When should I clear an auxilliary render target?

Question

I'm using a few different render targets in my game in addition to the back buffer. These other render targets are only used in a few places, for specific tasks.

I'm wondering when I should be clearing them. Right now I clear all of my render targets at the beginning of the frame, and it seems like I'm waiting for all the textures to clear before the rest of the drawing gets underway.

Would it be more efficient to clear these textures later in the frame, when they aren't being used? Is there any hope of the GPU sort of clearing them "on the side" while unrelated rendering is happening? Or are these tasks always sequential and will I always need to wait for clearing?

Answer 1

Clear them when you need them cleared.

As discussed in the comments, the performance degradation you are seeing is likely related to the placement of the calls in the relative call stream for the frame, and not specifically the fact that clearing the buffer is "slow" (it should be one of the faster operations available, since implementers know how often it will be called).

But it will take non-zero time, and there's really no point to clearing the buffers until you need to. So, if you are rendering to them every frame, clear them every frame. But if you only render to them once every few frames, you should consider moving to clear to just before the render into that buffer. It's possible this will, itself, have performance implications based on how you are using all the other related graphics objects, including the buffer itself, so you'll need to experiment and verify; perhaps you should clear immediately before, perhaps immediately after you're done using the buffer, and so on.

If there's no measurable performance difference, favor the option that is most logical.