If at all possible, it's best to avoid this type of thing, since it is incredibly slow. The GPU may have a long queue of rendering commands in-progress, and normally it is working in parallel to the CPU and things are going super fast. However when you ask for a pixel back, the CPU has to synchronise: it will have to sit and wait while the GPU finishes its current queue, then when it's finished read a single pixel then send it back to the CPU. Then the GPU is sitting idle waiting for the CPU to finish whatever it's doing with the logic and get back to issuing rendering commands at the end of the tick before it can start doing any work again. So normally the CPU and GPU are both busy and working at the same time, but as soon as you start asking for pixels back they both end up doing a lot of waiting and doing nothing. For this reason reading back just a single pixel per frame alone can drop you to 20 FPS or less. On mobile, reading back a single pixel once-off could noticably jank the game. If there's any conceivable way you can do it differently, you should.