Direct3D 11 missing GetRasterStatus, how do I detect the vertical blank period?

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I'm updating an application in which measurement of the time of presentation of a stimulus on a screen requires the greatest amount of accuracy. It is currently written with DirectDraw, which got put out to pasture a long while ago, and there's a need to update our graphics library.

The way which we measure the presentation time utilizes detecting the end of the Vertical Blank period. Specifically I need to know with, the greatest possible accuracy, when whatever was flipped onto the primary surface (or presented in the swap chain) is actually being drawn by the screen. Detecting the scan line can increase the certainty of that measurement, but I would be able to work with only detecting when the vertical blank period ended immediately after the Flip or Present was called.

Direct 3D 9 has the IDirect3DDevice9::GetRasterStatus Method that returns a D3DRASTER_STATUS struct which includes a InVBlank boolean, that describes if the device is in a vertical blank, as well as the current scan line. DirectDraw has similar functions (IDirectDraw::GetVerticalBlankStatus, also IDirectDraw::GetScanLine which returns DDERR_VERTICALBLANKINPROGRESS during Vertical Blank can be used to detect the VB).

However I have not been able to find any similar function in Direct3D11. Does anyone know if this functionality was moved or removed between Direct3D9 and Direct3D11, and if the latter, why?

vsync directx graphics c++
4 Answers

"Specifically I need to know with, the greatest possible accuracy, when whatever was flipped onto the primary surface (or presented in the swap chain) is actually being drawn by the screen."

Good luck.

There is actually no guarantee that anything you put into the present queue will ever be shown on screen (!!); you can manually drop frames w/ buffer sequencing present flags, or NVIDIA can do it for you (... thanks?)

Buffer Sequencing in DXGI

The DXGI Swapchain's flip queue is generally FIFO, but popular new driver overrides (i.e. FastSync) that users concerned with latency will most assuredly have enabled, favor CPU-side throughput over such trivial things as displaying any of the frames you draw :)

Normally you could count on IDXGISwapChain::Present (...) to begin blocking when the swapchain is full of undisplayed images and the driver is staging commands n-many frames ahead of the GPU, but with FastSync forced, Present never blocks and the render-ahead-queue flushes its work by overwriting any completed frames in the Swapchain that are waiting on VBLANK.

Back-to-back presents that complete quicker than screen refresh are under no obligation to (and will not) scan-out, thus their status in relation to VBLANK is meaningless.

Unless you implement rate limiting yourself to prevent the CPU from immediately staging the next frame after any call to Present, you need a different paradigm for measuring frame status altogether.

D3D9Ex / DXGI Supports Presentation Statistics in Flip / Fullscreen Exclusive:

Frames do not actually present to a user unless the following APIs say they do:

IDXGISwapChain::GetFrameStatistics (...) and IDXGISwapChain::GetLastPresentCount (...)

You can use frame stats to compute the length of the render queue / present latency in real-time, and your timing goals likely can be satisfied by tracking a present # against the accounting information for successfully sync'd frames.

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