Merge pull request matplotlib#26215 from meeseeksmachine/auto-backport-of-pr-25824-on-v3.7.x

Backport PR matplotlib#25824 on branch v3.7.x (pdf: Use explicit palette when saving indexed images)

Author: QuLogic

lib/matplotlib/backends/backend_pdf.py

@@ -1757,39 +1757,43 @@ def _writeImg(self, data, id, smask=None):
                'Subtype': Name('Image'),
                'Width': width,
                'Height': height,
-               'ColorSpace': Name({1: 'DeviceGray',
-                                   3: 'DeviceRGB'}[color_channels]),
+               'ColorSpace': Name({1: 'DeviceGray', 3: 'DeviceRGB'}[color_channels]),
                'BitsPerComponent': 8}
         if smask:
             obj['SMask'] = smask
         if mpl.rcParams['pdf.compression']:
             if data.shape[-1] == 1:
                 data = data.squeeze(axis=-1)
+            png = {'Predictor': 10, 'Colors': color_channels, 'Columns': width}
             img = Image.fromarray(data)
             img_colors = img.getcolors(maxcolors=256)
             if color_channels == 3 and img_colors is not None:
-                # Convert to indexed color if there are 256 colors or fewer
-                # This can significantly reduce the file size
+                # Convert to indexed color if there are 256 colors or fewer. This can
+                # significantly reduce the file size.
                 num_colors = len(img_colors)
-                # These constants were converted to IntEnums and deprecated in
-                # Pillow 9.2
-                dither = getattr(Image, 'Dither', Image).NONE
-                pmode = getattr(Image, 'Palette', Image).ADAPTIVE
-                img = img.convert(
-                    mode='P', dither=dither, palette=pmode, colors=num_colors
-                )
+                palette = np.array([comp for _, color in img_colors for comp in color],
+                                   dtype=np.uint8)
+                palette24 = ((palette[0::3].astype(np.uint32) << 16) |
+                             (palette[1::3].astype(np.uint32) << 8) |
+                             palette[2::3])
+                rgb24 = ((data[:, :, 0].astype(np.uint32) << 16) |
+                         (data[:, :, 1].astype(np.uint32) << 8) |
+                         data[:, :, 2])
+                indices = np.argsort(palette24).astype(np.uint8)
+                rgb8 = indices[np.searchsorted(palette24, rgb24, sorter=indices)]
+                img = Image.fromarray(rgb8, mode='P')
+                img.putpalette(palette)
                 png_data, bit_depth, palette = self._writePng(img)
                 if bit_depth is None or palette is None:
                     raise RuntimeError("invalid PNG header")
-                palette = palette[:num_colors * 3]  # Trim padding
-                obj['ColorSpace'] = Verbatim(
-                    b'[/Indexed /DeviceRGB %d %s]'
-                    % (num_colors - 1, pdfRepr(palette)))
+                palette = palette[:num_colors * 3]  # Trim padding; remove for Pillow>=9
+                obj['ColorSpace'] = [Name('Indexed'), Name('DeviceRGB'),
+                                     num_colors - 1, palette]
                 obj['BitsPerComponent'] = bit_depth
-                color_channels = 1
+                png['Colors'] = 1
+                png['BitsPerComponent'] = bit_depth
             else:
                 png_data, _, _ = self._writePng(img)
-            png = {'Predictor': 10, 'Colors': color_channels, 'Columns': width}
         else:
             png = None
         self.beginStream(