'Capture GPU frame' first frame for iOS app

Question

My application performs several rendering operations on the first frame (I am using Metal, although I think the same applies to GLES). For example, it renders to targets that are used in subsequent frames, but not updated after that. I am trying to debug some of draw calls from these rendering operations, and I would like to use the 'GPU Capture Frame' functionality to do so. I have used it in the past for on-demand GPU frame debugging, and it is very useful.

Unfortunately, I can't seem to find a way to capture the first frame. For example, this option is unavailable when broken in the debugger (setting a breakpoint before the first frame). The Xcode behaviors also don't seem to allow for capturing the frame once debugging starts. There also doesn't appear to even be an API for performing GPU captures, in Metal APIs or the CAMetalLayer.

Has anybody done this successfully?

Answer 1

I've come across this again, and figured it out properly now. I'll add this as a separate answer, since it's a completely different approach from my other answer.

First, some background. There are three components to capturing a GPU frame:

1. Telling Xcode that you want to capture a GPU frame. In typical documented use, you do this manually by clicking the GPU Frame Capture "camera" button in Xcode.
2. Indicating the start of the next frame to capture. Normally, this occurs at the next occurrence of MTLCommandBuffer presentDrawable:, which is invoked to present the framebuffer to the underlying view.
3. Indicating the end of the frame being captured. Normally, this occurs at the next-but-one occurrence of MTLCommandBuffer presentDrawable:.

In capturing the first frame, or activity before the first frame, only the third of these is available, so we need an alternate way to perform the first two items:

1. To tell Xcode to begin capturing a frame, add a breakpoint in Xcode at a line in your code somewhere before the point at which you want to start capturing a frame. Right-click the breakpoint, select Edit Breakpoint... from the pop-up menu, and add a Capture GPU Frame action to the breakpoint:

   

2. To indicate the start of the frame to capture, before the first occurrence of MTLCommandBuffer presentDrawable:, you can use the MTLCommandQueue insertDebugCaptureBoundary method. For example, you could invoke this method as soon as you instantiate the MTLCommandQueue, to immediately begin capturing everything submitted to the queue. Make sure the breakpoint in item 1 will be triggered before the point this code is invoked.

3. To indicate the end of the captured frame, you can either rely on the first normal occurrence of MTLCommandBuffer presentDrawable:, or you can add a second invocation of MTLCommandQueue insertDebugCaptureBoundary.

Finally, the MTLCommandQueue insertDebugCaptureBoundary method does not actually cause the frame to be captured. It just marks a boundary point, so you can leave it in your code for future debugging use. Wrap it in a DEBUG compilation conditional if you want it gone from production code.