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Global Illumination and Final Gather in Mental Ray for Maya

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Date Added: 9th December 2009
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The next stage is to fine tune the Global Illumination. To adjust the overall brightness of the scene, the light's Exponent value and Photon Intensity attributes are adjusted. Here is Maya Help manual definition for the Exponent value:


This is similar to decay -- the intensity increases as the value decreases. To increase the chances that indirect light will reach a greater distance, decrease the value.

Visible noise can occur with values less than 1. The default (2) simulates natural (quadratic) decay, but violates the conservation-of-energy law (that happens in the natural world), so bright spots from distant light sources could occur in unexpected locations.

Phrased another way, higher Exponent values will decrease brightness. Increments of 0.1 through to 3.5 will gradually decrease the brightness.

This is the same render with an Exponent value of 3. The image is much duller.

As the lights are set to Quadratic decay, and Final Gather will be used at the end, then only a little increase in Exponent value, up to 2.2 was used to darken the effect a little.

Then comes Photon Intensity adjustments. The Maya manual explains:

Energy (Photon Intensity)

The amount of light distributed by the light source. Each photon carries a fraction of the light source energy. 8000 is the default. 0 means no photons are emitted

If caustic effects are not bright enough, try increasing these values equally, in small increments (to 12000, to start) until you have better results.

If `hot spots' (blown out) areas occur, try decreasing these values equally to around 4000 or 50.

Basically it controls how bright the white spots are (the photons).

Here the Photon Intensity has been increased to 10,000 - just to slightly brighten things up.

One more light attribute to adjust - Global Illumination Photons. The Maya manual explains:

Global Illum Photons

The number of photons to be generated by the light source and therefore stored in the photon map. The default, 10000, is suitable for quick, low-quality renders. To improve the quality, increase this number, incrementally, to 20000 to start (render time will increase as a result). Generally, 100000 produces medium quality and 1000000 produces highly accurate effects.

This feature then controls quality. In other words, the more photons you can shoot, the better the quality. The aim is to increase this value to as high as possible, before render times are affected

Here, the number of Photons has been increased to 50,000. There are now more visible white spots. This will be fine tuned in the Render Globals Settings of the render.

To reduce the number of white spots (visible effect of photons), the Global Illum Accuracy is increased in the Render Globals Settings Mental Ray tab. Here is the Maya Help manual definition:

Global Illum Accuracy

Change the number of photons used to compute the local intensity of global illumination. The default number is 64; larger numbers make the global illumination smoother but increase render time.

A Global Illum Accuracy of 1 is shown here to show the individual photons.

A Global Illum Accuracy of 100 has been used here. Generally, Accuracy above 100 only increases render time.

To improve illumination quality and reduce the spotty effect, the Global Illum Radius is increased. The Maya manual explains:

Global Illum Radius

Controls the maximum distance at which mental ray for Maya considers photons for global illumination. When left at 0 (the default), mental ray for Maya calculates an appropriate amount of radius, based on the bounding box size of the scene. If the result is too noisy, increasing this value (to 1 to start, then by small increments up to 2) decreases noise but gives a more blurry result. To reduce the blur, you must increase the number of global illumination photons (Global illumination Accuracy) emitted by the light source.

Here, with Global Illum Accuracy set to 100, and Global Illum Radius is set to 0.1.

A Global Illum Radius value of 1 produced the best results.

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