Adding updated histogram entropy implementations (#598)

As of version 6.1.0, the Pillow imaging library’s C module includes an optimized native entropy method:

• https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst#610-2019-07-01

This change detects and selects this new native method, if it is available in the version of Pillow available to `sorl-thumbnails`.

Additionally, an optimized version of the existing Python entropy method is furnished as a fallback: rather than summing the entire histogram value set, the function calculates the product of the image dimensions and band count to arrive at the same number. It also uses generator expressions and the specialized `math.fsum(…)` and `math.log2(…)` library calls to improve on the performance of its predecessor without altering the algorithm.

The appropriate image-entropy function is then conditionally added to the `sorl.thumbnail.engines.pil_engine.Engine` class as a `staticmethod` at module-load time.

*) Additionally, this PR rounds out the smart-crop unit test

Co-authored-by: Camilo Nova <[email protected]>

sorl/thumbnail/engines/pil_engine.py

@@ -1,19 +1,42 @@
 from __future__ import unicode_literals, division
 
-import math
 from sorl.thumbnail.engines.base import EngineBase
 from sorl.thumbnail.compat import BufferIO
 
 try:
-    from PIL import Image, ImageFile, ImageDraw, ImageFilter
+    from PIL import Image, ImageFile, ImageDraw, ImageFilter, ImageMode
 except ImportError:
     import Image
     import ImageFile
     import ImageDraw
+    import ImageMode
 
 EXIF_ORIENTATION = 0x0112
 
 
+def color_count(image):
+    """ Return the number of color values in the input image --
+        this is the number of pixels times the band count
+        of the image.
+    """
+    mode_descriptor = ImageMode.getmode(image.mode)
+    width, height = image.size
+    return width * height * len(mode_descriptor.bands)
+
+
+def histogram_entropy_py(image):
+    """ Calculate the entropy of an images' histogram. """
+    from math import log2, fsum
+    histosum = float(color_count(image))
+    histonorm = (histocol / histosum for histocol in image.histogram())
+    return -fsum(p * log2(p) for p in histonorm if p != 0.0)
+
+
+# Select the Pillow native histogram entropy function - if
+# available - and fall back to the Python implementation:
+histogram_entropy = getattr(Image.Image, 'entropy', histogram_entropy_py)
+
+
 def round_corner(radius, fill):
     """Draw a round corner"""
     corner = Image.new('L', (radius, radius), 0)  # (0, 0, 0, 0))
@@ -204,6 +227,9 @@ def _entropy_crop(self, image, geometry_width, geometry_height, image_width, ima
 
         return image
 
+    # Add the histogram_entropy fumnction as a static method:
+    _get_image_entropy = staticmethod(histogram_entropy)
+
     def _scale(self, image, width, height):
         return image.resize((width, height), resample=Image.ANTIALIAS)
 
@@ -261,10 +287,3 @@ def _get_raw_data(self, image, format_, quality, image_info=None, progressive=Fa
             bf.close()
 
         return raw_data
-
-    def _get_image_entropy(self, image):
-        """calculate the entropy of an image"""
-        hist = image.histogram()
-        hist_size = sum(hist)
-        hist = [float(h) / hist_size for h in hist]
-        return -sum([p * math.log(p, 2) for p in hist if p != 0])

tests/thumbnail_tests/test_engines.py

@@ -340,9 +340,19 @@ def mean_pixel(x, y):
             for x, y in coords:
                 self.assertEqual(0 <= mean_pixel(x, y) < 5, True)
 
+    @unittest.skipIf(
+        'pil_engine' not in settings.THUMBNAIL_ENGINE,
+        'the other engines fail this test',
+    )
     def test_smart_crop(self):
-        # TODO: Complete test for smart crop
-        self.BACKEND.get_thumbnail('32x32', 'data/white_border.jpg', crop='smart')
+        th = self.BACKEND.get_thumbnail('data/white_border.jpg', '32x32', crop='smart')
+        self.assertEqual(th.x, 32)
+        self.assertEqual(th.y, 32)
+
+        engine = PILEngine()
+        im = engine.get_image(th)
+        self.assertEqual(im.size[0], 32)
+        self.assertEqual(im.size[1], 32)
 
     @unittest.skipIf(
         'pil_engine' not in settings.THUMBNAIL_ENGINE,