Volumic lights do not cast volumic shadows volume penetrates surface holes
Article needs peer review
Symptom
A scene contains Volumic Lights properties, as a result of applying Fast_Light_Effects and Fast_Volume_Effects. You notice that after rendering the scene, the volume penetrates the entire surface of the occluding object, instead of the predetermined holes, and the entire surface of the floor object receives the projected colour, and not the volumic shadow. How is this so, especially in light of the fact that the Fast_Light_Effects-Force Volumic Shadows parameter is turned on?
Figure I
Cause
Because Volumic Light effects require a shadow ray in order for volumic shadows to be rendered, the Light’s shadow parameter must be enabled. However, in the case of setting up a scene with Fast_Light_Effects, one must connect the Fast_Light_Effects node directly into the Light object’s Light shader input, and this will subsequently eliminate both the Soft_Light shader and therefore the Shadow parameter in the process.
Solution
How does one workaround this problem, especially if there is no Shadow parameter associated with the light? A Shadow parameter must be added to the Light object properties, by running this simple script :
SetValue "Spot.light.Fast_light_effects.si_default.factor", 0.0
SetValue "Spot.light.Fast_light_effects.si_default.shadow", True
You should now see this result :
Figure II
Notice that the volumic effect now only penetrates through the holes of the occluding object, and casts volume shadows in the appropriate areas on the floor object. If after executing the script, the result is still as in figure I, then verify the following :
- Is the Render or Region-Shadows-Raytraced type set to any option other than None. If it is set to None, then change the type to either Regular, Sort, or Segment, depending on the your scene’s specifications.
- If you only see the volumic shadows but not the volume itself, or small parts of the volumic effet: make sure that the - Fast_Volume_Effects-Maximum Ray Length value is set to a high enough value, thus reflecting the length of the light’s start/end falloff range; or the Fast_Light_Effects-Volumic Properties-Map size value is most likely too low, so increase this value (a good minimum value would be 100.0).
- In certain scene situations, if the Render or Region-Optimization-Raytraced Depth-Maximum depth value is too low, for example set to 2, then no volumic effect will be rendered; simply increase the Maximum depth value to a higher number, until the volumic effect renders in the scene. </UL></FONT></FONT>
References
Sources:
Dave Lajoie, Project Manager - Rendering, at Softimage.
Pierre Duranleau, 3D Graphics Support - Escalation, at Softimage.
About creating volumic effects :
Softimage XSI-Shaders, Lights, & Cameras</I> document : Chp 15, page 364.
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Applies To: XSI 3.0 on NT,Win2K,Irix,Linux
Posted: 11/29/2002
