Add alpha blending support

Pipfile

@@ -13,5 +13,6 @@ twine = "*"
 wheel = "*"
 pytest-benchmark = "*"
 tox = "*"
+pytest-mock = "*"
 
 [packages]

pilgram/css/blending/alpha.py

@@ -0,0 +1,117 @@
+# coding:utf-8
+
+# Copyright 2019 Akiomi Kamakura
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from PIL import ImageChops
+
+from pilgram.util import invert
+
+
+def split_alpha(im):
+    """Splits alpha channel
+
+    Arguments:
+        im: An image (RGB or RGBA).
+
+    Returns:
+        A tuple of the RGB and alpha image.
+    """
+
+    if im.mode == 'RGBA':
+        # NOTE: `merge` is slower than `convert` when using Vanilla Pillow
+        a = im.split()[3]
+        im = im.convert('RGB')
+        return im, a
+    elif im.mode == 'RGB':
+        return im, None
+    else:
+        raise ValueError('Unsupported mode: ' + im.mode)
+
+
+def alpha_to_rgb(im):
+    """Converts alpha image to rgb image.
+
+    Arguments:
+        im: An image (L only).
+
+    Returns:
+        A tuple of the RGB and alpha image.
+    """
+    if im.mode == 'L':
+        # NOTE: `merge` is slower than `convert` when using Vanilla Pillow
+        im = im.convert('RGB')
+        return im
+    else:
+        raise ValueError('Unsupported mode: ' + im.mode)
+
+
+def alpha_blend(im1, im2, blending):
+    """Simple alpha blending
+
+    The formula is defined as:
+
+        simple alpha compositing:
+        co = cs + cb x (1 - αs)
+
+        written as non-premultiplied:
+        αo x Co = αs x Cs + (1 - αs) x αb x Cb
+
+        now substitute the result of blending for Cs:
+        αo x Co = αs x ((1 - αb) x Cs + αb x B(Cb, Cs)) + (1 - αs) x αb x Cb
+                = αs x (1 - αb) x Cs + αs x αb x B(Cb, Cs) + (1 - αs) x αb x Cb
+
+    See the W3C document:
+    https://www.w3.org/TR/compositing-1/#blending
+
+    Arguments:
+        im1: A backdrop image (RGB or RGBA).
+        im2: A source image (RGB or RGBA).
+        blending: The blending method.
+
+    Returns:
+        The output image.
+    """
+
+    im1, a1 = split_alpha(im1)
+    if a1 is not None:
+        a1_rgb = alpha_to_rgb(a1)
+        a1_invert_rgb = alpha_to_rgb(invert(a1))
+
+    im2, a2 = split_alpha(im2)
+    if a2 is not None:
+        a2_rgb = alpha_to_rgb(a2)
+        a2_invert_rgb = alpha_to_rgb(invert(a2))
+
+    im_blended = blending(im1, im2)
+
+    if a1 is not None and a2 is not None:
+        im_hard_light_alpha = ImageChops.multiply(a1_rgb, a2_rgb)
+        im1_alpha = ImageChops.multiply(a1_rgb, a2_invert_rgb)
+        im2_alpha = ImageChops.multiply(a2_rgb, a1_invert_rgb)
+        im_blended = ImageChops.add(
+            ImageChops.multiply(im_hard_light_alpha, im_blended),
+            ImageChops.add(
+                ImageChops.multiply(im1_alpha, im1),
+                ImageChops.multiply(im2_alpha, im2)))
+    elif a1 is not None:
+        im_blended = ImageChops.add(
+            ImageChops.multiply(a1_rgb, im_blended),
+            ImageChops.multiply(a1_invert_rgb, im2))
+    elif a2 is not None:
+        im_blended = ImageChops.add(
+            ImageChops.multiply(a2_rgb, im_blended),
+            ImageChops.multiply(a2_invert_rgb, im1))
+
+    return im_blended