Merge pull request #1882 from wiredfool/pr_1840

Fix Transparency for PNG images

PIL/PngImagePlugin.py

@@ -74,8 +74,7 @@ _MODES = {
 }
 
 
-_simple_palette = re.compile(b'^\xff+\x00\xff*$')
-_null_palette = re.compile(b'^\x00*$')
+_simple_palette = re.compile(b'^\xff*\x00\xff*$')
 
 # Maximum decompressed size for a iTXt or zTXt chunk.
 # Eliminates decompression bombs where compressed chunks can expand 1000x
@@ -118,7 +117,7 @@ class ChunkStream(object):
             cid = s[4:]
             pos = self.fp.tell()
             length = i32(s)
-            
+
         if not is_cid(cid):
             raise SyntaxError("broken PNG file (chunk %s)" % repr(cid))
 
@@ -359,12 +358,14 @@ class PngStream(ChunkStream):
         s = ImageFile._safe_read(self.fp, length)
         if self.im_mode == "P":
             if _simple_palette.match(s):
+                # tRNS contains only one full-transparent entry,
+                # other entries are full opaque
                 i = s.find(b"\0")
                 if i >= 0:
                     self.im_info["transparency"] = i
-            elif _null_palette.match(s):
-                self.im_info["transparency"] = 0
             else:
+                # otherwise, we have a byte string with one alpha value
+                # for each palette entry
                 self.im_info["transparency"] = s
         elif self.im_mode == "L":
             self.im_info["transparency"] = i16(s)

Tests/test_file_png.py

@@ -203,6 +203,7 @@ class TestFilePng(PillowTestCase):
     def test_load_transparent_rgb(self):
         test_file = "Tests/images/rgb_trns.png"
         im = Image.open(test_file)
+        self.assertEqual(im.info["transparency"], (0, 255, 52))
 
         self.assert_image(im, "RGB", (64, 64))
         im = im.convert("RGBA")
@@ -215,16 +216,66 @@ class TestFilePng(PillowTestCase):
         in_file = "Tests/images/pil123p.png"
         im = Image.open(in_file)
 
+        # 'transparency' contains a byte string with the opacity for
+        # each palette entry
+        self.assertEqual(len(im.info["transparency"]), 256)
+
         test_file = self.tempfile("temp.png")
         im.save(test_file)
 
+        # check if saved image contains same transparency
+        im = Image.open(test_file)
+        self.assertEqual(len(im.info["transparency"]), 256)
+
+        self.assert_image(im, "P", (162, 150))
+        im = im.convert("RGBA")
+        self.assert_image(im, "RGBA", (162, 150))
+
+        # image has 124 unique alpha values
+        self.assertEqual(len(im.split()[3].getcolors()), 124)
+
     def test_save_p_single_transparency(self):
         in_file = "Tests/images/p_trns_single.png"
         im = Image.open(in_file)
 
+        # pixel value 164 is full transparent
+        self.assertEqual(im.info["transparency"], 164)
+        self.assertEqual(im.getpixel((31, 31)), 164)
+
         test_file = self.tempfile("temp.png")
         im.save(test_file)
 
+        # check if saved image contains same transparency
+        im = Image.open(test_file)
+        self.assertEqual(im.info["transparency"], 164)
+        self.assertEqual(im.getpixel((31, 31)), 164)
+        self.assert_image(im, "P", (64, 64))
+        im = im.convert("RGBA")
+        self.assert_image(im, "RGBA", (64, 64))
+
+        self.assertEqual(im.getpixel((31, 31)), (0, 255, 52, 0))
+
+        # image has 876 transparent pixels
+        self.assertEqual(im.split()[3].getcolors()[0][0], 876)
+
+    def test_save_p_transparent_black(self):
+        # check if solid black image with full transparency
+        # is supported (check for #1838)
+        im = Image.new("RGBA", (10, 10), (0, 0, 0, 0))
+        self.assertEqual(im.getcolors(), [(100, (0, 0, 0, 0))])
+
+        im = im.convert("P")
+        test_file = self.tempfile("temp.png")
+        im.save(test_file)
+
+        # check if saved image contains same transparency
+        im = Image.open(test_file)
+        self.assertEqual(len(im.info["transparency"]), 256)
+        self.assert_image(im, "P", (10, 10))
+        im = im.convert("RGBA")
+        self.assert_image(im, "RGBA", (10, 10))
+        self.assertEqual(im.getcolors(), [(100, (0, 0, 0, 0))])
+
     def test_save_l_transparency(self):
         in_file = "Tests/images/l_trns.png"
         im = Image.open(in_file)

docs/handbook/image-file-formats.rst

@@ -370,8 +370,13 @@ The :py:meth:`~PIL.Image.Image.open` method sets the following
     Gamma, given as a floating point number.
 
 **transparency**
-    Transparency color index. This key is omitted if the image is not a
-    transparent palette image.
+    For ``P`` images: Either the palette index for full transparent pixels,
+    or a byte string with alpha values for each palette entry.
+
+    For ``L`` and ``RGB`` images, the color that represents full transparent
+    pixels in this image.
+
+    This key is omitted if the image is not a transparent palette image.
 
 ``Open`` also sets ``Image.text`` to a list of the values of the
 ``tEXt``, ``zTXt``, and ``iTXt`` chunks of the PNG image. Individual
@@ -392,6 +397,9 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
     For ``P``, ``L``, and ``RGB`` images, this option controls what
     color image to mark as transparent.
 
+    For ``P`` images, this can be a either the palette index,
+    or a byte string with alpha values for each palette entry.
+
 **dpi**
     A tuple of two numbers corresponding to the desired dpi in each direction.