pick either png or pdf in knitr when both are available

Question

I like to generate both pdf and png image files when I create a (latex) document using knitr. This can be done using dev = c("pdf", "png"). However, I don't seem to be able to choose (on a per-figure) basis which of the two is picked in my latex figure environment. Currently the only way in which I can get, say, a png input file for Fig. 1 and a pdf input file for Fig. 2 is to generate only the required format (by using dev = "png", fig.ext = "png").

Is there a way in which I can still generate both, but at the latex level can select which one is shown? It could be solved easily by allowing an extension in the \includegraphics command, I suppose.

Any input appreciated...

Ron

Minimal example:

\documentclass[a4paper,12pt]{article}
%\VignetteEngine{knitr::knitr}

\DeclareGraphicsExtensions{.pdf,.png}

\begin{document}

%\maketitle
<<knitrInitialization,echo=FALSE>>=
require("knitr", quietly=TRUE)
opts_chunk$set(comment=NA,background='transparent',size='small',fig.width=6,fig.height=6,out.width='\\textwidth',dev=c('pdf','png'))  
@ 

%% this one generates two figures, and the pdf version is shown
%% because of the order in DeclareGraphicsExtensions
\begin{figure}[tb]
\centering
<<testPDF,echo=FALSE>>=
plot(1:10)
@
\caption{PDF figure}
\label{fig:pdf}
\end{figure}

%% if I want to show a png (e.g., because the pdf is too large) I can
%% only do that by not generating a pdf in the first place
\begin{figure}[tb]
\centering
<<testPNG,echo=FALSE,dev='png',fig.ext='png'>>=
plot(1:10)
@
\caption{PNG figure}
\label{fig:png}
\end{figure}


\end{document}

Answer 1

You can simply put

\DeclareGraphicsExtensions{.png,.pdf}

in your document when you want PNG to have precedence, and then

\DeclareGraphicsExtensions{.pdf,.png}

later if you want to go back to the PDF preference. This works in the body of a LaTeX document, not just in the header.