Better handle requesting no-vsync and make this usable with threaded

Just like we have e.g. QSG_NO_DEPTH_BUFFER, it is highly useful to
have a QSG_NO_VSYNC which is folded into swapInterval=0 which is then
mapped to QRhiSwapChain::NoVSync, i.e. both OpenGL and other APIs will
be happy. Thus one can, if so desired, try disabling vsync-based
blocking without modifying the application.

Now, since we know if there is at least one window that has
swapInterval == 0 set, the threaded render loop can recognize and
handle this: the same way it falls back to the system timer based
animation advancing when there are more than 1 visible windows, having
a no-vsync window can trigger the same. This way the animations
advancing too fast issue disappears (at the expense of potentially
more stuttering) whenever explicitly requesting the disabling of
vsync-based throttling.

In addition, expand the scenegraph docs around this topic.

Note however that this does nothing for animators (the render thread
animators). Those do not have a fallback option at all. If the render
thread spins too fast, they will advance too fast.

[ChangeLog][QtQuick] In addition to setting the swapInterval to 0 in
the QQuickWindow's QSurfaceFormat, one can now also set the
QSG_NO_VSYNC=1 environment variable to achieve the same (applies
globally to all Quick windows in the application). The threaded render
loop can now recognize that one or more windows have vsync-based
throttling disabled explicitly, and if so, it will fall back
automatically to using system timers to advance animations in order to
prevent animations from running too fast when the presentation rate is
not throttled by vsync.

Fixes: QTBUG-99950
Fixes: QTBUG-99951
Change-Id: Iddda81b467b48dbb8d87748868daa8d922ad1bd0
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/quick/scenegraph/qsgdefaultcontext.cpp b/src/quick/scenegraph/qsgdefaultcontext.cpp
index a25b34d22d..7ea7e55bdf 100644
--- a/src/quick/scenegraph/qsgdefaultcontext.cpp
+++ b/src/quick/scenegraph/qsgdefaultcontext.cpp
@@ -208,6 +208,7 @@ QSurfaceFormat QSGDefaultContext::defaultSurfaceFormat() const
     static bool useDepth = qEnvironmentVariableIsEmpty("QSG_NO_DEPTH_BUFFER");
     static bool useStencil = qEnvironmentVariableIsEmpty("QSG_NO_STENCIL_BUFFER");
     static bool enableDebug = qEnvironmentVariableIsSet("QSG_OPENGL_DEBUG");
+    static bool disableVSync = qEnvironmentVariableIsSet("QSG_NO_VSYNC");
     if (useDepth && format.depthBufferSize() == -1)
         format.setDepthBufferSize(24);
     else if (!useDepth)
@@ -221,6 +222,8 @@ QSurfaceFormat QSGDefaultContext::defaultSurfaceFormat() const
     if (QQuickWindow::hasDefaultAlphaBuffer())
         format.setAlphaBufferSize(8);
     format.setSwapBehavior(QSurfaceFormat::DoubleBuffer);
+    if (disableVSync) // swapInterval defaults to 1, it has no -1 special value
+        format.setSwapInterval(0);
     return format;
 }