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Global Illumination for the Beginner

By Rick Timmons
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| 0
Software used:
3ds Max, mental ray

## Step 2: The trouble with Photons

Take a VERY close look at the patterns that produce this splotchiness, especially on the sphere. You may need to zoom in on this. Looks like circles, doesn't it? They are, and those circles are the photons, and the pattern that they strike the diffuse surfaces produce this splotchiness. Photon size is directly calculated from the size of the entire scene and not the object it's striking. If you have zoomed in on the sphere, also inspect the area of the floor around the base of the sphere. You should just be able to detect a hint of green that's been bled onto the floor from the sphere. This is called "Color Bleed". Both Color Bleed and the splotchy patterns are known as "Artifacts". Typically, some would want to go to Final Gather, but let's try another method of dealing with this splotchiness. Close the rendered image and return to the Render Scene panel. In the Global Illumination section, click the option box to enable Maximum Sampling Radius, and enter a value of 12". Click the Render button to see what this new value does for your render. See image below:

No change? Of course not, the value of 12" was near the value calculated by mental ray as the photon size. Now, close the rendered image, enter a value of 48" and render again:

Much better, the walls are nice and smooth enough for us to move on to the next area that needs to be improved upon, the light levels. Close the render image window. In Max with mental ray there are four primary ways to increase the level of light in a scene:

1. Increase the intensity of light on each individual light or instanced group of lights through the Modify panel.
2. Increase the Global Energy Multiplier in Light Properties on the Render Scene panel.
3. Decrease the decay level in the same area as above.
4. Increase the "Bounce Factor".

## Step 2: The trouble with Photons

Any of the above four would work to brighten our scene, but only two of these will we look at. The first of these, Global Energy Multiplier, is fairly self-explanatory. Let's try it out. Slide down just a touch on the Render Scene panel to where you'll find the Light Properties area. Increase the Global Energy Multiplier to 1.5, then click Render at the bottom of the panel. Your scene should like this:

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