Conditional sections in Pandoc markdown - pandoc

Question

I want some sections of my document to be only for a given output format. For instance, I have an image that requires special LaTeX treatment and i want to use \includegraphics for it. But for the epub version, I want the standard behavior.
Is there a way in Pandoc markdown to specify a section that should be processed only in a given context (LaTex vs epub). Templates have conditionals. I did not see the feature for Pandoc markdown.
Or is there another way of dealing with this use case?

Answer 1

Here is a solution with the file preprocessor filepp. We define a function img(caption,path,options) that is replaced by ![caption](path) or by \includegraphics depending on a flag. Notice two things: you need to escape commas in the function arguments (see the third argument of the first plot); you need to have two unescaped commas, even if one argument is not specified (see the second example plot below).
test.md
#bigfunc img(caption,path,options)
#if "out_fmt" eq "tex"
\begin{figure}
\caption{\label{caption} caption}
\includegraphics[options]{path}
\end{figure}
#else
![caption](path)
#endif
#endbigfunc
Here is an image
img(Plot,Rplot.png,height=2\,width=2)
img(Plot,Rplot.png,)
We specify the ouput format in the filepp command and pipe it through pandoc:
filepp -m bigfunc.pm -D out_fmt=tex test.md | pandoc -o test.tex
filepp -m bigfunc.pm -D out_fmt=html test.md | pandoc -o test.html
tex output:
Here is an image
\begin{figure}
\Plot{\label{Plot} Plot}
\includegraphics[height=2,width=2]{Rplot.png}
\end{figure}
\begin{figure}
\Plot{\label{Plot} Plot}
\includegraphics[]{Rplot.png}
\end{figure}
html output:
<p>Here is an image</p>
<div class="figure">
<img src="Rplot.png" alt="Plot" />
<p class="caption">Plot</p>
</div>
<div class="figure">
<img src="Rplot.png" alt="Plot" />
<p class="caption">Plot</p>
</div>

Answer 2

There are a few possibilities:
Write a custom filter, that modifies the document's AST, see pandoc scripting.
Use a preprocessor, see this post about using gpp in front of pandoc.
In limited use cases you can use \renewcommand in your LaTeX template to change the behavior of a LaTeX command.