Pausing Quantum Renderer

Johan Vos johan.vos at
Wed Nov 18 20:45:29 UTC 2015


On Android, a JavaFX Application might transfer control to another app (and
become invisible) and enter the foreground later. In that case, the
GLSurface we are rendering on becomes invalid. In order to avoid problems
and save battery, we want to pause the renderer thread, but this turns out
to be more difficult than I expected.

When our app transfers control, we get a callback from Android. We
intercept this in javafxports, and we set the Screen width/height to 0/0 as
we don't want to render on the (invalid) surface while we lost control.
When we regain control, we resize the Screen and the app renders again.

The reason we set the width/height to 0/0 is because the PresentingPainter
will call SceneState.isValid() and this returns false in case getWidth() or
getHeight() are 0.

However, SceneState extends PresentableState and it overrides the update
method. It will call PresentatbleState.update() which will set the
viewWidth to the width of the new Screen (hence, 0) , but after that it
overwrites the viewWidth with camera.getViewWidth(). The latter still
contains the old value. A quick inspection shows that camera.setViewWidth()
is called when validate(...) is called on NGDefaultCamera, which is called
by ES2Context.updateRenderTarget() which happens during rendering, hence
*after* the PresentingPainter checks if the width is 0.

So immediately after we set the width of the Screen to 0 (on the FX App
Thread), a Pulse happens, and this one still things the screen is the
original size. While the pulse is happening, the android system destroys
our context, and the rendering fails. Moreover, the EGL system is in a
unpredictable state (recreating the surface fails).

A very dirty workaround for this is to wait for 1 pulse (with the new
pulselistener API this should be possible) before we return from the
callback method called by Android when the surface is about to be
destroyed. That way, we will have 1 bogus rendering on an existing (but
about-to-be-destroyed) surface.

But it would be better if there is some way to tell the quantum renderer to
immediately stop rendering. Existing pulses are no problem, as the
renderLock guarantuees that we set the size to 0 only when no other thread
(quantum renderer) has a lock on the renderLock.

- Johan

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