[vtkusers] Latest on lights

[Michael Halle]

The lights in the LightKit are CameraLights: they're stuck to the
camera, and move (automatically) as the camera moves.  The
DirectionAngle stuff means that azimuth 0 is at the camera, negative
azimuth is (I think) left of the camera, and positive right.
Elevation is straightforward.  I guess there are other choices, but
once you understand that the lights move with the camera, having the
values so that (azimuth=0, elevation=0) is the camera's location
looking toward the lookat point seems the most obvious to me.

The brightness of the light kit's lights are all keyed to the key
light.  The headlight and the fill light's intensities are expressed
as ratios of the key light's intensity.  That means that once you've
got your ratios down, you can control the brightness of the whole
scene with just the key light.

As for the headlight influencing the brightness of the other lights,
the above intensity interaction is all that's there: check the code
yourself.  (There is a setting that adjusts the intensity of each
light to compensate for changes in color, but that doesn't have
anything to do with the other lights.)

							--Mike