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Indirect Illumination with Mental Ray

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Date Added: 9th December 2009
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Next, select your light and make sure that it emits photons. Again, I have left the default settings for now (Fig.08).

Fig. 08

When rendered the image looks a whole lot brighter, but four noticeable issues have arisen (Fig.09 - render time = 0m 07s; Fig.10 - render time = 0m 06s, only a photon intensity of 1000 in this image for comparison):

  1. It's still not bright enough for me
  2. The scene looks way too splotchy
  3. I don't have enough colour bounce for my taste yet
  4. The render time went up by 2 seconds

We are going to have to take several steps to make this render look prettier. Let's tackle these 4 questions in order:

  1. It's still not bright enough for me (photon emission)
  2. The scene looks splotchy as all get-out (accurate scale)
  3. I don't have enough colour bounce for my taste yet (final gather)
  4. The render time went up by 2 second (tweaks made at the end)

To make the scene brighter, we have a few tricks up our sleeves. Sure we could just add more lights, or up the direct light intensity, but those are time-consuming solutions that will not be physically accurate. Instead, let's adjust the settings for Photon Emission:

Fig. 09

Fig. 10

Photon Emission: Photons are just like colourful super-bouncing balls tumbling from a big bucket at the top of a hill. They carry colour and light as tiny packets riding on a wave of energy. Everywhere they hit, they "bleed off" some of their colour onto where they hit, and then "pick up" some of the colour from that surface, bouncing and ricocheting until they come to a stop. At every hit, the photon is either absorbed based on the diffuseness of the surface, reflected based on the reflectance of the surface (the two combine to describe Albedo), or transmitted as through a transparent object. The bouncing process stops when the number of bounces equals the max photon depth as set in the Render Settings window or based on the exponent settings in the light. This bounce is akin to the end of the road for our super-bouncing balls, where they might gather in a big pile at the bottom of the hill. However, every place those balls (photons) hit as they went down the hill, they illuminated and spread colour, brightening the scene.

The Three values shown (Fig.11), in addition to the Max Photon Depth, give you all the variables you need to start controlling the 'splotchiness'.

Fig. 11

Photon Intensity - This value is effectively the "elasticity" of the rubber balls (photons) in the analogy. A higher number makes for super-bouncy balls which have a lot of energy, thus will hit a lot of places and make the scene brighter. A low number will yield bowling balls, photons which die very quickly and leave the scene very dark.

Exponent - This is essentially a decay-rate value. Leave at 2.000 to simulate the natural effects of how light decays in a Nitrogen-Oxygen based atmosphere such as our own. In the analogy, changing values for the exponent to a higher number is like dumping the balls into water or maybe even jelly. The extra friction from the medium as opposed to air will slow their speed down.

GI Photons - This is the number of "balls" (photons) being dumped into the scene. The more you have, obviously, the brighter a scene is going to get. I do not suggest changing this value until you are done with the process of tweaking the scene and wish to make it brighter. This keeps the number of variables lower for now, and thus the scene easier to tweak.

Let's up my Photon Intensity from 8,000 to 40,000. And, let's also up the Max Photon Depth from 5 to 10 and render again. This will make the scene even brighter since we are allowing each photon double the amount of bounces, and more initial energy (Fig.12 - render time = 0m 07s).

Fig. 12

Notice that the scene is now much brighter. If my scene was too bright here, I would turn my Photon Intensity values back down. Notice as well that my render time is still 7 seconds. Also notice how the scene looks like it is missing contact shadows, such as underneath the chair and along the wall. This will be fixed with final gather.

Well, now that the scene is brighter, its looks even more awful/splotchy because of the intensity of these photons doing some serious "disco effects" on the walls. The first way to fight this is to take an inventory of the scene size.

Determining Accurate Global Illumination Scale (Fig.13)
While GI can often be determined using a "guess and check" method, better understanding, the Accuracy and Radius values will allow us to accurately determine the amount of sampling needed to correctly light a scene indirectly. The Accuracy Value sets the maximum number of photon hits allowed in any scene, and while higher values will significantly increase render time, they will create much more accurate renders.

Fig. 13

The Radius Value is very important. This controls the max. distanced from a photon hit that the energy will be calculated. Leaving this at 0 will allow Mental Ray to calculate what it thinks is the "best" size, but will take longer to compute, and in most cases isn't the best. To determine an accurate number for this radius, I suggest employing the Distance tool under Create > Measure Tools > Distance Tool (Fig.14).

Fig. 14

The scene size here in my simple scene is 6.77 units. In my demo scene (if you remember from earlier), my scene size is roughly 24 units. What we do is take the scene size value, and use it as our Radius Value.

The Merge Distance is essentially a bubble; any photon inside that bubble gets blended with other photons in that bubble, creating a smoother result. For best results, set this at 01% of the scene size (in other words 01% of your radius value).

So, for my scene, my Radius is 24, my Merge Distance is 0.24. And here's what I get (Fig.15 - render time = 0m 13s).

Fig. 15

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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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