To use Global Illumination, you must use the mental ray renderer. In addition, you must employ virtual photons. In Maya, spot, point, area, and directional lights generate photons when the light's Emit Photons attribute is checked (see Figure 12.2). (You can find Emit Photons in the Caustic And Global Illumination subsection of the mental ray section in the light's Attribute Editor tab.)

Figure 12.2. The Caustic And Global Illumination subsection of a spot light's Attribute Editor tab

When a scene is rendered, photons are traced from each photon-emitting light to objects in the scene. If a photon "hits" a surface, it is absorbed, reflected, or transmitted (refracted) based on the qualities of the material assigned to the surface. If a photon is reflected or transmitted, it survives with only a portion of its original energy. The amount of energy is determined by the reflection coefficient, which is established by the Reflectivity attribute in Maya.

To simplify the photon-tracing process, many Global Illumination systems, including the one employed by mental ray, randomly cull photons in a process known as Russian Roulette. Photons that survive a surface hit through reflection or refraction are given an increased energy that is proportional to the potential energy of all the photons generated by the light source. The photons that are culled and thereby absorbed by the surface have their position, incident direction, and energy stored in a special photon map. The following scenario is a simplified example of the process:

The absorption, reflection, or transmission of a photon is affected by the shading components of the assigned material. Not only will changes to the Diffuse and Reflectivity attributes affect photon scattering, but Color, Eccentricity, and Specular Color have an equal impact. If a photon survives a hit and is reflected or transmitted, it continues through the scene until it hits another surface. Once again, the photon is absorbed, reflected, or transmitted. This process continues until the surviving photons are stopped by the Max Photon Depth attribute, which defines the number of hits permitted per photon. (Max Photon Depth is located in the Caustics And Global Illumination section of the mental ray tab in the Render Settings window.) Ultimately, the information stored in the photon map is combined with the direct illumination model, which in turn determines the color and intensity of each rendered pixel.

The number of photons generated by a given light is set by the light's Global Illum Photons attribute. You can lower or raise the default value of 10,000 to decrease or increase quality. In addition, you can change the qualities of photons generated by a light with the following attributes (all of which are found in the Caustic And Global Illumination subsection of the mental ray section in the light's Attribute Editor tab):

You can create a simple Global Illumination render with the following steps:

To save time, you can change the Quality Presets attribute to Preview: Global Illumination, which automatically activates Raytracing and Global Illumination in the Secondary Effects subsection and the Caustics And Global Illumination section. (For more information on the Rendering Features section, see Chapter 11.)

Global Illumination will often create a spotty render. Fortunately, you can fix this by adjusting such attributes as Radius and Accuracy, which are discussed in the next section of this chapter.

Keep these additional tips in mind when working with Global Illumination:

Global Illumination attributes are contained within the Caustics And Global Illumination section of the mental ray tab in the Render Settings window (see Figure 12.3). Of all the attributes, Radius and Accuracy are perhaps the most critical for creating a refined render.

Figure 12.3. The Caustics And Global Illumination section of the mental ray tab in the Render Settings window

Figure 12.4. Photon hits appear as colored circles. For the left render, each of the two lights generates 10,000 photons. For the right render, each of the two lights generates 200 photons.

In this situation, each circle corresponds to the location of one photon hit. The color of a given circle, however, is derived from the average energies of all the photon hits discovered within the circle. The areas in-between the circles receive no indirect illumination and are thus darker. The circles vary in color because the photon energy levels, stored as RGB, are influenced by the Color value of the materials they encounter while reflecting off and transmitting through surfaces. For example, if a material is green, the energy of the reflected or transmitted photon is biased toward green. Hence, the mingling of colors and color "bleeding" is possible.

Ideally, the individual photon hits should not be visible in the render but should blend seamlessly with each other and the direct illumination. If the default Radius value is producing spottiness, the following steps are recommended:

Balancing the number of photons in a scene with the Radius value is an important aspect of the Global Illumination process. An excess of photons will cause a redundant overlapping of photon hits and an unnecessarily slow render. On the other hand, a limited number of photons with a large Radius will lead to inaccurate indirect lighting calculations.

Although mental ray provides a Preview: Global Illumination option for the Quality Presets attribute, it does not provide a high-quality preset. Therefore, attributes should be raised individually to improve the render. For example, you can incrementally raise the Accuracy from 64 to 1024 to smooth out photon hits (see Figure 12.5).

Figure 12.5. (Top to Bottom) Accuracy set to 64 and 512. This scene is included on the CD as simple_global.ma.

Enable Map Visualizer and Photon Map File attributes are discussed in the next section. Accuracy and Radius, as found in the Photon Volume subsection, are detailed in "Preparing mental ray Shaders for Global Illumination" later in this chapter.

