I would like to make certain objects only render at certain parts of the screen. Basically exactly what the scissorRect can do. However, the exact same thing can also be achieved by setting the viewport to a smaller region and compensating the matrices for the offset. Does this give better performance? If I understand the documentation correctly the scissorTest is done per fragment while the viewport is done per vertex. This suggests that using the viewport instead of the scissorRect would potentially save significant amounts of fillrate. Is this true or am I misunderstanding this? ScissorRect is a lot easier to implement so I only want to use the viewport for this if I am actually gaining something that way.
(Note that this is for our 2D game, in which fillrate is currently the main performance bottleneck. The fragment shaders are extremely simple, so basically most performance goes into large amounts of overdraw of partially transparent objects.)