This is not a bug. 4 byte mask cannot be applied to 333 byte data

Tests/test_lib_image.py

@@ -26,12 +26,12 @@
     "BGR;24",
 )
 
+
 @pytest.mark.parametrize("mode", mode_names)
 def test_equal(mode):
-    num_img_bytes = len(Image.new(mode, (2, 2)).tobytes())
-    # alternatively, random.randbytes() in Python 3.9
-    data = secrets.token_bytes(num_img_bytes)
-    img_a = Image.frombytes(mode, (2, 2), data)
-    img_b = Image.frombytes(mode, (2, 2), data)
+    num_img_bytes = len(Image.new(mode, (4, 2)).tobytes())
+    data = b"0\xd2\xf9\x92Db\xbd]\x84\x86fd%\xfeR\x1b0\xd2\xf9\x92Db\xbd]\x84\x86fd%\xfeR\x1b"
+    img_a = Image.frombytes(mode, (4, 2), data)
+    img_b = Image.frombytes(mode, (4, 2), data)
     assert img_a.tobytes() == img_b.tobytes()
     assert img_a.im == img_b.im

src/_imaging.c

@@ -3760,7 +3760,7 @@ _compare_pixels(
     // Fortunately, all of the modes that have extra bytes in their pixels use four bytes for their pixels.
     UINT32 mask = 0xffffffff;
     if (
-        !strcmp(mode, "RGB") || !strcmp(mode, "BGR;24") ||
+        !strcmp(mode, "RGB") ||
         !strcmp(mode, "YCbCr") || !strcmp(mode, "HSV") || !strcmp(mode, "LAB")
     ) {
         // These modes have three channels in four bytes,
@@ -3769,14 +3769,6 @@ _compare_pixels(
         mask = 0xffffff00;
 #else
         mask = 0x00ffffff;
-#endif
-    } else if (!strcmp(mode, "BGR;15") || !strcmp(mode, "BGR;16")) {
-        // These modes have two channels in four bytes,
-        // so we have to ignore the last two bytes.
-#ifdef WORDS_BIGENDIAN
-        mask = 0xffff0000;
-#else
-        mask = 0x0000ffff;
 #endif
     } else if (!strcmp(mode, "LA") || !strcmp(mode, "La") || !strcmp(mode, "PA")) {
         // These modes have two channels in four bytes,
@@ -3799,7 +3791,9 @@ _compare_pixels(
             UINT32 *line_a = (UINT32*)pixels_a[y];
             UINT32 *line_b = (UINT32*)pixels_b[y];
             for (x = 0; x < xsize; x++, line_a++, line_b++) {
+                printf("mask x %d y %d linea %u lineb %u mask %u lineamask %u linebmask %u\n", x, y, *line_a, *line_b, mask, *line_a & mask, *line_b & mask);
                 if ((*line_a & mask) != (*line_b & mask)) {
+                    printf("fail");
                     return 1;
                 }
             }