Merge pull request #4231 from uploadcare/box-in-thumbnail

Fix thumbnail geometry when DCT scaling is used

Tests/test_image_draft.py

@@ -13,7 +13,11 @@ class TestImageDraft(PillowTestCase):
         im = Image.new(in_mode, in_size)
         data = tostring(im, "JPEG")
         im = fromstring(data)
-        im.draft(req_mode, req_size)
+        mode, box = im.draft(req_mode, req_size)
+        scale, _ = im.decoderconfig
+        self.assertEqual(box[:2], (0, 0))
+        self.assertTrue((im.width - scale) < box[2] <= im.width)
+        self.assertTrue((im.height - scale) < box[3] <= im.height)
         return im
 
     def test_size(self):

Tests/test_image_thumbnail.py

@@ -1,6 +1,6 @@
 from PIL import Image
 
-from .helper import PillowTestCase, hopper
+from .helper import PillowTestCase, fromstring, hopper, tostring
 
 
 class TestImageThumbnail(PillowTestCase):
@@ -58,3 +58,15 @@ class TestImageThumbnail(PillowTestCase):
         with Image.open("Tests/images/hopper.jpg") as im:
             im.thumbnail((64, 64))
             self.assertEqual(im.size, (64, 64))
+
+    def test_DCT_scaling_edges(self):
+        # Make an image with red borders and size (N * 8) + 1 to cross DCT grid
+        im = Image.new("RGB", (257, 257), "red")
+        im.paste(Image.new("RGB", (235, 235)), (11, 11))
+
+        thumb = fromstring(tostring(im, "JPEG", quality=99, subsampling=0))
+        thumb.thumbnail((32, 32), Image.BICUBIC)
+
+        ref = im.resize((32, 32), Image.BICUBIC)
+        # This is still JPEG, some error is present. Without the fix it is 11.5
+        self.assert_image_similar(thumb, ref, 1.5)

docs/releasenotes/7.0.0.rst

@@ -58,6 +58,20 @@ and :py:meth:`~PIL.ImageOps.fit` functions.
 See :ref:`concept-filters` to learn the difference. In short,
 ``Image.NEAREST`` is a very fast filter, but simple and low-quality.
 
+Image.draft() return value
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If the :py:meth:`~PIL.Image.Image.draft` method has no effect, it returns ``None``.
+If it does have an effect, then it previously returned the image itself.
+However, unlike other `chain methods`_, :py:meth:`~PIL.Image.Image.draft` does not
+return a modified version of the image, but modifies it in-place. So instead, if
+:py:meth:`~PIL.Image.Image.draft` has an effect, Pillow will now return a tuple
+of the image mode and a co-ordinate box. The box is the original coordinates in the
+bounds of resulting image. This may be useful in a subsequent
+:py:meth:`~PIL.Image.Image.resize` call.
+
+.. _chain methods: https://en.wikipedia.org/wiki/Method_chaining
+
 
 API Changes
 ===========
@@ -104,8 +118,12 @@ Use instead:
     with Image.open("hopper.png") as im:
         im.save("out.jpg")
 
-Better thumbnail aspect ratio preservation
+Better thumbnail geometry
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^^
 
 When calculating the new dimensions in :py:meth:`~PIL.Image.Image.thumbnail`,
 round to the nearest integer, instead of always rounding down.
+This better preserves the original aspect ratio.
+
+When the image width or height is not divisible by 8 the last row and column
+in the image get the correct weight after JPEG DCT scaling.

src/PIL/Image.py

@@ -1126,11 +1126,14 @@ class Image:
         """
         Configures the image file loader so it returns a version of the
         image that as closely as possible matches the given mode and
-        size.  For example, you can use this method to convert a color
+        size. For example, you can use this method to convert a color
         JPEG to greyscale while loading it.
 
+        If any changes are made, returns a tuple with the chosen ``mode`` and
+        ``box`` with coordinates of the original image within the altered one.
+
         Note that this method modifies the :py:class:`~PIL.Image.Image` object
-        in place.  If the image has already been loaded, this method has no
+        in place. If the image has already been loaded, this method has no
         effect.
 
         Note: This method is not implemented for most images. It is
@@ -2143,14 +2146,17 @@ class Image:
             x = max(round(x * size[1] / y), 1)
             y = round(size[1])
         size = x, y
+        box = None
 
         if size == self.size:
             return
 
-        self.draft(None, size)
+        res = self.draft(None, size)
+        if res is not None:
+            box = res[1]
 
         if self.size != size:
-            im = self.resize(size, resample)
+            im = self.resize(size, resample, box=box)
 
             self.im = im.im
             self._size = size

src/PIL/ImageFile.py

@@ -122,11 +122,6 @@ class ImageFile(Image.Image):
                 self.fp.close()
             raise
 
-    def draft(self, mode, size):
-        """Set draft mode"""
-
-        pass
-
     def get_format_mimetype(self):
         if self.custom_mimetype:
             return self.custom_mimetype

src/PIL/JpegImagePlugin.py

@@ -413,7 +413,8 @@ class JpegImageFile(ImageFile.ImageFile):
             return
 
         d, e, o, a = self.tile[0]
-        scale = 0
+        scale = 1
+        original_size = self.size
 
         if a[0] == "RGB" and mode in ["L", "YCbCr"]:
             self.mode = mode
@@ -436,7 +437,8 @@ class JpegImageFile(ImageFile.ImageFile):
         self.tile = [(d, e, o, a)]
         self.decoderconfig = (scale, 0)
 
-        return self
+        box = (0, 0, original_size[0] / float(scale), original_size[1] / float(scale))
+        return (self.mode, box)
 
     def load_djpeg(self):