Routine feature request: shrink png images

[gadenbuie]

It'd be helpful to add an action that

• Installs optipng
• Runs optipng on a list of directories provided to the action
• Commits any updates to those files back to the repo

The actual code might look something like this:

# Shrink images in man/figures
sudo apt-get install optipng
optipng man/figures/*.png
git commit man/figures/*.png -m "Shrink images in man/figures (Github Action)" || echo "No changes to man/figures/*.png"

but it'd be extra helpful to allow an array input of directories.

- name: Shrink pngs
  uses: rstudio/shiny-workflows/shrink-png@v1
  with:
    paths:
      - man/figures/*.png
      - vignettes/**.png

[gadenbuie]

Motivation and MVP: rstudio/bslib#521

[wch]

I like the idea of running optipng on files.

It would be nice if it could quickly detect and skip files that are already optimized. In my experience, if there are a large number of PNGs, it can take a nontrivial amount of time to run. That said, there's not likely to be a lot of PNGs in /man/figures and /vignettes.

[gadenbuie]

Good point @wch. Maybe we could limit optipng to rc- branches?

[wch]

It might make sense to just do it, if in practice it doesn't add a huge amount of time. We could also use a program that's faster than OptiPNG.

I came across an interesting discussion of PNG optimizers here: https://news.ycombinator.com/item?id=31655651
* Compression benchmarks: http://www.olegkikin.com/png_optimizers/
* Speed and compression benchmarks: https://css-ig.net/benchmark/png-lossless

It sounds like ECT is very good in terms of speed and file size, but it doesn't seem easy to install. oxipng also looks good, and I think is more widely available.

[wch]

Apparently oxipng is a rewrite of optipng in Rust. I just tested it against optipng, and it looks like it's about 30% faster on my machine. I think oxipng may be the way to go.