suggestion: offer rescaling / compression option to include_graphics() for external images

[maxheld83]

it would be really nice if knitr::include_graphics() (which is used heavily, and elegantly in bookdown() would allow me to specify a DPI and size, and then rescale the external image accordingly, as per this stack-exchange question.

Note: I mean external image, like some JPEG from a camera or something, not plot images generated by R.

It would be neat especially for any kind of website output, if knitr (or some other element of the toolchain) could take care of the resizing, so that users don't end up deploying huge images when bookdown is rendered to a website, etc.

(I guess, in principal, the same concern applies to PDFs, though file size is usually less of a concern there).

[yihui]

If anybody is able to implement it, I'll be happy to take a look at the pull request, otherwise I'm afraid I won't have the time to implement it in the near future.

[maxheld83]

Just two more thoughts on this in light of related blogdown issue and bookdown issue.

On the one hand (automatic) rescaling/resizing/cropping is on the roadmap for HUGO anyway. That'll probably be easier and/or more specialised than it could be in R. For blogdown related issues, this might be a better way.

On the other hand, it's not available yet, and the HUGO support wouldn't fix this for other knitr use cases.

[maxheld83]

I have now implemented this for my own purposes (inside blogdown), and it does work:

# resize and compress images ===
library(magick)
library(pbapply)  # just so we have a progress bar, because this thing takes a while

resize_n_compress <- function(file_in, file_out, xmax = 1920, quality = 0.7, cutoff = 100000) {
  # xmax <- 1920  # y pixel max
  # quality <- 0.7  # passed on to jpeg::writeJPEG()
  # cutoff <- 100000  # files smaller than this will not be touched
  # file_out <- "test.jpg"
  if (file.size(file_in) < cutoff) {  # in this case, just copy file
    if (!(file_in == file_out)) {
      file.copy(from = file_in, to = file_out, overwrite = TRUE)
    }
  } else {# if larger than cutoff
    # magick workflow
    image_raw <- image_read(path = file_in)
    if (image_info(image_raw)["width"] > xmax) {  # only resize if smaller
      image_resized <- image_scale(image = image_raw, geometry = as.character(xmax))
    } else {
      image_resized <- image_raw
    }
    image_write(image = image_resized, path = file_out, format = "jpeg", quality = quality)
  }
}

This is a bit clumsy, but it works.

This both increases the compression, and downscales the image.

While this works, it's pretty darn slow (at least on OS X for some reason, Travis CI was much faster ...), and I'm guessing this is because R isn't really meant/optimised for this kind of thing.
Handling huge, uncompressed images in RAM in R feels a bit painful, but it does work.

@yihui is this a direction you think might be worthwhile for me to pursue in a pull request?

I'm a bit confused as to where in the knitr, rmarkdown, blogdown/bookdown stack this is best included.

(Updated to now feature magick, which is a lot faster than imager).

[yihui]

I suggest you write your own wrapper function like

include_image = function(x, ...) {
  # first process x by yourself
  magic::whatever....
  # return
  knitr::include_graphics(x, ...)
}

I don't have much interest in provide built-in support for this.

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