What order does LaTeX check for image file extentions?

Question

I'm reading the tutorial on LaTeX by overleaf, and in their section Adding Images they have some example code:

\documentclass{article}
\usepackage{graphicx}
\graphicspath{ {images/} }
\begin{document}
The universe is immense and it seems to be homogeneous, 
in a large scale, everywhere we look at.
\includegraphics{universe}
There's a picture of a galaxy above
\end{document}

When explaining the \includegraphics{universe} part of that they say:

The \includegraphics{universe} command is the one that actually included the image in the document. Here universe is the name of the file containing the image without the extension, then universe.PNG becomes universe. The file name of the image should not contain white spaces nor multiple dots.

My question really comes down to this: If I had in that folder both universe.png and universe.jpg what would be chosen? Why?

Answer 1

In case the file has no extension (no . in the last part of the name after all path separators /), or in case the extension is not a known graphics extension (for example, if the file is called big.universe, universe is not a valid graphics extension), then graphics does something like:

    \@for\Gin@temp:=\Gin@extensions\do{%
      \ifx\Gin@ext\relax
        \Gin@getbase\Gin@temp
      \fi}%

The loop above will iterate in the \Gin@extensions list, and try to lookup a file named <base>.\Gin@temp. If the file exists, the extension \Gin@ext is set equal to \Gin@temp and it doesn't try other extensions, so the earlier items in \Gin@extensions have higher priority.

Each backend (output format) allows different graphics file formats. The usual ones:

• dvips (latex):

  \def\Gin@extensions{.eps,.ps,.eps.gz,.ps.gz,.eps.Z,.mps}
  

• pdftex and luatex:

  \def\Gin@extensions{%
    .pdf,.png,.jpg,.mps,.jpeg,.jbig2,.jb2,%
    .PDF,.PNG,.JPG,.JPEG,.JBIG2,.JB2%
    % .eps is usually in this list, if conversion to pdf is enabled
  }
  

• xetex (dvipdfmx):

  \def\Gin@extensions{% order here is like pdftex.def, except for PS
    .pdf,.PDF,.ai,.AI,%
    .png,.PNG,.jpg,.JPG,.jpeg,.JPEG,.jp2,.JP2,.jpf,.JPF,.bmp,.BMP,%
    .ps,.PS,.eps,.EPS,.mps,.MPS,%
    .pz,.eps.Z,.ps.Z,.ps.gz,.eps.gz}