Compressing PNG Output for R Packages with pngquant and ragg

Cassette collection. Photo by Jametlene Reskp.
Cassette collection. Photo by Jametlene Reskp.


Dealing with large-scale image outputs in R packages can be challenging, especially when it comes to passing CRAN checks. In this post, I will share my experience in using pngquant and ragg to compress the PNG output size for readme and vignettes. This allows R packages with many figures in their documentation to pass the CRAN checks without compromising image quality.

Problem description

I encountered a problem with my package, ggsci, which outputs approximately 30 example figures from both vignettes and README.Rmd. This exceeds the directory and total file size limits that R CMD check allows. For example, without any optimization, running R CMD check on ggsci will issue a check note like this:

installed size is  5.1Mb
sub-directories of 1Mb or more:
  doc    2.7Mb
  help   2.3Mb

The doc/ directory contains the HTML vignette with base64 encoded PNG images, while the help/ directory includes PNG outputs from README.Rmd in man/figures/. To avoid this check note, my goal is to compress the output images as much as possible without sacrificing observable image quality.

Initial solution: SVG

I first attempted using SVG output. For my figures, svglite provided the smallest file size (100 Kb vs. 300 Kb) compared to grDevices::svg() and gridSVG. This is likely because svglite does not encode text as polygons. However, 100 Kb per image was still too large. As my examples included scatterplots with many data points, further reducing the vector-based SVG file size would require significantly reducing the number of data points in the examples. Consequently, I did not take this approach.

Final solution: optimize PNG output with pngquant + ragg

Eventually, I came back to optimizing PNG outputs. I discovered that the ragg_png() device in the ragg package produced the smallest PNG outputs, approximately 120 Kb per image. By using pngquant for lossy compression, I was able to reduce the size further to around 30 Kb per image. My knitr chunk options are (please note that these might require tuning for your use case):

knitr::knit_hooks$set(pngquant = knitr::hook_pngquant)

  dev = "ragg_png",
  dpi = 72,
  fig.retina = 2,
  fig.width = 10.6667,
  fig.height = 3.3334,
  fig.align = "center",
  out.width = "100%",
  pngquant = "--speed=1 --quality=50"

Some technical explanations on why this works:

  1. Since CRAN does not re-do R CMD build nor re-render vignettes and will reuse the built vignettes in the uploaded tarball, it’s only necessary to have pngquant installed in the maintainer’s build environment running R CMD build. This ensures that the submitted tarball contains minimized images.
  2. Even with the pngquant hook set up, knitr can still render the R Markdown vignettes normally in environments without pngquant installed, so there will be no issues on CRAN build machines when they run R CMD check on the tarball uploaded by the package maintainer.
  3. The r-lib/actions GitHub Actions workflows do not have pngquant installed. In these workflows, R CMD build is ran first to build a tarball and R CMD check is used to check it. In this case, there will be a check note about the file and directory sizes. This is ok though because having check notes is still considered as passing for these workflows by default. You can change this default behavior by adjusting the error-on option to make it more or less strict.
  4. The output images of README.Rmd will not be regenerated on any machines besides the maintainer’s build environment. Regeneration only happens when the file is rendered manually. So they will just work ok.

That’s it. I hope these tips are useful for reducing your R package size without having to remove valuable figures from the documentation.