_font_blend.png")
+
+
 def test_standard_embedded_color(layout_engine):
     txt = "Hello World!"
     ttf = ImageFont.truetype(FONT_PATH, 40, layout_engine=layout_engine)
@@ -894,15 +907,15 @@ def test_float_coord(layout_engine, fontmode):
 def test_cbdt(layout_engine):
     try:
         font = ImageFont.truetype(
-            "Tests/fonts/NotoColorEmoji.ttf", size=109, layout_engine=layout_engine
+            "Tests/fonts/CBDTTestFont.ttf", size=64, layout_engine=layout_engine
         )
 
-        im = Image.new("RGB", (150, 150), "white")
+        im = Image.new("RGB", (128, 96), "white")
         d = ImageDraw.Draw(im)
 
-        d.text((10, 10), "\U0001f469", font=font, embedded_color=True)
+        d.text((16, 16), "AB", font=font, embedded_color=True)
 
-        assert_image_similar_tofile(im, "Tests/images/cbdt_notocoloremoji.png", 6.2)
+        assert_image_equal_tofile(im, "Tests/images/cbdt.png")
     except OSError as e:  # pragma: no cover
         assert str(e) in ("unimplemented feature", "unknown file format")
         pytest.skip("freetype compiled without libpng or CBDT support")
@@ -911,17 +924,15 @@ def test_cbdt(layout_engine):
 def test_cbdt_mask(layout_engine):
     try:
         font = ImageFont.truetype(
-            "Tests/fonts/NotoColorEmoji.ttf", size=109, layout_engine=layout_engine
+            "Tests/fonts/CBDTTestFont.ttf", size=64, layout_engine=layout_engine
         )
 
-        im = Image.new("RGB", (150, 150), "white")
+        im = Image.new("RGB", (128, 96), "white")
         d = ImageDraw.Draw(im)
 
-        d.text((10, 10), "\U0001f469", "black", font=font)
+        d.text((16, 16), "AB", "green", font=font)
 
-        assert_image_similar_tofile(
-            im, "Tests/images/cbdt_notocoloremoji_mask.png", 6.2
-        )
+        assert_image_equal_tofile(im, "Tests/images/cbdt_mask.png")
     except OSError as e:  # pragma: no cover
         assert str(e) in ("unimplemented feature", "unknown file format")
         pytest.skip("freetype compiled without libpng or CBDT support")

src/PIL/ImageDraw.py

@@ -32,6 +32,7 @@
 
 import math
 import numbers
+import struct
 
 from . import Image, ImageColor
 
@@ -542,7 +543,8 @@
                 # font.getmask2(mode="RGBA") returns color in RGB bands and mask in A
                 # extract mask and set text alpha
                 color, mask = mask, mask.getband(3)
-                color.fillband(3, (ink >> 24) & 0xFF)
+                ink_alpha = struct.pack("=i", ink)[3]
+                color.fillband(3, ink_alpha)
                 x, y = coord
                 self.im.paste(color, (x, y, x + mask.size[0], y + mask.size[1]), mask)
             else:

src/_imagingft.c

@@ -1047,8 +1047,8 @@ font_render(FontObject *self, PyObject *args) {
             if (yy >= 0 && yy < im->ysize) {
                 /* blend this glyph into the buffer */
                 int k;
-                unsigned char v;
                 unsigned char *target;
+                unsigned int tmp;
                 if (color) {
                     /* target[RGB] returns the color, target[A] returns the mask */
                     /* target bands get split again in ImageDraw.text */
@@ -1059,34 +1059,55 @@ font_render(FontObject *self, PyObject *args) {
                 if (color && bitmap.pixel_mode == FT_PIXEL_MODE_BGRA) {
                     /* paste color glyph */
                     for (k = x0; k < x1; k++) {
-                        if (target[k * 4 + 3] < source[k * 4 + 3]) {
-                            /* unpremultiply BGRa to RGBA */
-                            target[k * 4 + 0] = CLIP8(
-                                (255 * (int)source[k * 4 + 2]) / source[k * 4 + 3]);
-                            target[k * 4 + 1] = CLIP8(
-                                (255 * (int)source[k * 4 + 1]) / source[k * 4 + 3]);
-                            target[k * 4 + 2] = CLIP8(
-                                (255 * (int)source[k * 4 + 0]) / source[k * 4 + 3]);
-                            target[k * 4 + 3] = source[k * 4 + 3];
+                        unsigned int src_alpha = source[k * 4 + 3];
+
+                        /* paste only if source has data */
+                        if (src_alpha > 0) {
+                            /* unpremultiply BGRa */
+                            int src_red = CLIP8((255 * (int)source[k * 4 + 2]) / src_alpha);
+                            int src_green = CLIP8((255 * (int)source[k * 4 + 1]) / src_alpha);
+                            int src_blue = CLIP8((255 * (int)source[k * 4 + 0]) / src_alpha);
+
+                            /* blend required if target has data */
+                            if (target[k * 4 + 3] > 0) {
+                                /* blend RGBA colors */
+                                target[k * 4 + 0] = BLEND(src_alpha, target[k * 4 + 0], src_red, tmp);
+                                target[k * 4 + 1] = BLEND(src_alpha, target[k * 4 + 1], src_green, tmp);
+                                target[k * 4 + 2] = BLEND(src_alpha, target[k * 4 + 2], src_blue, tmp);
+                                target[k * 4 + 3] = CLIP8(src_alpha + MULDIV255(target[k * 4 + 3], (255 - src_alpha), tmp));
+                            } else {
+                                /* paste unpremultiplied RGBA values */
+                                target[k * 4 + 0] = src_red;
+                                target[k * 4 + 1] = src_green;
+                                target[k * 4 + 2] = src_blue;
+                                target[k * 4 + 3] = src_alpha;
+                            }
                         }
                     }
                 } else if (bitmap.pixel_mode == FT_PIXEL_MODE_GRAY) {
                     if (color) {
                         unsigned char *ink = (unsigned char *)&foreground_ink;
                         for (k = x0; k < x1; k++) {
-                            v = source[k] * convert_scale;
-                            if (target[k * 4 + 3] < v) {
-                                target[k * 4 + 0] = ink[0];
-                                target[k * 4 + 1] = ink[1];
-                                target[k * 4 + 2] = ink[2];
-                                target[k * 4 + 3] = v;
+                            unsigned int src_alpha = source[k] * convert_scale;
+                            if (src_alpha > 0) {
+                                if (target[k * 4 + 3] > 0) {
+                                    target[k * 4 + 0] = BLEND(src_alpha, target[k * 4 + 0], ink[0], tmp);
+                                    target[k * 4 + 1] = BLEND(src_alpha, target[k * 4 + 1], ink[1], tmp);
+                                    target[k * 4 + 2] = BLEND(src_alpha, target[k * 4 + 2], ink[2], tmp);
+                                    target[k * 4 + 3] = CLIP8(src_alpha + MULDIV255(target[k * 4 + 3], (255 - src_alpha), tmp));
+                                } else {
+                                    target[k * 4 + 0] = ink[0];
+                                    target[k * 4 + 1] = ink[1];
+                                    target[k * 4 + 2] = ink[2];
+                                    target[k * 4 + 3] = src_alpha;
+                                }
                             }
                         }
                     } else {
                         for (k = x0; k < x1; k++) {
-                            v = source[k] * convert_scale;
-                            if (target[k] < v) {
-                                target[k] = v;
+                            unsigned int src_alpha = source[k] * convert_scale;
+                            if (src_alpha > 0) {
+                                target[k] = target[k] > 0 ? CLIP8(src_alpha + MULDIV255(target[k], (255 - src_alpha), tmp)) : src_alpha;
                             }
                         }
                     }