Unfortunately, photon maps cannot be viewed with FCheck. Photon maps are not image files, but are data files that contain a 3D spatial search structure called a kd-tree. The kd-tree stores the location, incident direction, and energy of each absorbed photon. Nevertheless, mental ray provides a special window to examine data with the maps. Follow these steps:

You can control how photon hits are displayed by changing various settings within the mental ray Map Visualizer window (see Figure 12.7). Sections and attributes of the Map Visualizer window include Photon Visibility, Point Size, Normal Scale, and Direction Scale.

The Search Radius Scale attribute is designed for Final Gather renders utilizing irradiance and is discussed in "Using Irradiance" later in this chapter.

The Dgs_material shader provides a physically-accurate shading model by offering Diffuse, Glossy, and Specular attributes. Diffuse represents the diffuse component and determines the base color of the surface. Glossy and Specular control the look of reflections and refractions when raytracing; you can use them individually or in conjunction.

If Glossy is set to black, Specular controls the intensity of the reflection or refraction. The higher the Specular value, the more intense the reflection or refraction. The Specular attribute will not create specular highlights.

If Specular is set to black, Glossy controls the intensity of the reflections, refractions, and specular highlights. The higher the Glossy value, the more intense the reflection and refraction and the larger the specular highlight. In addition, Glossy provides distance-based degradation of reflections and refractions. The degradation is controlled by the Shiny attribute. High Shiny values create a sharp reflections and refractions for objects close to the assigned surface. Low values create the opposite effect. For example, in Figure 12.12, a cylinder is intersected with a plane that is assigned to a Dgs_material. In left render, the reflection and refraction of the cylinder are controlled by the Specular attribute and are thus sharp. In the right render, the reflection and refraction are controlled by the Glossy and Shiny attributes; the reflection and refraction are sharp near the intersection point, but quickly degrade as they travel farther away.

Figure 12.12. (Left) Dgs_material with reflection and refraction controlled by Specular attribute. (Right) Same material with reflection and refraction controlled by Glossy attribute. This scene is included on the CD as dgs_material.ma.

If Shiny is set to 0, Shiny U and Shiny V are enabled. Shiny U and Shiny V control the U and V scale of an anisotropic glossy highlight. The Transp attribute determines the transparency. The Index Of Refraction attribute sets the material's index of refraction (this is called Ior in version 8.5).

If Specular and Glossy are used in conjunction, their values are added together and thus produce more intense reflections and refractions. However, the sharpness of the reflection or refraction is controlled by Glossy and Shiny. Glossy, as a 3D term, refers to a surface that creates imperfect reflections (one that is less smooth than a specular surface and less rough than a diffuse surface).

The Dielectric_material shader re-creates a material that is a poor conductor of electricity. Real-world dielectric materials include ceramics, plastics, glass, and some liquids, such as water. In addition, the Dielectric_material shader replicates the interaction between photons and dielectric material boundaries (known as interfaces). Based on angles of incidence and refractive indices of the involved interface materials, a photon will either transmit (refract) through the second material or reflect internally through the first material (see Figure 12.13). The shader refers to the first material as the "inside" material and the second material as the "outside" material. The shader employs Fresnel equations and Snell's Law to determine transmission and reflectance qualities (for more information, see Chapter 4).

Figure 12.13. The mental ray Dielectric_material shading model

For additional realism, the shader incorporates Beer's Law, which relates the absorption of light by a material to the properties of the material. Thus, the Dielectric_material shader is perhaps the most physically accurate material available in Maya. The attributes for Dielectric_material follow:

Due to its physical accuracy, the Dielectric_material shader has a natural advantage when rendering glass. For example, when the water glass created in Chapter 11 is assigned to a Dielectric_material, it gains more accurate caustic hot spots at the base (see Figure 12.14). Global Illumination is not required.

The main disadvantage of Dielectric_material, and many other mental ray shaders, is the limited number of available attributes. For instance, there's no interactive way to alter the shape or color of the specular highlight of a Dielectric_material shader.

Figure 12.14. (Left) The water glass assigned to a Dielectric_material shader. (Right) The same glass assigned to an Anisotropic material. The left scene is included on the CD as dielectric_glass.ma.

Shaders that carry the "Mib" prefix represent common shading models. "Mib" shaders are found in the Materials section of the Create mental ray Nodes menu.

Mib_illum_lambert, Mib_illum_blinn, and Mib_illum_phong are stripped-down variations of their namesakes. Each of the shaders carries a Diffuse attribute, which is similar to the attribute carried by Maya materials. Since "Mib" shaders do not possess a Color attribute, you must adjust the Diffuse attribute to change the color. In addition, Mib_illum_lambert, Mib_illum_blinn, and Mib_illum_phong offer Ambience and Ambient attributes. Ambience is a multiplier for Ambient, which is equivalent to the Ambient Color attribute used by Maya materials.

