[vtkusers] Update: White image with dri volume rendering

[Lisa S. Avila]

Hello Larry,



>I don't understand why the raycaster uses texture mapping.
>I thought ray casting begins at the back of the volume and works its way
>up to the front adding intensity depending on the material it 'sees'.

The ray caster creates an image traversing each ray front to back (this way 
early termination can occur). The final image must then be displayed in the 
scene along with all the geometry that has already been rendered (and 
anything else that has yet to be rendered). This is where texture mapping 
comes in - the image is placed on a polygon and rendered into the scene in 
the correct location.

Lisa