chunk_hook: Built-in chunk hooks to extend knitr

In knitr: A General-Purpose Package for Dynamic Report Generation in R

Built-in chunk hooks to extend knitr

Description

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.

Usage

hook_pdfcrop(before, ...)

hook_optipng(...)

hook_pngquant(...)

hook_mogrify(...)

hook_plot_custom(before, options, envir)

hook_purl(before, options, ...)

Arguments

before, options, envir, ...

See References below.

Details

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 https://yihui.org/knitr/demo/cache/ for the meaning of cache = 2).

Note

The two hook functions hook_rgl() and hook_webgl() were moved from knitr to the rgl package (>= v0.95.1247) after knitr v1.10.5, and you can library(rgl) to get them.

References

https://yihui.org/knitr/hooks/#chunk-hooks

See Also

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

Examples

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

knitr documentation built on April 4, 2025, 5:15 a.m.