Edharris Softimage Xsi Rendering And Lighting Tutorials_update.pdf

Global Illumination

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:

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Global Illumination

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 light’s property page.

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Global Illumination

2. Click on the Photon tab to display the light’s 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)

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Global Illumination

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 .

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.

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Global Illumination

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.

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