Mib_illum_blinn adds Roughness and Index Of Refraction attributes. Roughness controls the size of the specular highlight. Index Of Refraction does not require transparency or raytracing to function; instead, it emulates the index of refraction that all real-world materials possess, including solids such as metals. (For example, titanium has a 2.16 refractive index.) Index Of Refraction affects the intensity of the specular highlight. To create an intense specular highlight with Mib_illum_blinn, raise Index Of Refraction above the default maximum of 2 and lower the Roughness value.

Mib_illum_phong is similar to Mib_illum_blinn, but replaces Roughness and Index Of Refraction with the Exponent attribute, which controls the size of the specular highlight.

Mib_illum_cooktorr uses the Cook-Torrance shading model and is a physically-based shader. The color of the shader's specular highlight is dependent on the material quality, light color, and the light's incident angle to the assigned surface. Thus, if the surface is animated, the specular highlight is prone to shift in color as it appears at different points along the surface. Mib_illum_cooktorr offers additional control over the specular highlight quality by splitting the Index Of Refraction into Red, Green, and Blue channels. You can assign a different value to each channel and thus make the highlight sensitive to light color. For example, if the light color is pure red and the Index Of Refraction Red attribute is set to 20, the highlight is intense. However, if the light shifts to a green color and the Index Of Refraction Green attribute is set to 1.5, the highlight is dull.

"Mib" shaders share the Mode attribute, which serves as a per-material light-linking system (see Figure 12.15). You can map lights and thereby list them with the Lights[n] attribute of the "Mib" material. You can follow two methods to achieve this:

Figure 12.15. The Mode attribute and Lights section of a "Mib" shader. Four lights are listed with the shader.

If Mode is set to Inclusive Linking, only the lights listed with the shader are used to render the assigned surface. If Mode is set to Exclusive Linking, only the lights not listed with the shader are used to render the assigned surface. If Mode is set to the default Maya Linking, standard Maya linking, as determined by the Relationship Editor, is utilized.

With Maya 8.5, the Mode attribute is dependent on numeric values. If Mode is set to 0, all the lights in the scene are used to render the shader. If Mode is set to 1, only the lights listed with the shader are used. If Mode is set to 2, only lights not listed with the shader are used.

"Misss" shaders undertake subsurface scattering (SSS) by allowing light to penetrate the surface of a translucent object. Translucency is emulated by the Translucence, Translucence Depth, and Translucence Focus attributes of common Maya materials, but is not as physically accurate as "Misss" shaders. Seven "Misss" shaders are offered by mental ray; the most commonly used ones are detailed in this section.

The Misss_fast_simple_maya shader follows its name by providing efficient SSS renders. To apply this shader, follow these steps:

The network created for Misss_fast_simple_maya, illustrated by Figure 12.17, is similar to those created for Misss_fast_skin_maya and Misss_fast_shader. The network includes the following key components:

The Misss_fast_skin_maya shader provides additional attributes to emulate human skin. These include epidermal and subdermal controls. Misss_fast_shader is designed to work with more complex networks that contain the additional SSS shaders, such as Misss_set_normal and Misss_skin_specular.

The mental ray renderer provides several lens shaders that affect the way in which light travels through the virtual camera lens. For example, Mia_lens_bokeh creates mental ray depth-of-field (see Figure 12.18). Bokeh is a photographic term that refers to the out-of-focus quality of an image.

Figure 12.18. Depth-of-field is provided by the Mia_lens_bokeh shader. This scene is included on the CD as mental_dof.ma.

To use this shader with Maya 2008, follow these steps:

Physical_lens_dof also creates depth-of-field, but offers only two attributes—Plane and Radius. In order to make the final render suitably smooth with Physical_lens_dof, high Min Sample Level and Max Sample Level values may be required. Physical_lens_dof is compatible with Maya 8.5 and 2008.

Maya cameras carry Lens Shader, Volume Shader, and Environment Shader attributes in the mental ray section of their Attribute Editor tab. As seen in the previous section, the Lens Shader attribute supports mental ray lens shaders. The Volume Shader attribute is designed for volume effects, and is discussed in the next section. You can use the Environment Shader attribute to create effects that automatically involve all objects within the scene. The first approach to using Environment Shader involves the application of an environment lighting system through the use of Mia_physicalsky. The second method involves using a mental ray environment shader.

Mia_physicalsky creates physically accurate outdoor lighting. Although Mia_physicalsky is listed in the lens shader category, you must map it to the Environment Shader attribute. In addition, you must map Mia_physicalsun to the Light Shader attribute of a directional light, which represents the incoming sunlight. The Light Shader attribute is located in the Custom Shaders subsection of the mental ray section in the light's Attribute Editor tab. The Mia_physicalsun shader is located in MentalRay Lights section of the Create Render Node window. Most important, Mia_physicalsky requires Final Gather to render (Final Gather is discussed later in this chapter).

