Scale image down, but not up in latex

Question

I have a command which includes an includegraphics command - I can pass an image to my command, and it will do some standard formatting for me before actually including the image. Some of the images that I'm including via this command are smaller than \textwidth, while some are larger. I'd like to scale the larger images down to \textwidth, while not scaling the smaller images up - this means I can't just do

\includegraphics[width=\textwidth]{img}

Is there a way to specify a maxwidth? Or, can I get the width of the image somehow so I can do something like

\ifthenelse{\imagewidth > \textwidth}{%
    \includegraphics[width=\textwidth]{img}}{%
    \includegraphics{img}}

Answer 1

After a few minutes of searching through CTAN manuals and Google results, I think I can safely say that what you want to do is either impossible or very hard. My only recommendation is that you have two commands, one for small images and one for large, or one command with an option.

There may be a way, but I leave it to other S.O. LaTeX wizards to provide a better answer.

Edit: I am wrong, see above.

Answer 2

To get the width of the image you can use this code:

\newlength{\imgwidth}
\settowidth{\imgwidth}{\includegraphics{img}}

You could use this in the document preamble to create a new command to automatically set the width:

\usepackage{graphicx}
\usepackage{calc}

\newlength{\imgwidth}

\newcommand\scalegraphics[1]{%   
    \settowidth{\imgwidth}{\includegraphics{#1}}%
    \setlength{\imgwidth}{\minof{\imgwidth}{\textwidth}}%
    \includegraphics[width=\imgwidth]{#1}%
}

and then, in your document:

\scalegraphics{img}

I hope this helps!