Coverage increased (+0.006%) to 78.141% when pulling b1f9173 on kkopachev:png-optimize into 97684a2 on thumbor:master.

@kkopachev we should add ssim tests for this optimization.

Not sure we need that. optimize seems to only tell zip_encoder in pillow to compress harder. Since png is lossless in general. Pillow does not do anything with image.
It just passes optimize flag down to ZipEncoder which sets max compression level

Apart from that, looking at the pillow source, I think there is optimize option available for gif as well ❔

Coverage increased (+0.009%) to 83.599% when pulling f2896de on kkopachev:png-optimize into c519d49 on thumbor:master.

New plan: instead of forcing max compression, expose config var. This is useful if people use more efficient optimizers after image operations. In case of PNG there is no reason to compress it, decompress and then compress it harder one more time.

Coverage increased (+0.009%) to 83.599% when pulling f2896de on kkopachev:png-optimize into c519d49 on thumbor:master.

Any updates here?

Guys, should I change something? Or explain a little more how that's useful?

Thanks @kkopachev . We were expecting a difference in output but as you stated this compression happens at the file itself and is not lossy.