Cannot set pixel density in standalone package for PNG output

Question

I want to output a PNG image with the standalone package. Here is a minimal working example:

\documentclass[preview, convert={density=300}]{standalone}

\begin{document}
    Hello world
\end{document}

When I inspect the file properties of the resulting PNG file, I still get 72 dpi. Even without using the preview option, it's still at 72 dpi. However, if I convert to JPG using convert={density=300, outext=.jpg}, I'd get 300 dpi.

How can this be fixed? My particular requirement would be to use the preview option and use the PNG image format.

Answer 1

The package manual states that “the following default settings are used: PNG format, a density of 300dpi, no explicit size […].”

So there's no surprise that density=300 doesn't result in a different output.

I wouldn't trust the “file properties” of the resulting file. How should a picture viewer (or your OS) know what an inch is? It's just pixels at the output side now.

Consider the following example where I typeset a black 1 inch by 1 inch square.

The output .png is 300 pixels by 300 pixels.

Code

\documentclass[
    preview,
    convert% uses default settings: density=300 and png
]{standalone}

\begin{document}
    \rule{1in}{1in}
\end{document}

Output

---

To complement egreg's comment:

Code

\documentclass[preview,convert]{standalone}
\usepackage{printlen}\uselengthunit{in}
\newlength{\myWidth} \newlength{\myHeight}
\sbox0{Hello world}
\setlength{\myWidth}{\wd0}\setlength{\myHeight}{\ht0}
\begin{document}
\printlength\myWidth${}\times{}$\printlength\myHeight\ (\the\wd0 ${}\times{}$\the\ht0)
\end{document}

Output