Thumbnail tuning

[homm]

There are two changes related to how Image.thumbnail works.

JPEG draft decoding

Image.thumbnail (or more precise Image.draft) instructs libJPEG to open image in draft mode. I don't know original motivation of libJPEG authors to add this mode. Looks like it gives very small performance win:

In [1]: from PIL import Image

In [2]: %timeit -n 20 i = Image.open('5k.jpg'); i.decoderconfig = (1, 0); i.load()
20 loops, best of 3: 245 ms per loop

In [3]: %timeit -n 20 i = Image.open('5k.jpg'); i.decoderconfig = (1, 1); i.load()
20 loops, best of 3: 234 ms per loop

Also it does't affect too much most of images, but for some images it introduces noticeable artifacts. For example this is cropped part of normal decoded image and draft. Source.

Docstring of Image.draft() method says:

Configures the image file loader so it returns a version of the image that as closely as possible matches the given mode and size.

I believe what this is exactly what this method should do. But not decrease quality of image decoding even if decoder options named the same as this method.

Bicubic as default filter

In Pillow 2.5 default filter was changed from NEAREST to ANTIALIAS. Antialias was chosen because all other filters gave poor quality for reduction. Starting from Pillow 2.7 we can replace ANTIALIAS with BICUBIC, because bicubic is a bit faster and ANTIALIAS doesn't give any advantages if we first scales down image with libJPEG (which uses supersampling, not convolutions).

[homm]

I'll resolve conflicts.

[homm]

Done.

[homm]

I'm worry about this PR. Will it be merged in 2.7? If you don't want to change thumbnail default filter, just let me know, I'll remove this part. But draft JPEG decoder mode should be definitely eliminated.

[aclark4life]

@homm Do you want it merged in 2.7? Have @wiredfool and/or @hugovk reviewed?

[homm]

I definitely want :)

[hugovk]

Looks fine for me but I'll let @wiredfool check too.

[wiredfool]

Ok, works for me.