Prevent audio/video resources in markdown/html to be dropped during LaTeX conversion

Prevent audio/video resources in markdown/html to be dropped during LaTeX conversion

[Alexander Neumann]

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Hello,

I have documents written in markdown and HTML and would like to convert 
them into LaTeX. Since markdown has no native support for media files such 
as audio and video, I had to make use of <audio> and <video> tags for this.
During the conversion to TeX, these "RawBlock" elements are completely 
ignored.

---

$ cat test.md
## Paragraph

A video below.

<video controls>
    <source src="video.mp4" type="video/mp4">
</video>

More text.

---

pandoc --to latex test.md --verbose
[INFO] Not rendering RawBlock (Format "html") "<video controls>"
[INFO] Not rendering RawBlock (Format "html") "<source src=\"video.mp4\" 
type=\"video/mp4\">"
[INFO] Not rendering RawBlock (Format "html") "</video>"
\hypertarget{paragraph}{%
\subsection{Paragraph}\label{paragraph}}

A video below.

More text.

---

I do not necessarily need videos in LaTeX (even though it might be handy 
for beamer presentations). I am just wondering if there is a way to 
compatibly annotate placeholder images or such a like. Or maybe some pandoc 
filters/template rules that replace <video> / <audio> blocks with (linked) 
thumbnails.

Best regards,
Alex

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Re: Prevent audio/video resources in markdown/html to be dropped during LaTeX conversion

[Jesse Rosenthal]

Alexander Neumann <aleneum-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:
> I am just wondering if there is a way to compatibly annotate
> placeholder images or such a like. Or maybe some pandoc
> filters/template rules that replace <video> / <audio> blocks with
> (linked) thumbnails.

Sure -- a filter could do this. Something like following (call it
script.lua):

~~~ 

function RawBlock (blk)
   if FORMAT == "latex" then
      if blk.format == "html" then
	 if blk.text == "<video controls>" then
	    return pandoc.RawBlock("latex", "\\begin{environment}")
	 elseif blk.text == "</video>" then
	    return pandoc.RawBlock("latex", "\\end{environment}")
	 else
	    local _, fp, _ = string.match(blk.text, "(<source src=\")([^\"]+)(.+)")
	    if fp then
	       return pandoc.RawBlock("latex", fp)
	    end
	 end
      end
   end
end

~~~

Given your test.md, we'd get:

~~~
$ pandoc test.md -t latex --lua-filter=script.lua
\hypertarget{paragraph}{%
\subsection{Paragraph}\label{paragraph}}

A video below.

\begin{environment}

video.mp4

\end{environment}

More text.
~~~

I do a bit of substitution for the center block (you can look up some
lua docs on it) and make up an environment name for it. Remember, in a
lua filter, if the function doesn't return anything, it leaves the
original untouched. Also, the opening FORMAT conditional makes it so it
only does this for LaTeX output.

There's probably a better way to do this than with the nested if-thens,
but this should get you started.

Best,
Jesse

Re: Prevent audio/video resources in markdown/html to be dropped during LaTeX conversion

[Alexander Neumann]

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Hi Jesse, 

thank you for your reply and your nice examples. Lua filters are indeed a 
nice way to deal with this.
Since I posted my question I have used lua filters to exclude notes and 
other markup not intended for the final document.

I had to solve the described situation otherwise though since I realized I 
had more conditions than I had previously stated here.
Specifically, the solution has to work with the pandoc filter 
`pandoc-crossref`.
A call like `pandoc -t markdown --lua-filter html_to_md.lua -F 
pandoc-crossref test.md` does not resolve the reference while adding the 
result of `html_to_md.lua` into the source file seems to work fine.
I could not figure out how to adjust the order of filters probably to deal 
with this.
Alternatively, I could have called pandoc multiple times.
I ended up (mis)using the multifigure syntax of pandoc-crossref:

```
Some text that refers to [@fig:figure_id].

<div id="fig:figure_id" class="subfigures">
![](vid/test.png)<video controls src="vid/test.mp4"></video>
    
A caption that describes the figure
</div>
```

This way, I was able embed videos in HTML by overlaying the thumbnail image 
with the actual video (via CSS) and have a resolved figure and references 
in the LaTeX export when the RawBlock is filtered out.
If I plan to use more than thumbnails in the LaTeX export, I'd go the way 
you have suggested with LaTeX environments.

Thanks again for your feedback,
Alex

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