Merge pull request #533 from wiredfool/fix_528

Fix for  transparency = 0 in PNG mode P images

CHANGES.rst

@@ -4,6 +4,9 @@ Changelog (Pillow)
 2.4.0 (unreleased)
 ------------------
 
+- Fixed saving mode P image as a PNG with transparency = palette color 0
+  [d-schmidt]
+
 - Improve heuristic used when saving progressive and optimized JPEGs with high quality values
   [e98cuenc]
 

PIL/PngImagePlugin.py

@@ -561,7 +561,7 @@ def _save(im, fp, filename, chunk=putchunk, check=0):
 
     transparency = im.encoderinfo.get('transparency',im.info.get('transparency', None))
     
-    if transparency:
+    if transparency or transparency == 0:
         if im.mode == "P":
             # limit to actual palette size
             alpha_bytes = 2**bits

Tests/test_file_png.py

@@ -251,6 +251,20 @@ def test_trns_rgb():
     im = roundtrip(im, transparency=(0, 1, 2))
     assert_equal(im.info["transparency"], (0, 1, 2))
 
+def test_trns_p():
+    # Check writing a transparency of 0, issue #528
+    im = lena('P')
+    im.info['transparency']=0
+    
+    f = tempfile("temp.png")
+    im.save(f)
+
+    im2 = Image.open(f)
+    assert_true('transparency' in im2.info)
+
+    assert_image_equal(im2.convert('RGBA'), im.convert('RGBA'))
+        
+    
 def test_save_icc_profile_none():
     # check saving files with an ICC profile set to None (omit profile)
     in_file = "Tests/images/icc_profile_none.png"