Deferred Clears

Take the following scenario:

  1. Application binds and clears FBO1
  2. Application binds FBO2 and renders to it
  3. Application binds FBO1 again and renders to it

Steps 2 and 3 each require a render pass for rendering. The clear in step 1 can potentially be done through loadOp of the render pass for step 3, assuming step 2 doesn't use the attachments of FBO1. This optimization is achieved in ANGLE by deferring clears.

When a clear is issued, one of the following happens:

  • If a render pass is already open, the framebuffer is cleared inline (using vkCmdClearAttachments)
  • If the clear is not to the whole attachment (i.e. is scissored, or masked), a draw call is used to perform the clear.
  • Otherwise the clear is deferred.

Deferring a clear is done by staging a Clear update in the vk::ImageHelper corresponding to the attachment being cleared.

There are two possibilities at this point:

  1. The vk::ImageHelper is used in any way other than as a framebuffer attachment (for example it's sampled from), or
  2. It's used as a framebuffer attachment and rendering is done.

In scenario 1, the staged updates in the vk::ImageHelper are flushed. That includes the Clear updates which will be done with an out-of-render-pass vkCmdClear*Image call.

In scenario 2, FramebufferVk::syncState is responsible for extracting the staged Clear updates, assuming there are no subsequent updates to that subresource of the image, and keep them as deferred clears. The FramebufferVk call that immediately follows must handle these clears one way or another. In most cases, this implies starting a new render pass and using loadOps to perform the clear before the actual operation in that function is performed. This also implies that the front-end must always follow a syncState call with a call to the backend (and for example cannot decide to no-op the call in between). That way, the backend has a chance to flush any deferred clears.

If the subsequent call itself is a clear operation, there are further optimizations possible. In particular, the previously deferred clears are overridden by and/or re-deferred along with the new clears.