Fixes DIB image format

The DIB image format uses the 40 byte BMP header, followed by 3 masks
for RGB channels. We were reading 4 masks, consuming the first pixel
of the image.

Mostly fixes issue #1293. Remaining issue: alpha channel is actually a
transparency mask.

PIL/BmpImagePlugin.py

@@ -109,7 +109,11 @@ class BmpImageFile(ImageFile.ImageFile):
                         for idx, mask in enumerate(['r_mask', 'g_mask', 'b_mask', 'a_mask']):
                             file_info[mask] = i32(header_data[36+idx*4:40+idx*4])
                     else:
-                        for mask in ['r_mask', 'g_mask', 'b_mask', 'a_mask']:
+                        # 40 byte headers only have the three components in the bitfields masks,
+                        # ref: https://msdn.microsoft.com/en-us/library/windows/desktop/dd183376(v=vs.85).aspx
+                        # See also https://github.com/python-pillow/Pillow/issues/1293
+                        file_info['a_mask'] = 0xff000000
+                        for mask in ['r_mask', 'g_mask', 'b_mask']:
                             file_info[mask] = i32(read(4))
                     file_info['rgb_mask'] = (file_info['r_mask'], file_info['g_mask'], file_info['b_mask'])
                     file_info['rgba_mask'] = (file_info['r_mask'], file_info['g_mask'], file_info['b_mask'], file_info['a_mask'])

Tests/test_file_bmp.py

@@ -70,6 +70,13 @@ class TestFileBmp(PillowTestCase):
         self.assertEqual(im.size, reloaded.size)
         self.assertEqual(reloaded.format, "JPEG")
 
+    def test_load_dib(self):
+        # test for #1293, Imagegrab returning Unsupported Bitfields Format
+        im = BmpImagePlugin.DibImageFile('Tests/images/clipboard.dib')
+        target = Image.open('Tests/images/clipboard_target.png')
+        self.assert_image_equal(im.convert('RGB'), target.convert('RGB'))
+        self.assert_image_similar(im, target, 1)
+
 
 if __name__ == '__main__':
     unittest.main()