Setting up a scene for Global Illumination using mental ray in SOFTIMAGE|XSI

By: Ed Harriss

 

Requirements for this Tutorial: SOFTIMAGE|XSI 1.0 or Higher.


In this tutorial you'll learn how to take a "raw" scene that looks like this:




and set it up for Global Illumination and Final Gathering so that it in the end it looks like this:




In order to follow the tutorial. You'll need this database. GlobalScenezip (3.6meg)
This file has been compressed with Winzip. It's possible that you might need to add the extension ".zip" to the end of the file in order for it to work with your de-compression program.

In that database you'll find two scenes: Global and GlobalFinished. The one that you'll be using to start off with is "Global."
"GlobalFinished" scene is the scene after all the steps in this tutorial have been applied to it.

Open up the "Global" scene. By loading it through the File menu.
The scene consists of: One light, a cube(Room) with a hole(Window) in it and one sphere.

To save time and unnecessary clutter in the tutorial, I've already done a few things to this scene:

1. The light has been placed and setup to cast shadows.
2. The Ambient Light has been turned down to Zero.
3. The Environment has been textured.
4. The Sphere has already been textured.


After loading the scene, change Viewport B into a camera view by choosing Cameras > Camera from the viewport dropdown menu.



For Global Illumination to work you need 3 things.

1. A light to emit the photon rays
2. Objects to interact with the emitted photon
3. Global Illumination effects in the renderer.




Step One: Defining a photon ray emitter(light)

1. Start by, selecting the light (the global-illumination source) and choose Modify > Shader from the Render toolbar. This displays the selected lightís property page.
2. Click on the Photon tab to display the lightís photon properties.
3. Define the light as a Global Illumination source by checking the Global Illumination check box.
This causes the light to emit photons.

 

 

 

 

 

 

 

 

 

4. Set the Energy Intensity for the light to 140000. Intensity is the option that controls how intense your Global Illumination effect is. The higher the number the brighter the effect will be. The farther objects are from the light the more energy that will be needed. Energy values don't affect rendering times, they just amplify brightness of the Global Illumination effect.

Energy Intensity Tips: This information is not intended to be used in this tutorial, but to serve as helpful hints for you when you are setting up your own Global Illumination scenes.

Tip 1: Set the Energy Intensity very high (40,000 to 50,000) when doing test renders. This makes the Global Illumination effect easier to see, therefore easier to set up.


Tip 2: The light's fall off settings will affect the the energy level of the photons. So, turn it off during test renders. (this setting is located in the Light Attenuation section of the Light Property Page) This will also make the Global Illumination effect easier to see and therefore easier to set up. When it comes time to fine tune the scene turn it back on and set it to "Use Light Exponent" and set the Light Exponent to 1 (this setting is located in the General section of the Light Property Page). This setting keeps the Global Illumination effect fairly bright while you continue to refine the look of your scene. When you've gotten close to your final look, turn the Light Exponent up, almost to 2. This will make the Global Illumination effect less noticeable but more realistic. (which is a good thing.... since in CG lighting, subtlety is the key)

5.Set the Number of Emitted Photons to 20000 for the global-illumination effect.
This value specifies the number of photons that the light emits. Lower values (like 1000), cause fuzzy puddles of light but render faster, so it's good for tests. High values will create smooth Global Illumination effects but render much slower and are better for final renderings.






Step Two: Transmitters and Receivers, Setting up objects to interact with the emitted photons

1. Select the Sphere.
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2. Right-click the Property button(XSI 1.0) or the Selection button(XSI 1.5) on the Selection panel
3. Select Viewing Properties from the drop-down menu.
This will open the Visibility property editor.
4. Select the Visibility tab.
This will display the render-visibility options.
5. Select the Global Illumination transmitters and receivers check boxes.
This will cause the object to reflect light as well as receive reflected light.


The red sphere will now bounce red light back into the scene because you have set it as a transmitter.
It will also accept light that has been bounced from other objects because you've set it as a receiver.




Repeat the steps for the Walls.

1. Select the Room. (Cube)
----
2. Right-click the Property button(XSI 1.0) or the Selection button(XSI 1.5) on the Selection panel
3. Select Viewing Properties from the drop-down menu.
This will open the Visibility property editor.
4. Select the Visibility tab.
This will display the render-visibility options.
5. Select the Global Illumination transmitters and receivers check boxes.
This will cause the object to reflect light as well as receive reflected light.


