question about FrameSkipper and time point to present on screen

[JackKaiXing]

I try to find out the accurate time point when content is presented to screen. but FrameSkipper is kind of confusing.

this is how we use filament:

1. after create engine and objects from gltfio, our project load a single GLB file.
2. before load success, we do not call beginFrame/render/endFrame; after load success, we are free to call them;
3. the first MAX_FRAME_LATENCY of call to FrameSkipper::Begin would return true; after that, fences is checked.(this is inside filament) And we wait for rendering present.

this is how i understand about FrameSkipper and time point of first present.

1. i found that before first call to Renderer::render(), filament has already start a loop to consume the command buffer, but this one does not have a command from swapchain->commit(...);
2. after the first commandbuffer is consumed, filament loop would continue to the next, which would has command from swapchain->commit();
3. the first MAX_FRAME_LATENCY of call to FrameSkipper::Begin/end has nothing to do with cpu/gpu sync; but if commandbuffer consumed before wait of fence from FrameSkipper at MAX_FRAME_LATENCY+1 call, it would present earlier than what FrameSkipper will ask to; and if the command buffer is not consumed, then MAX_FRAME_LATENCY+1 call would wait until present.

Does it right? thanks.

[pixelflinger]

FrameSkipper role is to prevent the CPU to push frames faster than the GPU can handle. The way filament should be used:

• render a frame at each vsync (use Choreographer on Android). This step is very important, you must be synchronized with h/w vsync
• then, when beginFrame() returns false, skip this frame. And try again at the next vsync

FrameSkipper uses a h/w fence at the end of each frame. Then it checks the N-2 fence, if it has not signaled, it returns false.

So basically FrameSkipper asks you to "skip" if the previous-previous frame has STILL not finished on the GPU.