How to or can I speed up animation using glut?


I am trying to program this stimulus for my experiment using glut, in which a complicated background and a superimposed simple patch will be presented. The luminance of the patch but not the background will udergo oscillation over time. The problem that I have now is to update the scene, after calling glutPostRedisplay, everything in the previous scene is cleared. And the time spent to draw the background (not the patch!!) is too long, of course partly also due to the relatively low performance of the machine . My question is how to, if I can, redraw only the patch part every time I update the scene?

Your help will be greatly appreciated!!

glScissor will restrict clears + drawing to a piece of the screen.
also with the wgl_arb_buffer_region (windows) extension u can store the screen (eg the background) somewhere for fast redraw later