knitr (version 1.42)

hook_pdfcrop: Built-in chunk hooks to extend knitr


Hook functions are called when the corresponding chunk options are not NULL to do additional jobs beside the R code in chunks. This package provides a few useful hooks, which can also serve as examples of how to define chunk hooks in knitr.


hook_pdfcrop(before, options, envir)

hook_optipng(before, options, envir)

hook_pngquant(before, options, envir)

hook_mogrify(before, options, envir)

hook_plot_custom(before, options, envir)

hook_purl(before, options, envir)


before, options, envir

See References below.


The function hook_pdfcrop() calls plot_crop() to crop the white margins of PDF plots.

The function hook_optipng() calls the program optipng to optimize PNG images. Note the chunk option optipng can be used to provide additional parameters to the program optipng, e.g. optipng = '-o7'.

The function hook_pngquant() calls the program pngquant to optimize PNG images. Note the chunk option pngquant can be used to provide additional parameters to the program pngquant, e.g. pngquant = '--speed=1 --quality=0-50'.

The function hook_mogrify() calls the program mogrify. Note the chunk option mogrify can be used to provide additional parameters to the program mogrify (with default -trim to trim PNG files).

When the plots are not recordable via grDevices::recordPlot() and we save the plots to files manually via other functions (e.g. rgl plots), we can use the chunk hook hook_plot_custom to help write code for graphics output into the output document.

The hook hook_purl() can be used to write the code chunks to an R script. It is an alternative approach to purl, and can be more reliable when the code chunks depend on the execution of them (e.g. read_chunk(), or opts_chunk$set(eval = FALSE)). To enable this hook, it is recommended to associate it with the chunk option purl, i.e. knit_hooks$set(purl = hook_purl). When this hook is enabled, an R script will be written while the input document is being knit. Currently the code chunks that are not R code or have the chunk option purl=FALSE are ignored. Please note when the cache is turned on (the chunk option cache = TRUE), no chunk hooks will be executed, hence hook_purl() will not work, either. To solve this problem, we need cache = 2 instead of TRUE (see for the meaning of cache = 2).


See Also

rgl::rgl.snapshot, rgl::rgl.postscript, rgl::hook_rgl, rgl::hook_webgl


Run this code
if (require("rgl") && exists("hook_rgl")) knit_hooks$set(rgl = hook_rgl)
# then in code chunks, use the option rgl=TRUE

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