LAmrUG

Los Angeles mental ray^® User Group

Agenda

7:00-7:45PM Reception & Refreshments
7:45-9:45PM Final Gathering
9:45-10:15PM Discussion

Notes

At the Culver Hotel, we had a nice "gathering" of about 20-25 to talk about Final Gathering. We covered a broad perspective from a slide presentation to demonstration of final gathering, as well as some finished animation for VFX. We filled the small room, and ran the presentation from a projector attached to a PC running RH9 using Open Office, and showed final gathering within XSI on an IBM laptop running XP. Then we showed some quicktime movies of film work from the laptop.

After collecting everyone into the room, Bart Gawboy reviewed some of the final gathering concepts and terminology, including the new finalgather options in version 3.2 of mental ray^®. This version is released in the latest versions of Maya and XSI. We also discussed some useful techniques for final gathering, including "Lighting Without Lights", and how to effect ambient occlusion. We also discussed HDRI, and the need for tools to make useful HDR images. The slides can be found in html form here.

In the second part of the meeting, Michael Isner from Softimage showed us how to use final gathering giving an artists viewpoint to working methodology. He showed us some nice uses of objects as light emitting sources for final gathering, as well as a useful displacement generating tool, which worked in conjunction with finalgathering, showing off the detail of the displacement. He also showed some of the new options such as falloff, and lighting from texture mapped objects. Of particular note was the nice effect of lighting particle clouds from several simple light emitting objects.

In the final part of the meeting, Jim Rothrock from BoyWonderFX showed us some high resolution movies of visual effects using primarily final gathering for lighting technique. The images were especially well suited because of the great detail in the geometry of CG elements. It also gave us a chance to exchange some valuable practical information for using final gathering in production.

Here's a list of some useful info we discussed:

• Final Gathering collects rays of incoming light energy over the point ray traced back from the eye/camera, which hits a surface with a diffuse component. When these final gather rays hit an object, the material shader is called.
• Final Gather Accuracy counts the number of rays sent out in the hemisphere above each finalgather point. It can also set the min and max radius between finalgather points on an object.
• FG Accuracy is a scene specified global. And FG points are view dependent.
• Most FG points are gathered in a preprocessing step between photon tracing and ray tracing. Use the diagnostic mode to see green dots indicating preprocessed points, and red dots indicating fg points generated while rendering.
• The difference between light energy (irradiance) coming in, and color (radiance) going out of a hemisphere over a diffuse surface using the typical lambertian model is π. Hence, most shaders collect irradiance, apply the diffuse color coefficients, and multiply by the factor of 1/π
• Native Maya shaders using mental ray rendering DO NOT. So for accurate finalgather lighting in Maya, make sure to set the irradiance factor to .318 (or 1/π). Eg, if you see blown out objects after turning on finalgather, check this first.
• Using physically accurate units of measurement ie, meters, watts, candelas, may add to your understanding of how to approach lighting issues in a given scene. This point could be argued from an artists perspective of just turning the knobs til it looks right. However, in some cases, this could save a lot of time tweaking parameters.