feat(webp): Allow finer grained control over WEBP compression settings

[jessey-git]

Description

WEBP uses the WebPConfig::factor setting to also control how hard it tries to compress when lossless compression is used. It also uses another WebPConfig::method setting to further tune both lossless and lossy schemes. [1]

Expose both of these settings so client applications using OIIO can adjust these values as they see fit.

This PR makes 4 changes:

• Adds support for using a lossless:factor value for the compression attribute. The 'factor' value controls the effort that WEBP uses during the compression process.
• Adds support for using a webp:method int attribute. The value of this attribute controls the WebPConfig::method setting. Note: this is different than the compression settings but will also affect speed and quality in various ways.
• Sets WebPPicture::use_argb to true when using lossless compression (per the WEBP documentation[2])
• Changes default lossless "quality" from 100 to 70. This results in much faster image output at a very minor increase in file size (e.g. a 2048x2048 test image goes from 1300ms/364kb to 800ms/371kb after this change)

Before - just 2 basic forms were possible:

Client uses "compression=lossless"
- OIIO uses lossless=true, method=6, quality=100, use_argb=false
Client uses "compression=webp:90"
- OIIO uses lossless=false, method=6, quality=90, use_argb=false

After - several additional variations are possible:

Client uses "compression=lossless"
- OIIO uses lossless=true, method=6, quality=70, use_argb=true
Client uses "compression=lossless:100" and "webp:method=4"
- OIIO uses lossless=true, method=4, quality=100, use_argb=true
Client uses "compression=webp:90"
- OIIO uses lossless=false, method=6, quality=90, use_argb=false
Client uses "compression=webp:90" and "webp:method=4"
- OIIO uses lossless=false, method=4, quality=90, use_argb=false

[1] https://developers.google.com/speed/webp/docs/api#advanced_encoding_api
[2] See notes around use_argb from the above link and see also the internal Encode helper function which keeps lossless and use_argb in sync: https://github.com/webmproject/libwebp/blob/main/src/enc/picture_enc.c#L241

Tests

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (Check if there is no
  need to update the documentation, for example if this is a bug fix that
  doesn't change the API.)
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[lgritz]

LGTM

[lgritz]

For my own curiosity: what's the difference between method and quality? They both are described as somehow trading size for time. Is method selecting between totally different algorithms?

[jessey-git]

There's little actual documentation about it... The WebPConfigInit and WebPConfigPreset functions will default to method=4 though.

There's a chart in the source about it but I don't have the necessary background to really understand what those various items mean: https://github.com/webmproject/libwebp/blob/main/src/enc/webp_enc.c#L74. So it seems like it controls some tuning mechanisms during Color encoding.

It also seems to control some "effort" mechanism when encoding Alpha -- the method eventually sets the effort_level here: https://github.com/webmproject/libwebp/blob/main/src/enc/alpha_enc.c#L39