Global Illumination, Quick and Dirty Reminders

Global Illumination, Quick and Dirty Reminders

This page is designed as a supplement to lectures. It is not intended to be a stand alone tutorial.

Render Settings>Common>Render Options. Turn OFF Enable Default Light.

To emit photons for Global Illumination, they have to be turned on in two places: on the light source's Mental Ray Tab and in the RenderSettings>IndirectLighting> Global Illumination.

Photons BOUNCE. They have to be contained within a space or they will fly all the way to the Atlantic, making render times long.

Emitted Photons do NOT illuminate on the first bounce. This makes sense if the light that is emitting photons is also illuminating the scene directly. If the light is only being used to emit photons than pay attention to how it is aimed and whether it is giving you the effect you want.

This is a test scene with one spot light aimed directly at the floor.

Render with just the direct effect of the lighting

Render with just the effect of Global Illumination (emitting photons from the light).

Note that the first bounce of the light is not rendering. Only the subsequent ones

Render with the spot light direct lighting and photon emission.

Just as a side note, this render is the same as the above, but Final Gather has been turned on. Note that Final Gather helps to smooth out and open up the entire image (though the render times increase dramatically).

With further tweaking of the solution, you can bring back the strength and drama of the Global Illumination effect, but still have it smoothed out by Final Gather.

You can view where the photons are hitting in the wire frame scene. This enables you to tumble around the view and see where they are concentrating or not.

RenderSettings>IndirectLighting>PhotonMap. Turn on "Enable Map Visualizer.

Window>RenderEditors>Mental Ray>Capitalize gives you the ability to show different aspects of how the photons are displayed in the wireframe view.

If you rerender the map will update. If you want to delete the visualization, you can select it in the Outliner and delete it. Then turn it off in the Render Settings.

Another way to visualize how many photons you have effecting the scene is to turn the Accuracy down to 1. This does not do any smoothing between photon hits.

There are 3 major ways to effect how a Global Illumination render will look.

1) The number of photons that the light is omitting. In the above solution, 100,000 photons are emitted. In this render only 25,000 are being emitted by the spotlight. The more photons emitted, the smoother the solution, but the render times increase.

2) The Intensity of each photon (or how bright they are). In the above image, the Photon Intensity is 1,000,000. In this image the Photon Intensity is 100,000

Increasing the intensity does not increase render times. It just makes them brighter. This means that whenever possible, you should try increasing the Photon Intensity to brighten a scene before increasing the number of photons emitted. This can keep your render times shorter.

3) Accuracy (in Render Globals) controls the smoothing effect between each individual photon hit. The higher the value, the more smoothing, but the longer render times.

In the above example, the Accuracy is set to 1 - which is only used to see individual photons. In this image the accuracy is set to 500. You can see the dramatic difference.

On the Light node, under Caustic and Global Illumination, you can change the Exponent. The default is "2.0" and this means that the photons will have a quadratic decay over distance, which is a realistic light falloff. Changing it to "1.0" will set a linear decay, which is less and your scene will be brighter. This setting would be good for long distances. If your scene needs more variation between light and dark areas, increase the Exponent beyond "2.0", this will cause dramatic falloff in the photons.

Other values, such as Radius, Merge Distance, and Photon Volume, are more advanced and will not be covered in this class. BUT NOTE, that they are set to ZERO! This actually does not mean that the Radius is ZERO, it means that Mental Ray is going to automatically calculate these based on your scene size. Tweaking these can help, but it requires understanding Global Illumination at a more advanced level.

Photon Tracing is similar to raytracing. You can set how many times the photon will bounce (reflect) or refract in the scene. Remember that photons react to the shader on the objects. If a surface has a diffuse setting of 0, it will be 100% absorbed - regardless of how you have the Photon Tracing set.

Tuning a Global Illumination solution is iterative. Start with low settings, save the image in your Render View, raise one setting higher, and see the effect. In general, this is the process I use:

Use the default settings and render

Increase Photon Intensity until you can see the image or have the brightness near what you want.

If the Photons hits are splotchy (and they will be) increase the number of Photons emitted.

Raise the Accuracy by 200 until you can't see much difference. Go back to the lower setting when you couldn't see a change.

Is there enough detail, or is it looking soft and blurry? If too soft, increase the number of photons.

Use the map visualizer to see how the photons are hitting in the scene. Adjust your light source? Add in additional lights?

You might now need to decrease your Photon Intensity (because you now have more photons illuminating the scene).

Again, raise accuracy by 200 until you can't see much difference.

Repeat until satisfied.

If you decide to add in Final Gather to help smooth out results:

Turn on Precompute Photon Lookup in RenderSettings>IndirectLighting>Final Gathering Map.

Try immediately decreasing by 1/2the number of Photons emitted . You probably can get by with less and this will save you render time.

You may want to increase the Photon Intensity so its effect is still evident.

All the normal Final Gather attributes come into play now and need to be tweaked.

With Global Illumination turned on, Final Gather will not use "Secondary Diffuse Bounces" Keep that at zero.

UMBC Department of Visual Arts, Advanced Maya Courses, Dan Bailey

Emitted Photons do NOT illuminate on the first bounce. This makes sense if the light that is emitting photons is also illuminating the scene directly. If the light is only being used to emit photons than pay attention to how it is aimed and whether it is giving you the effect you want. This is a test scene with one spot light aimed directly at the floor.
Render with just the direct effect of the lighting
Render with just the effect of Global Illumination (emitting photons from the light). Note that the first bounce of the light is not rendering. Only the subsequent ones
Render with the spot light direct lighting and photon emission.
Just as a side note, this render is the same as the above, but Final Gather has been turned on. Note that Final Gather helps to smooth out and open up the entire image (though the render times increase dramatically). With further tweaking of the solution, you can bring back the strength and drama of the Global Illumination effect, but still have it smoothed out by Final Gather.
You can view where the photons are hitting in the wire frame scene. This enables you to tumble around the view and see where they are concentrating or not. RenderSettings>IndirectLighting>PhotonMap. Turn on "Enable Map Visualizer. Window>RenderEditors>Mental Ray>Capitalize gives you the ability to show different aspects of how the photons are displayed in the wireframe view. If you rerender the map will update. If you want to delete the visualization, you can select it in the Outliner and delete it. Then turn it off in the Render Settings.
Another way to visualize how many photons you have effecting the scene is to turn the Accuracy down to 1. This does not do any smoothing between photon hits.
There are 3 major ways to effect how a Global Illumination render will look. 1) The number of photons that the light is omitting. In the above solution, 100,000 photons are emitted. In this render only 25,000 are being emitted by the spotlight. The more photons emitted, the smoother the solution, but the render times increase.
2) The Intensity of each photon (or how bright they are). In the above image, the Photon Intensity is 1,000,000. In this image the Photon Intensity is 100,000 Increasing the intensity does not increase render times. It just makes them brighter. This means that whenever possible, you should try increasing the Photon Intensity to brighten a scene before increasing the number of photons emitted. This can keep your render times shorter.
3) Accuracy (in Render Globals) controls the smoothing effect between each individual photon hit. The higher the value, the more smoothing, but the longer render times. In the above example, the Accuracy is set to 1 - which is only used to see individual photons. In this image the accuracy is set to 500. You can see the dramatic difference.