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.
This change is similar to D7232 and moves the scene_qpainter into a
dedicated plugin. Compared to the XRender case it's more complicated as
the platform plugins need to implement a platform specific backend.
The base implementation for this part used to be in scene_qpainter. As
the idea is to completly move it away from KWin core it would be point
less to still have the backend definition in KWin core, but it cannot
be in the scene plugin as otherwise all platforms need to link the
plugin.
To solve this a new platformsupport subdirectory is added which contains
the scene platform backend as a static library. For the OpenGL scene such
a static library will also be required.
Test Plan: SceneQPainter test still passes, nested compositor still works
Reviewers: #kwin, #plasma
Subscribers: plasma-devel
Tags: #plasma
Differential Revision: https://phabricator.kde.org/D7259
