glReadPixels is not problematic if you do it after your swap (which on ES requires an FBO I think). Then the only requirement is that it completes within a few milliseconds to make way for the next frame. That's certainly less CPU intensive than rendering every frame twice, and more accurate too, since you will get the actual pixels that went to screen instead of an estimation of them.
glReadPixels is not problematic if you do it after your swap (which on ES requires an FBO I think). Then the only requirement is that it completes within a few milliseconds to make way for the next frame. That's certainly less CPU intensive than rendering every frame twice, and more accurate too, since you will get the actual pixels that went to screen instead of an estimation of them.