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Indirect Illumination with Mental Ray

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Date Added: 9th December 2009
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The render time has gone up here (almost double), but the results are much cleaner. However, we still aren't even close yet. GI has done about all we can ask of it. Let's see what adding Final Gather to the mix will do.

Let's enable Final Gather in the Render Settings window (Fig.16).

Fig. 16

Final Gather

Final gather is method of simulating indirect illumination. When used in combination with global illumination, Final Gather lets you create the most realistic, physically accurate lighting conditions for a scene (using Global Illumination alone can sometimes give splotchy results, as we have seen).

When Final Gather is enabled, every object effectively becomes a source of ray-emitting light, mimicking the natural world in which objects influence the colour of their surroundings. When one light ray strikes an object, a series of secondary rays are diverted at random angles around it to calculate the light energy contribution from the surrounding objects. The light energy is then evaluated during the ray tracing process to add the effect of the bounced light. Unlike Global Illumination, Final Gather does not use photon maps for the calculation of light at a given point in scene. Instead, Mental Ray for Maya samples the surrounding area above every point in the scene. The illumination at those points is then computed as direct illumination. (If Global Illumination is also being used at the same time, Final Gather calculates the total incoming illumination in the scene (called irradiance).)

Final Gather rays are emitted in many directions from a sample point and stop according to the settings in the Final Gather section of the Render Globals Setting window. Because Final Gather rays do not bounce, secondary surfaces are not taken into consideration. (However, when rays hit geometry, material shaders may cast secondary reflection, refraction, or transparency rays, as long as those secondary rays are specular or glossy, not diffuse.)
Final gathering eliminates the low-frequency variation in the global illumination that often results if too few photons are used. (Performance is optimized because Mental Ray for Maya reuses and interpolates nearby final gathers.)

Final Gather and Global Illumination
You can combine Final Gather and Global Illumination techniques to:

a. Achieve realistic lighting and shadows more cost effectively
b. Reduce flicker in animations
c. Effectively illuminate interiors (global Illumination on its own can sometimes render splotchy results)
d. You can reduce the number of Global Illum. Photons, the Global Illum. Energy levels and the number of Final Gather Rays, resulting in less rendering time but more realistic lighting

Hemispherical Sampling
Final Gather works by collecting the photons emitted into a scene and samples them back together with the goal of smoothing the look of many "rubber balls", to extend the metaphor, into a more cohesive, smoothly lit scene. Final Gather samples hemispherical areas, which can be thought of as baskets for these photons to reside in at the end of their emission. Final Gather works best in diffusely lit scenes to collect brightness in dark corners, and while similar to GI (and often used in conjunction with it), FG uses its hemispherical sampling form of raytracing to collect and smooth the bounces which are already calculated by the GI.

Final Gather Settings: Final Gather settings should be tweaked in the order shown in Fig.17. These are listed in order of importance.

Fig. 17

1. Final Gather Accuracy. This value sets the number of rays fired from each primary ray. The default value is 100. 200 usually work for test renders. Go up to 500-1,000 at most for final renders.
2. Primary and Secondary Diffuse Scale. Can be used to multiply all computed FG values, and can be tinted any colour.
3. Max Radius (Fig.18). This value controls the size of the hemispherical sampling area. This number should be no larger than 1000% of the size (width) of the scene. This can be determined in the same way that was discussed a few paragraphs above. While I normally start at 1000% of the scene size; you can certainly experiment with other lower values such as 100%, or maybe even as low as 10% if you don't want as much colour bounce.

Fig. 18

4. Min Radius. This value controls the minimum size of the hemispherical sampling area. Usually 100% of the Max radius value is best, but you can go down to 10 or even 1% for less colour bounce. If both Max and Min are left at 0, which can be done by leaving off the "Use Radius Quality Control" checkbox, Mental Ray will try to determine the "best" values for the scene. This may take longer to render.With settings 1-4 in place, here's how my render now looks. It's pretty good, right? Splotchiness is gone; great colour bounces off that orange thingy; nice soft shadows where objects contact one-another. It's perhaps still a little too dull for me, though. Notice as well that our render time has now had a whole minute tacked on to it (Fig.19 - render time = 1m 11s). Yikes!

Fig. 19

5. Direct Illumination Shadow Effects (Fig.20). Does not change speed of render, should always be set to on. If off, will not be able to compute coloured or semi-transparent shadows well.

Fig. 20

6. Filter (Fig.21). Softens the render and reduces artefacts. Set to 0 by default. Values between 1 and 4 will soften the render.

Fig. 21

7. Falloff Start. This one is similar to the Min Radius (above). Here, values of around 10% of the scene work to show where you start ignoring elements around the current texel being illuminated, and start sampling the environment colour. Lower values often make for a brighter scene, since your environment (outside) is normally brighter than inside.

8. Falloff Stop. This one is similar to the Max Radius (above). Here, values of around 100% of the scene work to show where you ignoring all elements around the current texel being illuminated, and just sample the environment colour. Lower values often make for a brighter scene, since your environment (outside) is normally brighter than inside.If your Falloff Stop is a smaller number than your scene size, you start letting outside light pour in. Here I have set my Falloff Start and Stop to Values that are too low (Fig.22 - render time = 1m 11s).

Fig. 22

Here I have turned them back down to the settings shown above (the proper settings) (Fig.23 - render time = 1m 11s).

Fig. 23

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Readers Comments (Newest on Top)
Elise on Tue, 10 December 2013 10:26pm
The way you explain GI and FG is the clearest i've ever found. All of your definitions are humanly understandable - as opposed to Maya's aweful manuel. Thank you for creating this!
DoctorBeat on Sat, 22 September 2012 12:33pm
You saves my life man! Thank you
Jun on Thu, 26 July 2012 3:37am
Hi, This tips are very useful for me. It helps me understanding mental ray. Thanks very much!
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