How to get a bigger text area for a large image (or: reduced margins) on one page?

Question

In a document with the scrreprt class, I show an image with a caption. The image is set to fill the text height.

\documentclass{scrreprt}
\usepackage{graphicx}
\begin{document}
\begin{figure}
\centering
\includegraphics[height=1\textheight]{example-image-a4}
\caption{Image caption}
\end{figure}
\end{document}

I would like this image to be bigger than that, but still on the page. This is because the image shows an entire page of an old document which is originally in a larger format: I want to show its geometry and typography, so the whole page has to be visible, but at the same time, it is important that some detail remains discernible.

For this, I have two options:

1. Increase the size of the image via the height or width option.
2. Increase the size of the text area (or: reduce the margins) on this page only.

Trying option 1, I increased the height, but already at 1.1\textheight, the image shifts to the right and also covers the page number, while I expected it to be centered and located a bit further to the top of the page.

I don’t know how to achieve option 2: the question »How to set page geometry for a single page only?« doesn’t have an up-to-date answer, and this task seems to be non-trivial.

---

How can I place an image on a whole page in such a way that it

• is a bit bigger than the text area of the rest of the document,
• stays within the printable area and leaves a little room for margins,
• does not cover the page number?

Answer 1

You could trim the graphic. Then some parts of it will be "outside" the frame:

\listfiles
\documentclass{scrreprt}
\usepackage{graphicx,xcolor}
\usepackage[pass,showframe]{geometry}
\begin{document}
\begin{figure}
\centering
%red box for demo
\colorbox{red}{\includegraphics[width=\textwidth,trim=1cm 4cm 1cm 2cm]{example-image-a4}}
\caption{Image caption}
\end{figure}
\end{document}