Registering a custom link protocol as an image

Question

In org mode, is there a way to define how #+CAPTION: ... would work for custom link types? i.e, make org refer to them as images.

I created a custom link type called tex-fig (using org-link-set-parameters, as described here) that has custom behaviors when exporting to LaTeX and HTML, and I'd like to have figure captions for these tex-fig links. Currently, just doing

#+CAPTION: caption caption
[[tex-fig:path/to/file.tex]]

doesn't work when exporting to either backend. How can I define caption/float behavior for custom link types?

I've searched for hours, and it seems that the caption is handled through element properties, which are inaccessible through the :export callback when defining links by org-link-set-parameters (as the callback is given only literal strings, not elements or positions in the document). What are the alternative ways?

Answer 1

You posted already the solution for the HTML backend. So I will address only the LaTeX backend here.

Pittingly, you did not describe how you want to export tex-fig links to LaTeX. I assume here you want to use \import{...} for such figures as the org LaTeX export does for pgf figures.

For that case we can just re-use the .pgf branch of org-latex--inline-image for our purpose. We just have to take care that we

1. provide the right arguments
2. fake a .pgf file name which we correct in the resulting exported string

At first glance the code seems a bit bulky. But, the bits of the code are quite general. So, the gv setter for org-element-property can be used in general and I am surprised that the Org API didn't provide it already. Furthermore, my-org-let-info can be useful for other exporters as well.

(require 'org-element)

(defun my-org-export-tex-fig (path description backend)
  "Call the BACKEND specific tex-fig exporter.
The name of the exporter is `my-org-export-tex-fig:BACKEND'.
It is called with PATH and DESCRIPTION as arguments."
  (let ((sym (intern-soft (format "my-org-export-tex-fig:%s" backend))))
    (if sym
        (funcall sym path description)
      (user-error "Backend %s not implemented" backend))))

(defvar my-org-let-info nil
  "Save `info' arg of `org-latex-link'.")

(defun my-org-let-info (fun link desc info)
  "Save INFO locally in `my-org-latex-info'.
Just pass all args LINK, DESC, and INFO to FUN."
  (let ((my-org-let-info (list link info)))
    (funcall fun link desc info)))

(advice-add 'org-latex-link :around #'my-org-let-info)

(gv-define-simple-setter
 org-element-property
 (lambda (property element value)
   (org-element-put-property element property value))
 'fix-return)

(defun my-org-export-tex-fig:latex (path description)
  "LaTeX specific tex-fig exporter.
Export org links of form [[tex-fig:PATH][DESCRIPTION]] to LaTeX."
  (cl-letf* ((link (nth 0 my-org-let-info))
             ((org-element-property :path link)
              (concat path ".pgf"))
             (info (nth 1 my-org-let-info))
             (ret (org-latex--inline-image link info)))
    (replace-regexp-in-string "\\\\input{.*?\\(\\.pgf\\)}" "" ret t t 1)))

(org-link-set-parameters
 "tex-fig" :export #'my-org-export-tex-fig)

The code is tested with Emacs 26.3 called as emacs -Q.

Answer 2

Well, I've succeeded in HTML exporting by simply telling it that tex-fig should be treated as an image:

(add-to-list 'org-html-inline-image-rules
             (cons "tex-fig" "\\.tex"))

But there seem to be no way to do it in LaTeX. I've read the source code, and while exporting a link through LaTeX it first checks if the link can be exported through a custom protocol, and only if not it tries as an image - so there is no way to export a custom link AS an image. This might not be the intended behavior...