Rendering to CAMetalLayer from dedicated render thread / loop

Question

In Windows World, a dedicated render thread would loop something similar to this:

void RenderThread()
{
    while (!quit)
    {
        UpdateStates();
        RenderToDirect3D();
        // Can either present with no synchronisation,
        // or synchronise after 1-4 vertical blanks.
        // See docs for IDXGISwapChain::Present
        PresentToSwapChain();
    }
}

What is the equivalent in Cocoa with CAMetalLayer? All the examples deal with updates being done in the main thread, either using MTKView (with it's internal timer) or using CADisplayLink in the iOS examples.

I want to be in control of the whole render loop, rather than just receiving a callback at some non-specified interval (and ideally blocking for V-Sync if it's enabled).

Answer 1

At some level, you're going to be throttled by the availability of drawables. A CAMetalLayer has a fixed pool of drawables available, and calling nextDrawable will block the current thread until a drawable becomes available. This doesn't imply you have to call nextDrawable at the top of your render loop, though.

If you want to draw on your own schedule without getting blocked waiting on a drawable, render to an off-screen renderbuffer (i.e., a MTLTexture with dimensions matching your drawable size), and then blit from the most-recently-drawn texture to a drawable's texture and present on whatever cadence you prefer. This can be useful for getting frame timings, but every frame you draw and then don't display is wasted work. It also increases the risk of judder.

Your options are limited when it comes to getting callbacks that match the v-sync cadence. Your best is almost certainly a CVDisplayLink scheduled in the default and tracking run loop modes, though this has caveats.

You could use something like a counting semaphore in concert with a display link if you want to free-run without getting too far ahead.

If your application is able to maintain a real-time framerate, you'll normally be rendering a frame or two ahead of what's going on the glass, so you don't want to literally block on v-sync; you just want to inform the window server that you'd like presentation to match v-sync. On macOS, you do this by setting the layer's displaySyncEnabled to true (the default). Turning this off may cause tearing on certain displays.