OpenGL context issues when observing the VR camera

[cpinter]

If the VR camera is observed, and the callback makes changes in the MRML scene, then the OpenGL context produces strange symptoms, such as showing nothing, or even a previously shown scene in one VR view while the other is showing the proper scene.

To reproduce:

• Add observation to VR camera

    vrViewWidget = slicer.modules.virtualreality.viewWidget()
    rendererCollection = vrViewWidget.renderWindow().GetRenderers()
    vrRenderer = rendererCollection.GetItemAsObject(0)
    self.vrCamera = vrRenderer.GetActiveCamera()
    self.addObserver(self.vrCamera, vtk.vtkCommand.ModifiedEvent, self.onVrCameraModified)

• Set a matrix

  def onVrCameraModified(self,caller,event):
    avatarTransformNode = self.headAvatarModelNode.GetParentTransformNode()

    # Move and orient avatar model to VR camera
    import numpy as np
    position = self.vrCamera.GetPosition()
    positionVector = np.array(position)
    focalPointVector = np.array(self.vrCamera.GetFocalPoint())
    viewUpVector = np.array(self.vrCamera.GetViewUp())
    # Z
    zAxis = focalPointVector - positionVector
    zAxisLength = np.linalg.norm(zAxis)
    zAxis = zAxis / zAxisLength
    # X
    xAxis = np.array(viewUpVector)
    # Y
    yAxis = np.cross(zAxis, xAxis)
    # Matrix
    avatarTransformMatrix = vtk.vtkMatrix4x4()
    for z in xrange(3):
      avatarTransformMatrix.SetElement(z,2,zAxis[z])
    for y in xrange(3):
      avatarTransformMatrix.SetElement(y,1,yAxis[y])
    for x in xrange(3):
      avatarTransformMatrix.SetElement(x,0,xAxis[x])
    for p in xrange(3):
      avatarTransformMatrix.SetElement(p,3,position[p])
    avatarTransformNode.SetMatrixTransformToParent(avatarTransformMatrix)

If a QTimer.singleShot triggers a function that does this calculation and SetMatrixTransformToParent, then everything works well.

Maybe if the VR camera is modified with a different timing, then this issue wouldn't come up. For example it could be done as the first step and not while doing the rendering, or an event could be invoked that sets the camera.