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  • Vlad Zahorodnii's avatar
    Introduce RenderLoop · b8a70e62
    Vlad Zahorodnii authored
    At the moment, our frame scheduling infrastructure is still heavily
    based on Xinerama-style rendering. Specifically, we assume that painting
    is driven by a single timer, etc.
    
    This change introduces a new type - RenderLoop. Its main purpose is to
    drive compositing on a specific output, or in case of X11, on the
    overlay window.
    
    With RenderLoop, compositing is synchronized to vblank events. It
    exposes the last and the next estimated presentation timestamp. The
    expected presentation timestamp can be used by effects to ensure that
    animations are synchronized with the upcoming vblank event.
    
    On Wayland, every outputs has its own render loop. On X11, per screen
    rendering is not possible, therefore the platform exposes the render
    loop for the overlay window. Ideally, the Scene has to expose the
    RenderLoop, but as the first step towards better compositing scheduling
    it's good as is for the time being.
    
    The RenderLoop tries to minimize the latency by delaying compositing as
    close as possible to the next vblank event. One tricky thing about it is
    that if compositing is too close to the next vblank event, animations
    may become a little bit choppy. However, increasing the latency reduces
    the choppiness.
    
    Given that, there is no any "silver bullet" solution for the choppiness
    issue, a new option has been added in the Compositing KCM to specify the
    amount of latency. By default, it's "Medium," but if a user is not
    satisfied with the upstream default, they can tweak it.
    b8a70e62