Refactor out passed in palette handling, add ImagePalette as an option, document palette format

PIL/GifImagePlugin.py

@@ -27,6 +27,8 @@
 from . import Image, ImageFile, ImagePalette, ImageChops, ImageSequence
 from ._binary import i8, i16le as i16, o8, o16le as o16
 
+import itertools
+
 __version__ = "0.9"
 
 
@@ -333,15 +335,22 @@ def _normalize_palette(im, palette, info):
     :param info: encoderinfo 
     :returns: Image object
     """
+    source_palette = None
+    if palette:
+        # a bytes palette
+        if isinstance(palette, (bytes, bytearray, list)):
+            source_palette = bytearray(palette[:768])
+        if isinstance(palette, ImagePalette.ImagePalette):
+            source_palette = bytearray(itertools.chain.from_iterable(
+                                zip(palette.palette[:256],
+                                    palette.palette[256:512],
+                                    palette.palette[512:768])))
+            
     if im.mode == "P":
-        if palette and isinstance(palette, bytes):
-            source_palette = palette[:768]
-        else:
+        if not source_palette:
             source_palette = im.im.getpalette("RGB")[:768]
     else:  # L-mode
-        if palette and isinstance(palette, bytes):
-            source_palette = palette[:768]
-        else:
+        if not source_palette:
             source_palette = bytearray(i//3 for i in range(768))
         im.palette = ImagePalette.ImagePalette("RGB",
                                                palette=source_palette)
@@ -436,7 +445,6 @@ def _save_all(im, fp, filename):
 
 def _save(im, fp, filename, save_all=False):
     im.encoderinfo.update(im.info)
-
     # header
     try:
         palette = im.encoderinfo["palette"]

Tests/test_file_gif.py

@@ -455,24 +455,40 @@ class TestFileGif(PillowTestCase):
         # generate an L mode image with a separate palette
         
         im = hopper('P')
-        im_l = im.split()[0]
-        palette = im.palette.getdata()
+        im_l = Image.frombytes('L', im.size, im.tobytes())
+        palette = bytes(bytearray(im.getpalette()))
 
         out = self.tempfile('temp.gif')
         im_l.save(out, palette=palette)
 
         reloaded = Image.open(out)
-        self.assert_image_equal(reloaded, im)
 
+        self.assert_image_equal(reloaded.convert('RGB'), im.convert('RGB'))
+    
     def test_palette_save_P(self):
         # pass in a different palette, then construct what the image
         # would look like.
+        # Forcing a non-straight grayscale palette.
 
         im = hopper('P')
-        palette = ImagePalette.ImagePalette('RGB')
+        palette = bytes(bytearray([255-i//3 for i in range(768)]))
 
         out = self.tempfile('temp.gif')
-        im.save(out, palette=palette.getdata())
+        im.save(out, palette=palette)
+
+        reloaded = Image.open(out)
+        im.putpalette(palette)
+        self.assert_image_equal(reloaded, im)
+
+    def test_palette_save_ImagePalette(self):
+        # pass in a different palette, as an ImagePalette.ImagePalette
+        # effectively the same as test_palette_save_P
+
+        im = hopper('P')
+        palette = ImagePalette.ImagePalette('RGB', list(range(256))[::-1]*3)
+
+        out = self.tempfile('temp.gif')
+        im.save(out, palette=palette)
 
         reloaded = Image.open(out)
         im.putpalette(palette)

docs/handbook/image-file-formats.rst

@@ -124,8 +124,12 @@ are available::
     eliminating unused colors. This is only useful if the palette can
     be compressed to the next smaller power of 2 elements.
 
-**palette**
-    Use the specified palette for the saved image.
+**palette** 
+    Use the specified palette for the saved image. The palette should
+    be a bytes or bytearray object containing the palette entries in
+    RGBRGB... form. It should be no more than 768 bytes. Alternately,
+    the palette can be passed in as an
+    :py:class:`PIL.ImagePalette.ImagePalette` object.
 
 Reading local images
 ~~~~~~~~~~~~~~~~~~~~