From 8bc19ac78cfa9d434e773279d4f8059ecd28116f Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Sat, 24 Apr 2021 11:14:18 +1000
Subject: [PATCH] Do not alter pixels outside mask when drawing text on an
 image with transparency

---
 Tests/images/transparent_background_text.png  | Bin 1271 -> 1326 bytes
 .../images/transparent_background_text_L.png  | Bin 0 -> 350 bytes
 Tests/test_image_paste.py                     |   2 +-
 Tests/test_imagefont.py                       |   3 +++
 src/libImaging/Paste.c                        |   2 +-
 5 files changed, 5 insertions(+), 2 deletions(-)
 create mode 100644 Tests/images/transparent_background_text_L.png

diff --git a/Tests/images/transparent_background_text.png b/Tests/images/transparent_background_text.png
index 40acd92b62243953ebc0910629922eef112424e1..8ddd65cc68b5b65bdeb47a9701c6dcf50c564b36 100644
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diff --git a/Tests/images/transparent_background_text_L.png b/Tests/images/transparent_background_text_L.png
new file mode 100644
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diff --git a/Tests/test_image_paste.py b/Tests/test_image_paste.py
index 3740fbcdc..1d3ca8135 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),
-                    (255, 255, 255, 0) if mode == "RGBA" else (127, 254, 127, 0),
+                    (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 883c14170..e843351ab 100644
--- a/Tests/test_imagefont.py
+++ b/Tests/test_imagefont.py
@@ -131,6 +131,9 @@ class TestImageFont:
         target = "Tests/images/transparent_background_text.png"
         assert_image_similar_tofile(im, target, 4.09)
 
+        target = "Tests/images/transparent_background_text_L.png"
+        assert_image_similar_tofile(im.convert("L"), target, 0.01)
+
     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 03b17f571..a1bf18a92 100644
--- a/src/libImaging/Paste.c
+++ b/src/libImaging/Paste.c
@@ -436,7 +436,7 @@ fill_mask_L(
                          strcmp(imOut->mode, "La") == 0 ||
                          strcmp(imOut->mode, "LA") == 0 ||
                          strcmp(imOut->mode, "PA") == 0) &&
-                        i != 3) {
+                        i != 3 && channel_mask != 0) {
                         channel_mask =
                             255 - (255 - channel_mask) * (1 - (255 - out[3]) / 255);
                     }