Fixed bug with rounding pixels to palette

This fixes a bug with rounding pixels to the nearest palette color. Specifically, there is a typo with computing the minimum distance from a palette color to a given cache box. This is causing palette colors to be incorrectly excluded from consideration.

Tests/test_image_entropy.py

@@ -9,7 +9,7 @@ def test_entropy():
     assert round(abs(entropy("L") - 7.063008716585465), 7) == 0
     assert round(abs(entropy("I") - 7.063008716585465), 7) == 0
     assert round(abs(entropy("F") - 7.063008716585465), 7) == 0
-    assert round(abs(entropy("P") - 5.0530452472519745), 7) == 0
+    assert round(abs(entropy("P") - 5.082506854662517), 7) == 0
     assert round(abs(entropy("RGB") - 8.821286587714319), 7) == 0
     assert round(abs(entropy("RGBA") - 7.42724306524488), 7) == 0
     assert round(abs(entropy("CMYK") - 7.4272430652448795), 7) == 0

Tests/test_image_getcolors.py

@@ -16,7 +16,7 @@ def test_getcolors():
     assert getcolors("L") == 255
     assert getcolors("I") == 255
     assert getcolors("F") == 255
-    assert getcolors("P") == 90  # fixed palette
+    assert getcolors("P") == 96  # fixed palette
     assert getcolors("RGB") is None
     assert getcolors("RGBA") is None
     assert getcolors("CMYK") is None

Tests/test_image_histogram.py

@@ -10,7 +10,7 @@ def test_histogram():
     assert histogram("L") == (256, 0, 662)
     assert histogram("I") == (256, 0, 662)
     assert histogram("F") == (256, 0, 662)
-    assert histogram("P") == (256, 0, 1871)
+    assert histogram("P") == (256, 0, 1551)
     assert histogram("RGB") == (768, 4, 675)
     assert histogram("RGBA") == (1024, 0, 16384)
     assert histogram("CMYK") == (1024, 0, 16384)

docs/releasenotes/9.2.0.rst

@@ -76,7 +76,9 @@ TODO
 Other Changes
 =============
 
-TODO
-^^^^
+Fixed bug with rounding pixels to palette
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-TODO
+Fixed a bug when rounding image pixels to palette colors. This affects the results of
+``Image.quantize`` and of ``Image.convert`` when converting from mode "RGB" to "P",
+"L", or "1", regardless of whether dithering is enabled.

src/libImaging/Palette.c

@@ -200,15 +200,15 @@ ImagingPaletteCacheUpdate(ImagingPalette palette, int r, int g, int b) {
 
         /* Find min and max distances to any point in the box */
         r = palette->palette[i * 4 + 0];
-        tmin = (r < r0) ? RDIST(r, r1) : (r > r1) ? RDIST(r, r0) : 0;
+        tmin = (r < r0) ? RDIST(r, r0) : (r > r1) ? RDIST(r, r1) : 0;
         tmax = (r <= rc) ? RDIST(r, r1) : RDIST(r, r0);
 
         g = palette->palette[i * 4 + 1];
-        tmin += (g < g0) ? GDIST(g, g1) : (g > g1) ? GDIST(g, g0) : 0;
+        tmin += (g < g0) ? GDIST(g, g0) : (g > g1) ? GDIST(g, g1) : 0;
         tmax += (g <= gc) ? GDIST(g, g1) : GDIST(g, g0);
 
         b = palette->palette[i * 4 + 2];
-        tmin += (b < b0) ? BDIST(b, b1) : (b > b1) ? BDIST(b, b0) : 0;
+        tmin += (b < b0) ? BDIST(b, b0) : (b > b1) ? BDIST(b, b1) : 0;
         tmax += (b <= bc) ? BDIST(b, b1) : BDIST(b, b0);
 
         dmin[i] = tmin;