[vtkusers] light source in cammera

[Wolfram H Volpi]

I am trying to understand how the light source stays in the camera during
vtkCamera::Azimuth and vtkCamera::Elevation when a scene is rendered.

When I render a scene using vtkVolumeRayCastIsosurfaceFunction, the
specular isosurface looks nice and shiny.
As I rotate through Elevation and Azimuth the light source seems to stay
with the camera.

I am writing a class that raycasts a synthetic scene directly from a
mathematical model (no interpolation is used).  And would like the same
light-in-camera effect.

Is vtkEncodedGradientEstimator::Update() involved?  If so, where is
vtkEncodedGradientEstimator::Update() called from?

Thank you for your help,
Wolfv.