Explain zoom levels

[iangilman]

Particularly the difference between viewport zoom levels and image zoom levels. This seems like a reasonable page to do it on:

https://openseadragon.github.io/examples/viewport-coordinates/

Here's something I wrote up recently that can get us started:

There are 3 different systems of zoom levels at work... there's the tile levels (if you're using something like DZI), the viewport zoom, and the image zoom. Viewport zoom is 1 when the width of the content and the width of the viewer are the same. As you zoom in, viewport zoom goes up, so when the width of the content is twice the width of the viewer, the viewport zoom is 2. When it's 4 times the width, viewport zoom is 4, etc. Image zoom is 1 when you're looking at the pixels of the image 1:1. If you're zoomed out so you're only seeing half of the pixels, then image zoom is 0.5, etc.

With DZI, the top level of tiles is whatever folder has the biggest number... that's 1:1 pixels with the original image. The next number down has half the pixels, the next number has half again. You shouldn't have to think about that when using the viewer, though.