Friday, October 8, 2010

Google's WEBP image format and World of Warcraft

I got my hands on Google's new image file format today and started testing it on some images I had lying around. I specifically wanted to see how it did with bad JPEGs of rendered sceenes. Hard edges and re-processed artifacts are something that JPEG traditionally handles very poorly, so there's some real, practical benefit to having a new format that can do these things well.

I took 9.5MB of input screenshots from my library and got 3.9MB of images out at 75% quality. Now, 75% in JPEG is pretty poor, but these don't look all that bad at all. At least not worse overall than the input (WoW compresses its screenshots pretty heavily).

More interestingly, however, here are some visual comparisons. All of these images are PNG format, converted either from the original JPEG input or from the converted WEBP at the given resolution.

Original screenshot (cropped region saved as PNG from original JPEG). Full file size: 541KB.
This is the 75% quality version of the same image as WEBP. Full file size: 257KB.
And this is the same WEBP conversion again, this time at 85%. Full file size: 347KB.
A second original image, again cropped and saved as PNG from a JPEG original screenshot. Full file size: 189KB. (re-compressed at 75% quality as JPEG, it was 132KB).
A 75% quality WEBP conversion. Full file size: 72KB.
An 85% quality WEBP conversion. Full file size: 109KB.

The lessons I learned from this are:
  • WEBP does have some visual loss, even at 85% when it comes to highly saturated color, especially red.
  • The file size drop is quite dramatic, even over a re-compressed JPEG at a lower quality.
  • Overall, the look of the WEBP files is impressively smooth.
  • I noticed (not in the samples, above) that splotchy regions of similar color, but varying value were sometimes flattened to the point that information was clearly being thrown away. At 75% quality this was a substantial change, but at 85% quality, it was hardly noticeable.
I really look forward to this format being available in the majority of browsers, so I can safely start using it!

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