Consider transparency when drawing text on an RGBA image

Author: radarhere

Tests/images/transparent_background_text.png

@@ -236,7 +236,7 @@ def test_color_mask_L(self):
                 [
                     (127, 191, 254, 191),
                     (111, 207, 206, 110),
-                    (127, 254, 127, 0),
+                    (255, 255, 255, 0) if mode == "RGBA" else (127, 254, 127, 0),
                     (207, 207, 239, 239),
                     (191, 191, 190, 191),
                     (207, 206, 111, 112),

Tests/test_image_paste.py

@@ -150,6 +150,18 @@ def test_render_equal(self):
 
         assert_image_equal(img_path, img_filelike)
 
+    def test_transparent_background(self):
+        im = Image.new(mode="RGBA", size=(300, 100))
+        draw = ImageDraw.Draw(im)
+        ttf = self.get_font()
+
+        txt = "Hello World!"
+        draw.text((10, 10), txt, font=ttf)
+
+        target = "Tests/images/transparent_background_text.png"
+        with Image.open(target) as target_img:
+            assert_image_similar(im, target_img, 4.09)
+
     def test_textsize_equal(self):
         im = Image.new(mode="RGB", size=(300, 100))
         draw = ImageDraw.Draw(im)

Tests/test_imagefont.py

@@ -379,9 +379,13 @@ fill_mask_L(Imaging imOut, const UINT8* ink, Imaging imMask,
             UINT8* mask = (UINT8*) imMask->image[y+sy]+sx;
             for (x = 0; x < xsize; x++) {
                 for (i = 0; i < pixelsize; i++) {
-                    *out = BLEND(*mask, *out, ink[i], tmp1);
-                    out++;
+                    UINT8 channel_mask = *mask;
+                    if (strcmp(imOut->mode, "RGBA") == 0 && i != 3) {
+                        channel_mask = 255 - (255 - channel_mask) * (1 - (255 - out[3]) / 255);
+                    }
+                    out[i] = BLEND(channel_mask, out[i], ink[i], tmp1);
                 }
+                out += pixelsize;
                 mask++;
             }
         }