Optimise OpenGL ES 2.0 2D drawing using dirty rectangles

Is it possible to optimise OpenGL ES 2.0 drawing by using dirty rectangles?

In my case, I have a 2D app that needs to draw a background texture (full screen on iPad), followed by the contents of several VBOs o开发者_开发问答n each frame. The problem is that these VBOs can potentially contain millions of vertices, taking anywhere up to a couple of seconds to draw everything to the display. However, only a small fraction of the display would actually be updated each frame.

Is this optimisation possible, and how (or perhaps more appropriately, where) would this be implemented? Would some kind of clipping plane need to be passed into the vertex shader?

If you set an area with glViewport, clipping is adjusted accordingly. This however happens after the vertex shader stage, just before rasterization. As the GL cannot know the result of your own vertex program, it cannot sort out any vertex before applying the vertex program. After that, it does. How efficent it does depents on the actual GPU.

Thus you have to sort and split your objects to smaller (eg. rectangulary bounded) tiles and test them against the field of view by yourself for full performance.