[vtk-developers] VTK and Qt5 / QML

[c.sell at byterefinery.de]

Hello all,

I am looking into the possibility of replacing the 3D rendering engine  
in my Qt5 / QML based mobile tablet-oriented application with VTK.  
What I need is a first class QtQuick integration (not a hack or  
workaround) that is 100% stable in every context (not an unusual  
requirement, I admit). To my amazement, nothing of that kind seems to  
exist (correct me if I'm wrong).

I went on to investigate what had been done so far and to implement my  
first prototypes, using the QQuickFrameBufferObject approach. From the  
very start this felt like an uphill battle, because VTK seems to come  
from a Windowing background and is quite tightly integrated with  
concepts that are not valid in a QML context.

I'll describe my findings together with the 2 prototypical QML items I  
implemented:

1st was an adaptation of the DICOM example which now runs as a QML  
item. All user interaction is handled by QML and forwarded to VTK  
(which is one thing that doesn't come naturally with VTK), and after  
some non-elegant tweaking I was able to have the UI move from slice to  
slice and re-render upon mouse wheel events as expected. The problem  
here was, that QML insists on keeping mouse event handling and OpenGL  
rendering on separate threads, with one "rendering thread" dedicated  
to OpenGL. However, the pre-existing VTK Interactors directly call  
Render() after reconfiguring the UI from the mouse event data, which  
is an absolute QML No-Go. I had to introduce a RenderDelegate that  
works somewhat like this:

QML mouse event:
     tell RenderDelegate about the event
     call update() on the QML item, which triggers rer-endering on the  
dedicated thread
on the QML rendering thread:
     call Render() on the RenderWindow
     inside the RenderDelegate, look at whether a mouse event is  
pending, and call the corresponding VTK mouse handler
     call the renderer's default Render() function while setting the  
delegate's "Used" flag to false

2nd was a QML item that displays a model loaded from a 3ds file. My  
goal here was to move the model around using mouse drag events. I took  
the lessons learned from the first example, threw in a  
vtkInteractorStyleTrackballCamera and hoped for the best. First thing  
I found was that I needed a specialized RenderWindowInteractor, which  
I provided. When I realized that the most important requirement for  
that class was to provide access to timer functionality, I already got  
wary, as throwing around events and doing stuff offline while on the  
QML rendering thread is never a good idea. My fears came true when I  
finished wiring everything together: I was able to move the model  
using the mouse, but after a few moments things got whacky with error  
output written to the console and finally a segemtation fault from  
deep inside VTK.

I still need to investigate the second example, but would like to  
synchronize with the community at this point in order to avoid  
errors/misconceptions on my side, to seek help if possible and to  
offer my contribution to the VTK code base once this becomes  
functional. My first impression is that there are some issues that lie  
on an architectural level and cannot be (elegantly) dealt with on the  
QML side alone. Any comments?

thanks,
Christian Sell