1# Deferred Clears 2 3Take the following scenario: 4 51. Application binds and clears FBO1 62. Application binds FBO2 and renders to it 73. Application binds FBO1 again and renders to it 8 9Steps 2 and 3 each require a render pass for rendering. The clear in step 1 can potentially be done 10through `loadOp` of the render pass for step 3, assuming step 2 doesn't use the attachments of FBO1. 11This optimization is achieved in ANGLE by deferring clears. 12 13When a clear is issued, one of the following happens: 14 15- If a render pass is already open, the framebuffer is cleared inline (using 16 `vkCmdClearAttachments`) 17- If the clear is not to the whole attachment (i.e. is scissored, or masked), a draw call is used to 18 perform the clear. 19- Otherwise the clear is deferred. 20 21Deferring a clear is done by staging a `Clear` update in the `vk::ImageHelper` corresponding to the 22attachment being cleared. 23 24There are two possibilities at this point: 25 261. The `vk::ImageHelper` is used in any way other than as a framebuffer attachment (for example it's 27 sampled from), or 282. It's used as a framebuffer attachment and rendering is done. 29 30In scenario 1, the staged updates in the `vk::ImageHelper` are flushed. That includes the `Clear` 31updates which will be done with an out-of-render-pass `vkCmdClear*Image` call. 32 33In scenario 2, `FramebufferVk::syncState` is responsible for extracting the staged `Clear` updates, 34assuming there are no subsequent updates to that subresource of the image, and keep them as 35_deferred clears_. The `FramebufferVk` call that immediately follows must handle these clears one 36way or another. In most cases, this implies starting a new render pass and using `loadOp`s to 37perform the clear before the actual operation in that function is performed. This also implies that 38the front-end must always follow a `syncState` call with a call to the backend (and for example 39cannot decide to no-op the call in between). That way, the backend has a chance to flush any 40deferred clears. 41 42If the subsequent call itself is a clear operation, there are further optimizations possible. In 43particular, the previously deferred clears are overridden by and/or re-deferred along with the new 44clears. 45