How to do backend-dependent actions in org-mode export?

Question

How do I specify different actions for different export backends in org-mode?

To be more specific, I would like to export an pdf figure in latex export, while using a png in html export (in fact, a png thumbnail linking to the pdf, if that is of relevance):

#+CAPTION: cation of figure
#+NAME:    myfig
# case export to latex: [[file:./figure.pdf]]
# case export to html:  [[file:./figure.pdf][file:./figure.png]]

I tried macros and this answer, which did not work. There are also answers on similar cases for tikz pdf/svg/png export like this thread, which do not seem to be transferable to my case.

Note: I initially posted this question on stackoverflow. It occured to me later that emacs stackexchange is a more appropriate site for this question. So I deleted it on stackoverflow (there were no answers) to repost it here.

Answer 1

I finally found a way to do this, using a macro provided by fniessen:

#+MACRO: if-latex-else (eval (if (org-export-derived-backend-p org-export-current-backend 'latex) "$1" "$2"))

If more/other export-backends than LaTeX and HTML are used, a condition statement can be used instead:

#+MACRO: if-latex-html-else (eval (cond ((org-export-derived-backend-p org-export-current-backend 'latex) "$1") ((org-export-derived-backend-p org-export-current-backend 'html) "$2") (t "$3") ))

Usage would be something like

{{{if-latex-html-else([[file:./figure.pdf]], [[file:./figure.pdf][file:./figure.png]], /Figure *figure.pdf* is supposed to be here/)}}}

Not as short as a simple switch, but substantially better than providing native LaTeX/HTML/whatever code for every figure that is to be included.

Answer 2

Using macros as the accepted answer is a great option, which I also used following this question. But there is one alternative that I started using and that some may prefer: define a src block with emacs-lisp and a call it with #+CALL for each figure we want to include.

It works like this, add the block below to your org-mode document (it can be inside a heading with the :noexport: tag) and change it according with your preferences.

#+NAME: get-filename-by-backend
#+begin_src emacs-lisp :var filename="default" :exports none :results raw
  (concat "[[file:"
          (case org-export-current-backend
            (html (concat filename ".svg"))
            (twbs (concat filename ".svg"))
            (latex (concat filename ".pdf"))
            (t (concat filename ".png")))
          "]]")
#+end_src

Now, whenever you want to include a figure just add something like

#+NAME: a_name_for_this_figure_call
#+CALL: get-filename-by-backend(filename="figs/my_nice_figure")

and execute the block with C-c C-c. This will add the result as

#+RESULTS: a_name_for_this_figure_call
[[file:figs/my_nice_figure.png]]

Now you can add #+NAME: figure_label, #+CAPTION: The figure caption., #+ATTR_LATEX: ..., etc above the #+RESULTS: line.

The trick here is that we get a png image when we are not exporting, which can be nicely displayed with org-toggle-inline-images. However, during the export the #+CALL: line is re-evaluated and we get a pdf image for latex export and an svg image for HTML export.

The advantage of this solution, compared with a macro solution, goes beyond just displaying the image inline. Since you can pass a variable to a src block that receives the result of another src block it means that you can have a src block using your preferred language that generates "backend dependent" export code.