Let’s consider two scenarios. First, a front-lit setup, where the camera and the sun are positioned on the same side of the volume. As you can see, setting the Phase Function to a positive value of 0.7 (Forward Scattering) produces the most realistic result. The default value, 0.0 (Isotropic Scattering), also produces a reasonable result in this setup. However, a Phase Function of -0.7 (Backward Scattering) makes the volume appear unrealistically dense and overly thick.
Here is the second scenario: backlighting, where the light source is positioned behind the volume relative to the camera. Again, a positive value, which produces forward scattering, gives the most realistic result. In contrast, -0.7 produces stronger backscattering, making the clouds appear overly dense and unrealistic.
The diagram also suggests that, in a front-lit setup, positive values can render faster than negative ones. It also shows that the front-lit setup renders faster than the back-lit setup.
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