Maya 5 Final Gathering Tips

Please note that this has been addressed in Maya 6.

When using final gathering with Maya 5, there is an important variable for controlling the amount of the final gathering effect on a material. Named Irradiance Color under a material's mental ray tab, it defaults to 1.0 1.0 1.0. Normally, this should be set to 0.318 0.318 0.318.

0.318 is the value of 1/π. When a shader gathers irradiance with mi_compute_irradiance, it should normally divide the result by π, since the irradiance is the energy gathered over the hemisphere above that point. Dividing by π makes it a color at the point. In Maya, this factor is brought out to the UI, so you can decrease or increase its effect. Unfortunately, the default value in Maya 5 of 1 1 1 creates extreme final gathering lighting effects. In Maya 6, this factor is taken into account, so that the default of 1 1 1 is properly divided by pi. When reading Maya 5 scenes, Maya 6 will multiply Irradiance Color by pi, so that you're renders will look the same.

Here we have a simple scene: a cube first without final gathering, and then with final gathering as defaulted

   

By the way, I set the fg rays (accuracy) in the above pics at 100, just to get a rough feel for the lighting in the scene.

When writing your own shaders, be aware of this factor. You can see a good example of its use in the base dgs shader supplied with Maya.

If you want more info on final gathering, check out our slide presentation of last June's meeting focusing on final gathering here.