From 145761cb2d346a2383b26ffbfe52c16634b1ca25 Mon Sep 17 00:00:00 2001
From: Luv Goel <luvgoel555@gmail.com>
Date: Fri, 23 Oct 2026 08:55:07 +0530
Subject: [PATCH 1/2] convert: honor dither argument with ADAPTIVE palette
 (#5836)

Image.convert("P", palette=ADAPTIVE, colors=N) built the palette via
quantize(), which maps each pixel to the nearest palette entry without
dithering, then returned early -- silently dropping the dither argument.

Build the adaptive palette first, then re-convert with the requested
dither so Floyd-Steinberg is applied. Add a regression test asserting
NONE vs FLOYDSTEINBERG produce different histograms.
---
 Tests/test_image_convert.py | 17 +++++++++++++++++
 src/PIL/Image.py            |  8 +++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/Tests/test_image_convert.py b/Tests/test_image_convert.py
index 6573e30ebe7..b522c5f1cc3 100644
--- a/Tests/test_image_convert.py
+++ b/Tests/test_image_convert.py
@@ -377,3 +377,20 @@ def test_matrix_identity() -> None:
     # Test list
     converted_im = im.convert("RGB", list(identity_matrix))
     assert_image_equal(converted_im, im)
+
+def test_adaptive_convert_honors_dither(tmp_path: Path) -> None:
+    # Regression test for #5836: convert("P", palette=ADAPTIVE, colors=N,
+    # dither=...) must apply dithering, not just nearest-color mapping.
+    im = hopper("RGB")
+
+    no_dither = im.convert("P", palette=Image.Palette.ADAPTIVE, colors=8, dither=Image.Dither.NONE)
+    with_dither = im.convert("P", palette=Image.Palette.ADAPTIVE, colors=8, dither=Image.Dither.FLOYDSTEINBERG)
+
+    # Both should produce valid palette images with the same palette size.
+    assert no_dither.mode == "P"
+    assert with_dither.mode == "P"
+    assert len(no_dither.getpalette()) == len(with_dither.getpalette()) == 24
+
+    # Dithered and non-dithered results must differ. Histograms should spread
+    # differently because dithering distributes error across neighboring pixels.
+    assert no_dither.histogram() != with_dither.histogram()
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index f8a89cbb2eb..f6a5a5b7190 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -1204,7 +1204,13 @@ def convert_transparency(
                     raise ValueError(msg)
 
         if mode == "P" and palette == Palette.ADAPTIVE:
-            im = self.im.quantize(colors)
+            # quantize() maps each pixel to the nearest palette entry without
+            # dithering, so it silently drops the dither argument. Build the
+            # adaptive palette first, then re-convert with dither so the
+            # dither argument is honored (see #5836).
+            adaptive = self.im.quantize(colors)
+            palette_im = self._new(adaptive)
+            im = self.im.convert(mode, dither, palette_im.im)
             new_im = self._new(im)
             from . import ImagePalette
 

From 0e8fbbe7aca3c6e38585c0fe82e7e64e07928032 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Fri, 23 Oct 2026 03:26:05 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 Tests/test_image_convert.py | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/Tests/test_image_convert.py b/Tests/test_image_convert.py
index b522c5f1cc3..eb8d515f3ea 100644
--- a/Tests/test_image_convert.py
+++ b/Tests/test_image_convert.py
@@ -378,13 +378,21 @@ def test_matrix_identity() -> None:
     converted_im = im.convert("RGB", list(identity_matrix))
     assert_image_equal(converted_im, im)
 
+
 def test_adaptive_convert_honors_dither(tmp_path: Path) -> None:
     # Regression test for #5836: convert("P", palette=ADAPTIVE, colors=N,
     # dither=...) must apply dithering, not just nearest-color mapping.
     im = hopper("RGB")
 
-    no_dither = im.convert("P", palette=Image.Palette.ADAPTIVE, colors=8, dither=Image.Dither.NONE)
-    with_dither = im.convert("P", palette=Image.Palette.ADAPTIVE, colors=8, dither=Image.Dither.FLOYDSTEINBERG)
+    no_dither = im.convert(
+        "P", palette=Image.Palette.ADAPTIVE, colors=8, dither=Image.Dither.NONE
+    )
+    with_dither = im.convert(
+        "P",
+        palette=Image.Palette.ADAPTIVE,
+        colors=8,
+        dither=Image.Dither.FLOYDSTEINBERG,
+    )
 
     # Both should produce valid palette images with the same palette size.
     assert no_dither.mode == "P"