From e10cab42f1112645e1cf90b0e460e441e2682f1f Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Sun, 19 Apr 2020 20:56:17 +1000
Subject: [PATCH 1/2] Consider transparency when drawing text on an RGBA image

---
 Tests/images/transparent_background_text.png | Bin 0 -> 1271 bytes
 Tests/test_image_paste.py                    |   2 +-
 Tests/test_imagefont.py                      |  12 ++++++++++++
 src/libImaging/Paste.c                       |   8 ++++++--
 4 files changed, 19 insertions(+), 3 deletions(-)
 create mode 100644 Tests/images/transparent_background_text.png

diff --git a/Tests/images/transparent_background_text.png b/Tests/images/transparent_background_text.png
new file mode 100644
index 0000000000000000000000000000000000000000..40acd92b62243953ebc0910629922eef112424e1
GIT binary patch
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diff --git a/Tests/test_image_paste.py b/Tests/test_image_paste.py
index 1d3ca8135..3740fbcdc 100644
--- a/Tests/test_image_paste.py
+++ b/Tests/test_image_paste.py
@@ -236,7 +236,7 @@ class TestImagingPaste:
                 [
                     (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),
diff --git a/Tests/test_imagefont.py b/Tests/test_imagefont.py
index 0e642cde2..b9dec9530 100644
--- a/Tests/test_imagefont.py
+++ b/Tests/test_imagefont.py
@@ -150,6 +150,18 @@ class TestImageFont:
 
         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)
diff --git a/src/libImaging/Paste.c b/src/libImaging/Paste.c
index 0bda25739..69353ce46 100644
--- a/src/libImaging/Paste.c
+++ b/src/libImaging/Paste.c
@@ -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++;
             }
         }

From 9390e5636a21ef6dbd68f787eb975bf40b255fa9 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Mon, 20 Apr 2020 18:53:37 +1000
Subject: [PATCH 2/2] Also consider other alpha modes

---
 src/libImaging/Paste.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/libImaging/Paste.c b/src/libImaging/Paste.c
index 69353ce46..d0610cfc5 100644
--- a/src/libImaging/Paste.c
+++ b/src/libImaging/Paste.c
@@ -380,7 +380,13 @@ fill_mask_L(Imaging imOut, const UINT8* ink, Imaging imMask,
             for (x = 0; x < xsize; x++) {
                 for (i = 0; i < pixelsize; i++) {
                     UINT8 channel_mask = *mask;
-                    if (strcmp(imOut->mode, "RGBA") == 0 && i != 3) {
+                    if ((
+                        strcmp(imOut->mode, "RGBa") == 0 ||
+                        strcmp(imOut->mode, "RGBA") == 0 ||
+                        strcmp(imOut->mode, "La") == 0 ||
+                        strcmp(imOut->mode, "LA") == 0 ||
+                        strcmp(imOut->mode, "PA") == 0
+                    ) && i != 3) {
                         channel_mask = 255 - (255 - channel_mask) * (1 - (255 - out[3]) / 255);
                     }
                     out[i] = BLEND(channel_mask, out[i], ink[i], tmp1);