close #2274: add a function download_image() to include an image via a URL for non-HTML output formats

Author: yihui

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: knitr
 Type: Package
 Title: A General-Purpose Package for Dynamic Report Generation in R
-Version: 1.43.15
+Version: 1.43.16
 Authors@R: c(
     person("Yihui", "Xie", role = c("aut", "cre"), email = "xie@yihui.name", comment = c(ORCID = "0000-0003-0645-5666")),
     person("Abhraneel", "Sarma", role = "ctb"),

NAMESPACE

@@ -35,6 +35,7 @@ export(convert_chunk_header)
 export(current_input)
 export(dep_auto)
 export(dep_prev)
+export(download_image)
 export(engine_output)
 export(extract_raw_output)
 export(fig_chunk)

NEWS.md

@@ -10,6 +10,8 @@
 
 - Updated the package vignette `vignette('knit_print', 'knitr')` to mention that package authors no longer have to make **knitr** a hard dependency if they want to define S3 methods for `knitr::knit_print` with R >= 3.6.0 (thanks, @cderv, #1929).
 
+- Added a new function `download_image()` to download an image from a URL and include it via `include_graphics()`. This is mainly for including online images when the output format is not HTML (e.g., LaTeX), because the URL will not work as the image path, and it has to be downloaded beforehand (thanks, @bayeslearner, #2274).
+
 ## BUG FIXES
 
 - Make the internal function `add_html_caption()` work with Quarto <= v1.3.353 (thanks, @giabaio, #2261).

R/plot.R

@@ -490,6 +490,29 @@ include_graphics = function(
   structure(path, class = c('knit_image_paths', 'knit_asis'), dpi = dpi)
 }
 
+#' Download an image from the web and include it in a document
+#'
+#' When including images in non-HTML output formats such as LaTeX/PDF, URLs will
+#' not work as image paths. In this case, we have to download the images. This
+#' function is a wrapper of \code{xfun::\link[xfun]{download_file}()} and
+#' \code{\link{include_graphics}()}.
+#' @param url The URL of an image.
+#' @param path The download path (inferred from the URL by default). If the file
+#'   exists, it will not be downloaded (downloading can take time and requires
+#'   Internet connection). If you are sure the file needs to be downloaded
+#'   again, delete it beforehand.
+#' @param use_file Whether to use the URL or the download path to include the
+#'   image. By default, the URL is used for HTML output formats, and the file
+#'   path is used for other output formats.
+#' @param ... Other arguments to be passed to \code{\link{include_graphics}()}.
+#' @export
+#' @examplesIf interactive()
+#' knitr::download_image('https://www.r-project.org/Rlogo.png')
+download_image = function(url, path = xfun::url_filename(url), use_file = !pandoc_to('html'), ...) {
+  if (!file.exists(path) && use_file) xfun::download_file(url, path)
+  include_graphics(if (use_file) path else url, ...)
+}
+
 # calculate the width in inches for PNG/JPEG images given a DPI
 raster_dpi_width = function(path, dpi) {
   if (!file.exists(path) || is.na(dpi)) return()