Fix thumbnail geometry when DCT scaling is used

Tests/test_image_draft.py

@@ -13,7 +13,11 @@ def draft_roundtrip(self, in_mode, in_size, req_mode, req_size):
         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):
@@ -47,3 +47,14 @@ def test_no_resize(self):
         with Image.open("Tests/images/hopper.jpg") as im:
             im.thumbnail((64, 64))
             self.assert_image(im, im.mode, (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", (255, 255)), (1, 1))
+
+        thumb = fromstring(tostring(im, "JPEG", quality=99, subsampling=0))
+        thumb.thumbnail((32, 32), Image.BICUBIC)
+
+        ref = im.resize((32, 32), Image.BICUBIC)
+        self.assert_image_similar(thumb, ref, 2)

docs/releasenotes/7.0.0.rst

@@ -47,6 +47,20 @@ Setting the size of TIFF images
 Setting the size of a TIFF image directly (eg. ``im.size = (256, 256)``) throws
 an error. Use ``Image.resize`` instead.
 
+Image.draft() return value
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``Image.draft()`` method used to return ``None`` or the image itself.
+Unlike other `chain methods`_, ``draft()`` modifies the image in-place
+rather than returning a modified version.
+
+In the new version, ``draft()`` returns ``None`` if it has no effect or
+a tuple of new image mode and the box of original coordinates in the
+bounds of resulting image otherwise
+(the box could be useful in subsequent ``resize()`` call).
+
+.. _chain methods: https://en.wikipedia.org/wiki/Method_chaining
+
 
 API Changes
 ===========

src/PIL/Image.py

@@ -1126,12 +1126,15 @@ def draft(self, mode, size):
         """
         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, or to extract a 128x192
         version from a PCD file.
 
+        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 @@ def thumbnail(self, size, resample=BICUBIC):
             x = int(max(x * size[1] / y, 1))
             y = int(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 @@ def __init__(self, fp=None, filename=None):
                 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

@@ -409,7 +409,8 @@ def draft(self, mode, size):
             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
@@ -432,7 +433,8 @@ def draft(self, mode, size):
         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):