pngquant for smaller vignettes

[bastistician]

Re minimalism: knitr supports a hook_pngquant for smaller vignette outputs, but litedown seems to lack such a built-in compression feature. I sense conflicting interests as litedown aims to be minimal by itself. OTOH, "overall" minimalism could be strengthened by shrinking vignettes engine-wise.

FWIW, as a workaround, I added the following chunk at the end of the Rmd source file, meant to be evaluated only when building the package:

```{r, compact-PNGs, include=FALSE, purl=FALSE, eval=USE_PNGQUANT}
system2("pngquant",
        c("--skip-if-larger", "--speed=1", "--ext=.png", "--force",
          "--", shQuote(litedown::get_context("plot_files"))))
```

Works for me (and shrinks my vignette from 80K to 54K), but this kludge might not be suitable in all cases.

[yihui]

That's a good idea. I think I'll support it. Thanks for the suggestion!

[yihui]

Thanks to this request, I ended up with writing an R package for compressing images: https://github.com/yihui/tinyimg I have been interested in learning more about Rust and the interface to R for a long time, and this gave me a good excuse to finally try it out (with the help of AI).

I tried to find a good threshold for lossy optimization but wasn't able to figure it out: https://pkg.yihui.org/tinyimg/examples/benchmark.html The threshold 2.3 for "just noticeable difference" appears to be very conservative. I feel users will have to find out the best threshold on a case-by-case basis, if they are not satisfied by lossless optimization or the 2.3 threshold for lossy optimization.

With the {tinyimg} package, your code chunk can be written as this:

if (requireNamespace('tinyimg')) tinyimg::tinypng(litedown::get_context("plot_files"))

This assumes all plots are PNG, though.