LaTeX - how to detect scale of \includegraphics

Question

I have a general question, so without any example code.

I'm implementing part of document containing gallery of images.

Generally, I have two types of images:

1) document, typically scanned A4, which shall be displayed to full page, required scaling is to keep maximum width and detect the height: \includegraphics[width=\textwidth]{image.jpg}

2) photo, which shall be scaled down to maximize amount of images at one page - height is fixed and width is detected: \includegraphics[height=4cm]{image.jpg}

For most images it works OK, but for very rare exceptions I have problem with non-standard dimensions:

1) very high document which doesn't fit to page height if I specify width=\textwidth

2) very wide photo which doesn't fit to text width if I specify height=XXcm

Question is: how to implement something like below description?

1) width = \textwidth if height fits page height, otherwise scale image (= keep aspect ratio) to image height = {page height - some spare buffer for caption}

2) height = {some height} if width fits \textwidth, otherwise scale image to image width = \textwidth

Note: dimensions of images can vary from image to image and all above cases could be randomly in one chapter.

Answer 1

If you specify both height and width keys, and specify keepaspectratio then the values are taken as upper bounds and the image is scaled so that both height and width fit within the specified areas, so you need something like

\includegraphics[width=\textwidth, height=.75\textheight,keepaspectratio]{..}

if the caption takes .25\textheight