Support for saving lossless WebP

[dhotson]

Hello. :)

This PR adds initial support for saving lossless WebP.

Sharp usually saves WebP in lossy mode with a default quality of 80.
A bug that was fixed in libvips 8.3 means lossless WebP output is now possible.

To enable this in sharp, I've introduced an options object parameter to the webp method. e.g.

var image = sharp('file.png')
  .webp({ lossless: true });

I'd be happy to make changes to the API—please just let me know.

Also, I haven't added test cases since my test environment isn't working quite right. Once I've got that worked out—any thoughts on the kinds of tests you would like to see?

Todo

• extra options parameter for toFormat() and other methods?
• unit tests
• documentation?

Example image output

PNG (lossless)

---

WebP (lossy - quality:80)

---

WebP (lossless)

[coveralls]

Coverage decreased (-1.1%) to 96.998% when pulling 52b18a2 on dhotson:webp-save-lossless into c4df560 on lovell:outfit.

[lovell]

This is a great start, thanks Dennis, the proposed API looks good and your todo list is sensible.

You'll be pleased to know I'm currently working on libvips v8.3.0-pre binaries for the CI environments to use, including the addition of WebP support on Windows.

[felixbuenemann]

@dhotson There's more nice stuff coming for WebP in jcupitt/libvips#430.

One of the features added in that PR is near lossless encoding, which applies filtering to the image to improve compression. With moderate filtering you can likely reach ~50% of PNG24 filesize.

Another interesting one is the smart_subsample option, which greatly improves the perceived sharpness of fine details in lossy WebP images by using a smarter colorspace conversion that reduces color bleeding caused by chroma subsampling. With quality 85 and smart_subsample: true the lossless sample above would likely look much better at smaller filesize than lossless.

I tested several compression settings on the 600x600 RGBA sample image linked in the PR above:

• 123K for PNG24
• 83K WebP Lossless
• 55k WebP Near Lossless
• 13k WebP Lossy quality 85%, alpha quality 30%, smart subsampling on

The comparison only show PNG24 vs. WebP Lossy vs. WebP Lossy with smart subsample, which looks just as good as the PNG to me, while the normal lossy image looks poor even with quality 100%.

[dhotson]

@felixbuenemann nice work! Should I close this PR? Does your PR include lossless webp as an option?

[felixbuenemann]

@dhotson Please don't close this. I was talking about a libvips PR not sharp.

[dhotson]

Ah oops! I thought that linked PR was this repo. Reopening.. :)

[lovell]

@dhotson When you next rebase your working branch against the outfit branch you'll pick up the commits for #369, which include changes that allow for the CI environments to work/pass again.

[coveralls]

Coverage decreased (-0.9%) to 97.959% when pulling 289aac9 on dhotson:webp-save-lossless into 051d022 on lovell:outfit.

[lovell]

The outfit branch has now been merged into the master branch ahead of v0.15.0.

Let me know when you're ready to continue with this and we can work out the most suitable destination branch at the time.

[lovell]

Oh, looks like Github auto-closed this PR when I removed the outfit branch.

[dhotson]

Ah no problem—is this still something that you're interested in? I'm not sure where it stands alongside some of the other things that @felixbuenemann mentioned.

[lovell]

Yes, I'm still interested in exposing WebP lossless available in libvips v8.3.1 as well as the new goodness such as smart_subsample that will be in the forthcoming libvips v8.4.0.

[Nantris]

Oddly, lossy webP on a 12kb image is resulting in a 21kb final image, whereas losseless is giving me a 5kb image at better quality?