Trilinear RC lighting is very dark

[aowen87]

Describe the bug

When performing trilinear ray casting/compositing, enabling the lighting option results in a very dark rendering (from any angle). The ambient, diffuse, specular, and shininess options are available, but they have no noticeable affect (which may be due to the fact that it's just so dark).

Helpful additional information

• [ ] Did VisIt crash?
• [ ] Did you get wrong results?

Impact

• [ ] High - User productivity significantly degraded
• [x] Medium - User productivity partially degraded
• [ ] Low - User productivity trivially degraded

Likelihood

• [ ] High - occurrence is not specific to platform, data, config, a combination of operators, queries, etc.
• [x] Medium - neither low nor high likelihood
• [ ] Low - occurrence is specific to combination of multiple factors such as configuration, data, platform, etc.

To Reproduce

Steps to reproduce the behavior. For example:

1. Open globe.silo (or any of our test datasets)
2. Add a volume plot, and set the rendering method to trilinear compositing.
3. Enable lighting, and render.

Expected behavior

The rendering shouldn't be so dark.

Attachments

This is globe.silo rendered with trilinear compositing and lighting enabled:

This is noise.silo rendered with trilinear compositing and lighting enabled:

This is noise.silo rendered with rasterization and lighting enabled. The camera position is the same as above:

[aowen87]

This appears to be related to calling AddLightingHeadlight instead of AddLighting. This is the only volume rendering method I can find that uses this method; all the others use AddLighting. If I switch the trilinear method to AddLighting, the shading/lighting looks correct.

[aowen87]

In the rendering options for ray casting, there is a reduction factor with a default value of "Lower", which generates the lighting power used in phong shading. It looks like this lighting power has a much greater affect on shading in AddLightingHeadlight than the standard AddLighting. As a result, the default setting makes it look black. If you change this setting to a higher value, the shading looks correct.

[aowen87]

Incorporating the lighting power into diffuse only (similar to how it's handled in AddLighting) gives a much smoother and more reasonable result.