matplotlib · QuLogic · Jun 29, 2023 · Jun 13, 2023
diff --git a/lib/matplotlib/backends/backend_pdf.py b/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(

diff --git a/lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.pdf b/lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_axes/imshow.pdf b/lib/matplotlib/tests/baseline_images/test_axes/imshow.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_axes/imshow_clip.pdf b/lib/matplotlib/tests/baseline_images/test_axes/imshow_clip.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_backend_pdf/grayscale_alpha.pdf b/lib/matplotlib/tests/baseline_images/test_backend_pdf/grayscale_alpha.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/bbox_image_inverted.pdf b/lib/matplotlib/tests/baseline_images/test_image/bbox_image_inverted.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/figimage.pdf b/lib/matplotlib/tests/baseline_images/test_image/figimage.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/image_alpha.pdf b/lib/matplotlib/tests/baseline_images/test_image/image_alpha.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/image_interps.pdf b/lib/matplotlib/tests/baseline_images/test_image/image_interps.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/image_placement.pdf b/lib/matplotlib/tests/baseline_images/test_image/image_placement.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/image_shift.pdf b/lib/matplotlib/tests/baseline_images/test_image/image_shift.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/imshow.pdf b/lib/matplotlib/tests/baseline_images/test_image/imshow.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/imshow_masked_interpolation.pdf b/lib/matplotlib/tests/baseline_images/test_image/imshow_masked_interpolation.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/log_scale_image.pdf b/lib/matplotlib/tests/baseline_images/test_image/log_scale_image.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/no_interpolation_origin.pdf b/lib/matplotlib/tests/baseline_images/test_image/no_interpolation_origin.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/rotate_image.pdf b/lib/matplotlib/tests/baseline_images/test_image/rotate_image.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_patheffects/patheffect1.pdf b/lib/matplotlib/tests/baseline_images/test_patheffects/patheffect1.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_streamplot/streamplot_colormap.pdf b/lib/matplotlib/tests/baseline_images/test_streamplot/streamplot_colormap.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_tightlayout/tight_layout5.pdf b/lib/matplotlib/tests/baseline_images/test_tightlayout/tight_layout5.pdf
diff --git a/lib/matplotlib/tests/test_backend_pdf.py b/lib/matplotlib/tests/test_backend_pdf.py
@@ -131,6 +131,30 @@ def test_composite_image():
         assert len(pdf._file._images) == 2
 
 
+def test_indexed_image():
+    # An image with low color count should compress to a palette-indexed format.
+    pikepdf = pytest.importorskip('pikepdf')
+
+    data = np.zeros((256, 1, 3), dtype=np.uint8)
+    data[:, 0, 0] = np.arange(256)  # Maximum unique colours for an indexed image.
+
+    rcParams['pdf.compression'] = True
+    fig = plt.figure()
+    fig.figimage(data, resize=True)
+    buf = io.BytesIO()
+    fig.savefig(buf, format='pdf', dpi='figure')
+
+    with pikepdf.Pdf.open(buf) as pdf:
+        page, = pdf.pages
+        image, = page.images.values()
+        pdf_image = pikepdf.PdfImage(image)
+        assert pdf_image.indexed
+        pil_image = pdf_image.as_pil_image()
+        rgb = np.asarray(pil_image.convert('RGB'))
+
+    np.testing.assert_array_equal(data, rgb)
+
+
 def test_savefig_metadata(monkeypatch):
     pikepdf = pytest.importorskip('pikepdf')
     monkeypatch.setenv('SOURCE_DATE_EPOCH', '0')

diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -753,11 +753,7 @@ def test_log_scale_image():
     ax.set(yscale='log')
 
 
-# Increased tolerance is needed for PDF test to avoid failure. After the PDF
-# backend was modified to use indexed color, there are ten pixels that differ
-# due to how the subpixel calculation is done when converting the PDF files to
-# PNG images.
-@image_comparison(['rotate_image'], remove_text=True, tol=0.35)
+@image_comparison(['rotate_image'], remove_text=True)
 def test_rotate_image():
     delta = 0.25
     x = y = np.arange(-3.0, 3.0, delta)