My iOS app draws 2D curves in an opengles view. The scene is very expensive to render (can take up to 1-2 seconds), which means that, AFAIK, I can't change the scale, redraw, and re-render for incremental changes in scale (due to pinch-and-zoom). I currently draw directly on a buffer that is rendered to the screen.
I think one way I can achieve zooming is by rendering to a texture at a given resolution and then render a quad with part of that texture (potentially at a different scale and translated). My guess is that it'll double the memory I'm currently using (if I of course keep the texture at the same resolution as the screen). Can someone confirm that? Is there another way to do zooming without redrawing while not doubling graphical memory usage?
Now, if I want to maintain a decent quality, I'll have to re-render at different resolutions. My initial thinking was to "manually" create a mipmap with e.g. 2 levels (1 texture for 100-150% zooming, and another one for 150-200% zooming). This time, I'll have 1 buffer + 2 textures. I can of course, re-render on panning but I don't think the user experience will be great. Any thoughts on how I can improve that from a user experience and/or memory perspective?
Since you already need that long to draw the scene I would suggest you to create tiling. You could draw the scene in different resolutions at load time and save the output to some images (save the image files into some temporary directory). With this approach you should have minimum memory consumption and the user experience should be great.
If you do this you should also consider if you even want to use openGL to present the scene since you have some very nice methods for presenting a large image on an image view, scroll view. Doing it this way you can actually skip all the GL - UIView bindings and presenting. You can move all the GL work to some separate thread which means if a scene should change you can do that in background allowing the user to work on the current scene uninterrupted. Also if you expect the user to be "swapping" between the scenes you can keep them saved and reuse them without any performance impact at all.