The room will now bounce light back into the scene because you have set it as a transmitter.
It will also accept light that has been bounced from other objects (like the sphere) because you've set it as a receiver.



Step Three: Turning on Global Illumination and Final Gathering for the render region.


1. Draw a render region in the viewport that shows a camera view. (Viewport B)
Press the "Q" key on your keyboard and drag across that viewport from left to right.
This will allow us to see the results of the render as we tweak it.

Note: Global Illumination effects can take quite a long time to render.
If you've got an older computer you might want to turn the auto refresh for the render region off. This will prevent it from updating everytime you move a slider or click a button.

If needed press the render region button and toggle Auto refresh off



Edit the render options:





2. Choose Render > Region > Options from the Render toolbar to open the View Rendering Options property editor.


Or


Right-click the render regionís frame
to display its pop-up menu and choose Properties to open the View Rendering Options property editor.





3. Click the Optimization tab,
4. Select Grid as the Acceleration Method.
Changing the acceleration method to Grid will speed up renders that use global illumination.


5. Click the Photon tab,
6. Select the Global Illumination check box.
The selected effect is now visible in the render region.
(If you've disabled auto refresh click the refresh button to update the render region.)



Your Image should look like this:


Not so good eh?
The Global Illumination effect need to be fine tuned.
Final Gathering needs to be added.



Fine tuning your Scene.

The Global Illumination effects are set up in your scene, but they have not been tuned enough to yield good results.

Under the global illumination check box there are two "tuning" settings:
Accuracy and Radius.

 

Accuracy: Represents the number of photons that the renderer looks for within the area represented by the Radius(see below). It then uses what it finds to define the look of that area. Low accuracy numbers will yield grainy images, High accuracy numbers will give you smother more precise results but will take more time to render. If you set the accuracy value higher than the number of photons emitted by all the lights in the scene, your image will never render because the photon calculation won't finish.

Radius: This is the size(in Softimage units) of the effect that the photon produces when it hits an object. Larger the numbers produce bigger effects, but usually less accurate results. (unless you scene is scaled up very large and you've used really high accuracy settings) High radius values combined with low accuracy values can give you a good "quick and dirty" approximation of what your scene will look like. (Great for test renders) Low radius values combined with high accuracy values will provide the most accurate representation of the final image but will take much longer to render.

The default settings for radius are 0. Don't be deceived by this low setting. It's not really producing photons that are 0 in radius. In this case they are actually close to 1.3 in radius. Since it's a "default", the 0 setting is the only one that doesn't accurately represent the size of the radius. The setting of zero causes SOFTIMAGE|XSI to make a "best guess" and a lot of the time it's guess is pretty good.


Global Illumination alone can be used to produce good quality images. But if you combine the power of Global Illumination with Final Gathering, the results can be much more realistic than if you used Global Illumination alone.

7. Turn on the Final Gathering effect by checking the Final Gathering Check Box.

Final gathering calculates illumination using rays cast by the objects in the scene, rather than the light.

Like Global Illumination, it too has an accuracy setting. This setting defines how many rays each pixel in the image will shoot contribute to the overall look of the scene.

The Radius settings define an area around each pixel that is sampled for indirect and direct illumination. A good combination of low Min radius values and High Max Radius values can yield decent results. My personal preference is to set both numbers very low for my final render. Like .02 and .01. (Or lower, if you've got the time)




Your image should look like this:



Getting closer, but the image is filled with artifacts.
With a little more tuning it'll look much better.

8. Turn the Global Illumination Accuracy up to 50

9. Turn the Global Illumination Radius down to 0.5



10. Turn the Final Gathering Accuracy to 200
11. Turn the Final Gathering Min Radius to 0.05
12. Turn the Final Gathering Max Radius to 0.5

Depending on the speed of your computer it may take quite some time to render this image.
Now would be a good time to take a break. =)

Your image should look like this:



Looks pretty good. You may see some artifacts if you render the image large enough, adjusting the Final Gathering accuracy, turning the min/max radius down and turning up the number of photons that the light emits can clean up the artifacts and will usually fix that problem.

 

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