Fix thumbnail geometry when DCT scaling is used

[homm]

Image.thumbnail() method uses .draft(None, size) which could scale down an image with an integer ratio. The problem is if the original image size is not dividable by this ratio, the last line or row of the resulting image gets higher weight.

This could be fixed by using the box argument for resizing. For example, if the original image was 257×257 pixels and we want 30×30. The image will be scaled down 8 times to 33×33 and then resized to 30×30. But 257 / 8 = 32.125, not 33. Now using box we can extract an interesting region (0, 0, 32.125, 32.125).

im = Image.new('RGB', (257, 257), 'red')
im.paste(Image.new('RGB', (255, 255), 'white'), (1, 1))
 
thumb = fromstring(tostring(im, "JPEG", quality=95))
thumb.thumbnail((30, 30), Image.BICUBIC)
thumb.save('_out.thumb.png')
 
im.resize((30, 30), Image.BICUBIC).save('_out.ref.png')

Thumbnail, fixed version, reference without DCT scaling:

Thumbnail, fixed version, reference without DCT scaling:

[homm]

Both values are not handled: https://github.com/python-pillow/Pillow/blob/master/src/libImaging/JpegDecode.c#L238

While 1 is more correct.

[homm]

This is breaking change. Previously, draft() could return None or self. This is very strange behavior, I don't think anyone actually used it. Now draft() returns the results of its work or None.

I'll add this in docs.

[radarhere]

Suggested change

	im = self.resize(size, resample, box=box)
	im = self.resize(size, resample, box)

I suggest not specifying the argument name - it's not necessary, and the variable name already clearly indicates the purpose.

[homm]

In truth, I think this is a case where we could use Keyword-Only Arguments.

It seems obvious that we can omit the argument name when you watch on it and so you know that it exists and what exactly it is doing. But once you omit it, you can't know exactly where your variable goes.

[homm]

Rebased

[homm]

@radarhere Does this description look good for you?

[radarhere]

I've created uploadcare#60 with my suggestion.