Question: how is a region extracted?

[PD Dr. M. Weihrauch]

I would like to understand the following:

If you extract a certain region from a zoom level via OpenSlide, how
does OpenSlide do that?

I am asking for this reason: we are using the "SZI" format (basically a
ZIP file with a deep zoom pyramid inside) and I would like to write a
function to extract a region. Obviously, I would have to stitch together
certain tiles from a zoom level and later crop the result, so that I can
get a specific region.

How is that with e. g. ndpi or svs formats? Don't they also have a tiled
pyramid, so does OpenSlide also stitch together tiles or are the images
on each zoom level there as a whole so that you read pixel-wise into a
new image?

Thanks a lot

Martin