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Photon Illumination of DGS Materials in Mental Ray for Maya

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Date Added: 9th December 2009
Software used:

Final Gather

Final gathering is a less accurate means of calculating indirect illumination, but can be used to improve the quality of global illumination. Global illumination is more affective on shiny and transparent materials requiring accurate ray tracing of light. However, it is not as affective on matte materials. Final Gather can be used to improve lighting and shading effects on more diffuse surfaces by eliminating photon map artifacts.

Final Gather is turned on in the Render Global Settings, under the Final Gather section. The default number of Final Gather Rays is 1000 per sample point. This controls the number of rays shot and can be too high for test renders. The Final Gather Rays value is set 100 for the first test render. Also, with final gathering, fewer Global Illum Photons are need and lower Global Illum Accuracy will still produce sufficient results. For each of the lights, the Global Illum Photons was reduced to 50,000 and the Global Illum Accuracy reduced to 100. This speeds up render time, without affecting results.

Introducing final gather has in fact darkened the image. The wall is now a source of ray-emitting light and its brightness is determined by the tone of the material. The wall and reflector board where both initially set to a diffuse value of 0.8. Increasing their diffuse values for both dgs material and dgs photon material produces the following brighter image.

This image has the backdrop material set to 0.9 and the reflector board set to 1.0. The Final Gather Accuracy has also been increased to 200. Note how Final Gather now casts a clearer shadow for the ball.

The Min/Max Radius controls the size of the sampling region used by final gather rays. The default value 0 produces an approximation based on the largest object in the scene. The values will vary based on the sizes of geometry in the scene. A guideline is to set the Max Radius to 10% of the scene's overall dimension, and then set the Min Radius to 10% of the Max Radius. By setting Export Verbosity to Info Messages in the Translation section, it is possible to record the scene diameter in the Output Window. In this example it's about 90 units.

Here the Max Radius has been set to 9 and the Min Radius to 0.9. The results are a little smoother than the last render.
The Filter option controls speckle elimination and prevents samples with extreme brightness from incorrectly illuminating the scene.

Increasing this value to 2 has eliminated some of the brightness from the hotspot in the background.
And so that's using DGS Materials with Global Illumination and Final Gather.

Information for this article has come from the following sources:

- A DGS tutorial posted on Highend3d.

- The Diffuse, Glossy and Specular diagrams where recreated from Jeremy Birn's book 'Digital Lighting & Rendering'. A must have book for anyone serious about 3d graphics. It is available from Amazon.

- Many of the individual attribute explanations were aided by the Maya Help manual.

- Maya version 6 was used for all renders and the files are available for download.

- 01_set-up.mb
- 02_mat.mb
- 03_tweeking.mb
- 04_tweeking.mb
- 05_final.mb

The final render has been produced with a few creative touches. One of the negative fill boards was rotated to directly illuminate the sphere. The sphere's Saturation value of its Specular component was set to 1.0, with Hue set to primary red. The camera was moved in a little cloaser and the backdrop Value reduced to 0.85. Production quality settings were used and the area lights sampling increased to 5 x 5.

So, are DGS materials better than standard materials? Yes, once you get the hang of them. The main practical advantage is that DGS photon materials are a must when using Global Illumination as they render must faster. Personally, I find the idea of describing a material in terms of its relative roughness or smoothness to be much more intuitive.

Have fun experimenting with this great new technology.

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