Merge pull request #364 from wiredfool/premultiply

Alpha Premultiplication support for transform and resize

PIL/Image.py

@@ -1309,6 +1309,9 @@ class Image:
         if self.mode in ("1", "P"):
             resample = NEAREST
 
+        if self.mode == 'RGBA':
+            return self.convert('RGBa').resize(size, resample).convert('RGBA')
+
         if resample == ANTIALIAS:
             # requires stretch support (imToolkit & PIL 1.1.3)
             try:
@@ -1606,6 +1609,9 @@ class Image:
         :returns: An Image object.
         """
 
+        if self.mode == 'RGBA':
+            return self.convert('RGBa').transform(size, method, data, resample, fill).convert('RGBA')
+
         if isinstance(method, ImageTransformHandler):
             return method.transform(size, self, resample=resample, fill=fill)
         if hasattr(method, "getdata"):
@@ -1613,6 +1619,7 @@ class Image:
             method, data = method.getdata()
         if data is None:
             raise ValueError("missing method data")
+            
         im = new(self.mode, size, None)
         if method == MESH:
             # list of quads
@@ -1620,7 +1627,7 @@ class Image:
                 im.__transformer(box, self, QUAD, quad, resample, fill)
         else:
             im.__transformer((0, 0)+size, self, method, data, resample, fill)
-
+            
         return im
 
     def __transformer(self, box, image, method, data,

Tests/test_image_transform.py

@@ -57,6 +57,40 @@ def test_mesh():
     assert_image_equal(blank, transformed.crop((w//2,0,w,h//2)))
     assert_image_equal(blank, transformed.crop((0,h//2,w//2,h)))
 
+def _test_alpha_premult(op):
+     # create image with half white, half black, with the black half transparent.
+    # do op, 
+    # there should be no darkness in the white section.
+    im = Image.new('RGBA', (10,10), (0,0,0,0));
+    im2 = Image.new('RGBA', (5,10), (255,255,255,255));
+    im.paste(im2, (0,0))
+    
+    im = op(im, (40,10))
+    im_background = Image.new('RGB', (40,10), (255,255,255))
+    im_background.paste(im, (0,0), im)
+    
+    hist = im_background.histogram()
+    assert_equal(40*10, hist[-1])
+
+    
+def test_alpha_premult_resize():
+
+    def op (im, sz):
+        return im.resize(sz, Image.LINEAR)
+    
+    _test_alpha_premult(op)
+    
+def test_alpha_premult_transform():
+    
+    def op(im, sz):
+        (w,h) = im.size
+        return im.transform(sz, Image.EXTENT,
+                            (0,0,
+                             w,h), 
+                            Image.BILINEAR)
+
+    _test_alpha_premult(op)
+
 
 def test_blank_fill():
     # attempting to hit

libImaging/Convert.c

@@ -289,6 +289,31 @@ rgba2rgba(UINT8* out, const UINT8* in, int xsize)
     }
 }
 
+/* RGBa -> RGBA conversion to remove premultiplication 
+   Needed for correct transforms/resizing on RGBA images */
+static void
+rgba2rgbA(UINT8* out, const UINT8* in, int xsize)
+{
+    int x;
+    unsigned int alpha;
+    for (x = 0; x < xsize; x++, in+=4) {
+        alpha = in[3];
+        if (alpha) {
+            *out++ = CLIP((255 * in[0]) / alpha);
+            *out++ = CLIP((255 * in[1]) / alpha);
+            *out++ = CLIP((255 * in[2]) / alpha);
+        } else {
+            *out++ = in[0];
+            *out++ = in[1];
+            *out++ = in[2];
+        }
+        *out++ = in[3];
+
+    }
+}
+
+
+
 /* ---------------- */
 /* CMYK conversions */
 /* ---------------- */
@@ -619,6 +644,8 @@ static struct {
     { "RGBA", "CMYK", rgb2cmyk },
     { "RGBA", "YCbCr", ImagingConvertRGB2YCbCr },
 
+    { "RGBa", "RGBA", rgba2rgbA },
+
     { "RGBX", "1", rgb2bit },
     { "RGBX", "L", rgb2l },
     { "RGBA", "I", rgb2i },