Question

I need to limit the drawing of an object to a rectangle. I can’t just change the viewport to match the rectangle becouse the ModelView matrix (that should change the rectangle, but not the content) may not be identity. A solution that would work is to draw to a FBO that match the rectangle, then draw the FBO to the screen, but it seems to slow. Is there any better option to do that?

Answer 1

If I understood you correctly, glScissor should be the function you are looking for. It crops the rendering to a selected sub-rectangle of the viewport. This does not modify the viewport. So the objects cover the same size on the screen, it just prevents you from drawing any pixels outside of the scissor region. If this is not what you want and you want the sub-rectangle to contain the whole scene and thus your objects to shrink, then changing the viewport is the solution of choice.

EDIT: If you want the rectangle to be transformable and especially rotatable (and therefore not a rectangle anymore on the screen), then rendering into an FBO and using this as texture on a quad is probably the best solution. Otherwise you could probably also just modify the vertex coordinates after projection, thus multiplying the transformation matrix of the target rectangle with the projection matrix and using this as new projection matrix, but I’m not completely sure about that (but at least something similar should do it.