fix #2415: respect empty fig.alt, i.e., still generate the alt attribute even if fig.alt = ''; discard the attribute only when fig.alt = NA

Author: yihui

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: knitr
 Type: Package
 Title: A General-Purpose Package for Dynamic Report Generation in R
-Version: 1.50.6
+Version: 1.50.7
 Authors@R: c(
     person("Yihui", "Xie", role = c("aut", "cre"), email = "xie@yihui.name", comment = c(ORCID = "0000-0003-0645-5666", URL = "https://yihui.org")),
     person("Abhraneel", "Sarma", role = "ctb"),

NEWS.md

@@ -22,6 +22,10 @@
 
 - HTML screenshots are taken inside the current working directory by default if it is writable, instead of `tempdir()`, which may not be accessible to certain browsers (thanks, @markschl, #2416). The directory can be customized via the global R option `knitr.html_screenshot.tmpdir`, e.g., if you want the old behavior, you may set `options(knitr.html_screenshot.tmpdir = tempdir())`.
 
+## MINOR CHANGES
+
+- Empty alt text for images is allowed now (thanks, @Olivia-Box-Power, #2415). With the chunk option `fig.alt = ''`, the HTML output will be `<img alt = "" ... />`. Previously, the `alt` attribute was omitted in this case. If you still want to omit the `alt` attribute, you may use `fig.alt = NA`.
+
 # CHANGES IN knitr VERSION 1.50
 
 ## NEW FEATURES

R/hooks-html.R

@@ -39,17 +39,18 @@ hook_animation = function(options) {
 
 .img.tag = function(src, w, h, caption, extra) {
   ext = tolower(file_ext(src))
-  if (length(caption) != 1 || caption == '') caption = NULL
   tag = 'img'; extra2 = NULL; att = 'src'
   if (ext == 'pdf') {
     extra2 = 'type="application/pdf"'; tag = 'embed'
   } else if (ext == 'svg' && getOption('knitr.svg.object', FALSE)) {
     extra2 = 'type="image/svg+xml"'; tag = 'object'; att = 'data'
   }
+  if (length(caption) != 1 || is.na(caption) || (tag != 'img' && caption == ''))
+    caption = NULL
   res = paste0(c(
     paste0('<', tag),
     sprintf('%s="%s%s"', att, opts_knit$get('base.url') %n% '', .upload.url(src)),
-    sprintf('%s="%s"', if (tag %in% c('embed', 'object')) 'title' else 'alt', caption),
+    sprintf('%s="%s"', if (tag == 'img') 'alt' else 'title', caption),
     .img.attr(w, h, c(extra, extra2))
   ), collapse = ' ')
   paste0(res, if (tag == 'object') '></object>' else ' />')
@@ -59,7 +60,7 @@ hook_animation = function(options) {
   cap = options$fig.cap %n% {
     if (is.null(pandoc_to())) sprintf('plot of chunk %s', options$label) else ''
   }
-  if (length(cap) == 0) cap = ''
+  if (length(cap) == 0 || is.na(cap)) cap = ''
   if (alt) {
     alt = options$fig.alt %n% cap
     return(if (escape) html_escape(alt) else alt)

tests/testit/test-hooks-md.R

@@ -22,7 +22,7 @@ assert('include_graphics() includes custom images correctly', {
   (img_output('a.png') %==% '![](a.png)')
   (img_output(c('a.png', 'b.png'), list(fig.show = 'hold')) %==% '![](a.png)![](b.png)')
   (img_output('a.png', list(fig.cap = 'foo bar')) %==% '![foo bar](a.png)')
-  (img_output('a.png', list(out.width = '50%')) %==% '<img src="a.png" width="50%" />')
+  (img_output('a.png', list(out.width = '50%')) %==% '<img src="a.png" alt="" width="50%" />')
   (img_output('a.pdf', list(out.width = '300px')) %==% '<embed src="a.pdf" width="300px" type="application/pdf" />')
 })
 
@@ -101,6 +101,11 @@ assert("Include a plot by pandoc md", {
     sprintf("![%s](1.png){width=%s %s}", cap, w, ex))
 })
 
+assert('empty alt text is preserved and NA alt is discarded', {
+  (hook_plot_md(x, opts_chunk$merge(list(fig.alt = ''))) %==% '<img src="1.png" alt=""  />')
+  (hook_plot_md(x, opts_chunk$merge(list(fig.alt = NA, out.width = '100'))) %==% '<img src="1.png" width="100" />')
+})
+
 assert("fig.alt does not break office document", {
   old = opts_knit$get('rmarkdown.pandoc.to')
   opts_knit$set(rmarkdown.pandoc.to = "docx")