I am trying to do GPU box selection (by using imageStore in the fragment shader as suggested to me in an earlier question). Basically I would like to call imageStore only on pixels that lie inside a user-defined screen rectangle. How do I setup my camera for the selection render pass such that everything outside the selection rectangle is clipped?
Do you want it to be an orthogonal perspective camera ?
Perspective I think. The user is creating the screen rectangle when looking through a perspective camera.
You've got a little design flaw there, be more specific about the feature, since for example user can pick a small rectangle in upper left corner, and there is no perspective observation to that corner without rotating the camera view and zooming, thus changing entire observation.
If you want to achieve that, find center-of-rectangle ray, rotate the view to align with it (just world space "at" position), and zoom. The amount of zoom is simply lessening the FOV. You can compute the FOV as rectangle world space size dividing near plane world space size.
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