[vtkusers] Dealing with QTBUG-40889

[clinton at elemtech.com]

----- On Jul 1, 2016, at 7:07 AM, Elvis Stansvik <elvis.stansvik at orexplore.com> wrote: 

> I'm in the unfortunate situation of having to support Qt 5.5.1, and I'm hit by

> https://bugreports.qt.io/browse/QTBUG-40889

> which was fixed in 5.6 with

> https://codereview.qt-project.org/#/c/126136/

> I've been trying to find a way to work around this bug using a combination of
> native and non-native event filters in Qt, but haven't really found a good
> solution.

> The problem is I want to use the built-in interactor styles in VTK, such as
> vtkInteractorStyleTrackballCamera, and these makes calls to Render() as fast as
> the mouse events arrive.

> I have to ask: Is this really a good idea, shoudn't the rendering be governed by
> a timer during the interaction (say dolly), to be more robust against a flood
> of mouse events? The reason Qt has even buffered events (and hence opened up
> for compression, barring that bug) is that the main thread is overworked. And
> this is due to VTK rendering at every mouse move event.

> If I make my own completely custom interactor style, I of course have full
> control and can let a timer govern the rendering, but I was hoping to leverage
> the built-in ones. And it's not possible to subclass them and disable just the
> Render calls unfortunately.

> At the moment I don't quite know what to do. Moving the camera around in a VTK
> window is like syrup, and our product is to run using the Qt 5.5.1 that is
> packaged in *buntu Xenial :(

> Thanks for any advice, especially if you've worked around this problem yourself
> somehow.

> Elvis

I think this problem is related to not following the Qt recommended way of doing rendering with mouse events. 
What Qt 5.5 would like to do is give you 2 or maybe 3 mouse events per render. 

To do that with VTK, you can implement a render callback for the interactor, which calls QVTKWidget::update(), then overload QVTKWidget::paintEvent() to call vtkRenderWindow::Render() instead of vtkRenderWindowInteractor::Render(). 

This is what I have done to gain back the responsiveness. 
I've had thoughts in the past about proposing a vtkRenderWindowInteractor::RequestRender() function which the interactor style classes can use, which might help solve this problem. But then with Qt 5.6, the problem goes away. 

Clint 
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