From 8210645e4b0b80578a3abd9d2e31811c36430ecc Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Wed, 23 Jun 2021 19:28:46 +1000
Subject: [PATCH] If all 256 colors are in use, then there is no need for an
 additional color for background or transparency

---
 Tests/test_imagepalette.py |  3 +++
 src/PIL/GifImagePlugin.py  | 10 +++++++++-
 src/PIL/Image.py           | 41 +++++++++++++++++++++++++-------------
 3 files changed, 39 insertions(+), 15 deletions(-)

diff --git a/Tests/test_imagepalette.py b/Tests/test_imagepalette.py
index 5152abf56..829b47641 100644
--- a/Tests/test_imagepalette.py
+++ b/Tests/test_imagepalette.py
@@ -31,8 +31,11 @@ def test_getcolor():
 
     assert palette.getcolor((0, 0, 0)) == palette.getcolor((0, 0, 0, 255))
 
+    # An error is raised when the palette is full
     with pytest.raises(ValueError):
         palette.getcolor((1, 2, 3))
+    # But not if the image is not using one of the palette entries
+    palette.getcolor((1, 2, 3), image=Image.new("P", (1, 1)))
 
     # Test unknown color specifier
     with pytest.raises(ValueError):
diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index e84bd6fe1..135c9c68f 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -760,7 +760,15 @@ def _get_background(im, infoBackground):
             # WebPImagePlugin stores an RGBA value in info["background"]
             # So it must be converted to the same format as GifImagePlugin's
             # info["background"] - a global color table index
-            background = im.palette.getcolor(background, im)
+            try:
+                background = im.palette.getcolor(background, im)
+            except ValueError as e:
+                if str(e) == "cannot allocate more than 256 colors":
+                    # If all 256 colors are in use,
+                    # then there is no need for the background color
+                    return 0
+                else:
+                    raise
     return background
 
 
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index fedfe9f0d..aebc34bc7 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -979,19 +979,27 @@ class Image:
                         if isinstance(t, tuple):
                             try:
                                 t = trns_im.palette.getcolor(t, self)
-                            except Exception as e:
-                                raise ValueError(
-                                    "Couldn't allocate a palette color for transparency"
-                                ) from e
-                    trns_im.putpixel((0, 0), t)
-
-                    if mode in ("L", "RGB"):
-                        trns_im = trns_im.convert(mode)
+                            except ValueError as e:
+                                if str(e) == "cannot allocate more than 256 colors":
+                                    # If all 256 colors are in use,
+                                    # then there is no need for transparency
+                                    t = None
+                                else:
+                                    raise ValueError(
+                                        "Couldn't allocate a palette color for transparency"
+                                    ) from e
+                    if t is None:
+                        trns = None
                     else:
-                        # can't just retrieve the palette number, got to do it
-                        # after quantization.
-                        trns_im = trns_im.convert("RGB")
-                    trns = trns_im.getpixel((0, 0))
+                        trns_im.putpixel((0, 0), t)
+
+                        if mode in ("L", "RGB"):
+                            trns_im = trns_im.convert(mode)
+                        else:
+                            # can't just retrieve the palette number, got to do it
+                            # after quantization.
+                            trns_im = trns_im.convert("RGB")
+                        trns = trns_im.getpixel((0, 0))
 
             elif self.mode == "P" and mode == "RGBA":
                 t = self.info["transparency"]
@@ -1050,9 +1058,14 @@ class Image:
             if new_im.mode == "P":
                 try:
                     new_im.info["transparency"] = new_im.palette.getcolor(trns, new_im)
-                except Exception:
+                except ValueError as e:
                     del new_im.info["transparency"]
-                    warnings.warn("Couldn't allocate palette entry for transparency")
+                    if str(e) != "cannot allocate more than 256 colors":
+                        # If all 256 colors are in use,
+                        # then there is no need for transparency
+                        warnings.warn(
+                            "Couldn't allocate palette entry for transparency"
+                        )
             else:
                 new_im.info["transparency"] = trns
         return new_im