[vtk-developers] swap interval and mouse input problems

[Clinton Stimpson]

Hi,

On certain platforms with certain Xlib or OpenGL implementations, I'm seeing 
interaction lags when using VTK in Qt applications.  I see this with both 
QVTKWidget and QVTKWidget2.

With a swap interval = 1 to prevent tearing, the SwapBuffers() can stall, which 
can result in mouse input events being queued by the X server.

Here is bug report describing the same issue
https://bugreports.qt.io/browse/QTBUG-39370

It appears the best fix is to request a render in response to high frequency 
mouse input, rather than doing a render immediately.  This fixes the queuing of 
mouse events which can come at a higher frequency than the monitor refresh 
rate.  This will allow consuming multiple mouse events between a render.  This 
requesting of renders makes application behavior consistent regardless of the 
swap interval setting.   However, VTK doesn't appear to have a mechanism for 
'requesting' a render.

As an experiment, I changed vtkRenderWindowInteractor::Render to request a 
render rather than do one, and then call vtkRenderWindow::Render when an 
actual render needs to happen.  This fixes the mouse lag problem.
This approach to fixing the problem changes the meaning of Render(), and also 
doesn't fit with vtkRenderView::Render().  It seems vtkRenderView could be 
changed to rely on vtkRenderWindow's StartEvent event instead of the 
interactor's RenderEvent.

Would it make more sense to propose that a new function be added to request a 
render?  RequestRender()?

Thanks,
Clint