Both Mia_physicalsky and Mia_physicalsun carry attributes that re-create atmospheric conditions, such as Haze, Red-Blue Shift, and Horizon Height. When adjusting the attribute values, it's recommended that the same values be applied to any attribute that matches between the two shaders. As an example, the shaders are applied to a scene in Figure 12.19. The decaying shadows, specular caustics at the base of the house, and the green haze over the background are automatically generated.

Figure 12.19. Mia_physicalsky and Mia_physicalsun shaders provide physically accurate outdoor lighting. A simplified version of this scene is included on the CD as physical_sky.ma.

The mental ray renderer provides six environment shaders. You can map each shader to the Environment Shader attribute of the rendering camera. They function in a manner similar to Maya environment textures. For example, Mib_lookup_spherical creates an infinite sphere to which a texture can be mapped. The mapped texture appears in reflections and refractions (assuming the reflective/refractive surface is not surrounded by other surfaces). Although the texture appears in the background of the render, it is not present in the alpha channel. In this way, Mib_lookup_spherical functions like the Env Sphere texture. Mib_lookup_spherical, however, cannot be mapped with a Maya texture. Instead, you must map the shader's Texture attribute with a mentalrayTexture node. In fact, when you click the checkered Map button beside the Texture attribute, a mentalrayTexture node is connected automatically. You can then choose a bitmap file by clicking the mentalrayTexture node's Image Name attribute Browse button.

Mib_lookup_cube6 is equivalent to Env Cube. Mib_lookup_cube1 uses a cube projection, but wraps a single texture around the six walls. Mib_lookup_cylindrical is a cylindrical variation. Mib_lookup_background fits a texture to the camera view on a virtual plane. Mia_envblur, available with Maya 2008, is designed to blur other environment shaders. For example, Mia_envblur is mapped to the Environment Shader attribute of a camera. In turn, Mib_lookup_spherical is mapped to the Environment attribute of Mia_envblur. The result is a blurred environment texture that appears in reflections (see Figure 12.20).

Figure 12.20. Mia_envblur and Mib_lookup_spherical create a blurred environment texture that appears in reflections. This scene is included on the CD as mia_envblur.ma.

Mib_volume and Parti_volume shaders create a volume fog effect within a closed surface. You can find both shaders in the Volumetric Materials section of the Create mental ray Nodes menu. To apply either shader, you must map it to the Volume Shader attribute of the shading group node assigned to a surface. If the shading group node is also connected to a Maya material through the Surface Material attribute, the Transparency value of the Maya material must be increased for the volume material to be seen.

Mib_volume is an extremely simple fog that carries two attributes: Color and Max. Color represents the color of the fog and Max controls the density. A Max value of 0 will equal 100 percent density. A Max value greater than 8 will create close to 0 percent density.

Parti_volume is a more advanced material that supplies additional attributes to control light scattering and nonuniformity.

If a mental ray shader is used with Global Illumination or caustics, it will be ignored by the photon tracing process unless a connection is made to the Photon Shader attribute of the shading group to which the shader belongs. Maya 8.5 and Maya 2008 treat this necessity in slightly different ways.

With version 2008, some mental ray shaders, such as Dgs_material and Transmat, are automatically connected to both the Material Shader and Photon Shader attributes of a shading group node when they are created. Other shaders, such as those with the "Mib" prefix, are only connected to the Material Shader. With version 8.5, all shaders are connected to the Material Shader attribute, leaving Photon Shader open. In fact, mental ray provides four "sister" photonic shaders that may be used in this situation: Dgs_material_photon, Dielectric_material_photon, Transmat_photon, and Parti_volume_photon. Each corresponds directly to its material or volumetric material namesake. For example, if you want to photon trace with Dielectric_material, you can map Dielectric_material_photon to the Photon Shader attribute of the shading group node (see Figure 12.21). Dgs_material_photon, Dielectric_material_photon, and Transmat_photon are located in the Photonic Materials section of the Create mental ray Nodes menu. Parti_volume_photon is located in the Photon Volumetric Materials section.

Whether a sister photonic shader or a standard shader is mapped to the Photon Shader attribute of the shading group, it is important to match input attributes. That is, the attributes fed to Material Shader and Photon Shader should match. For example, if Dgs_material has a Shiny value of 50 and is mapped to Material Shader, then Dgs_material_photon should have a Shiny value of 50 as it is mapped to Photon Shader.

The mental ray renderer also provides a generic photon shader, Mib_photon_basic, which functions when paired with Dgs_material, Dielectric_material, and various "Mib" materials. Although this pairing cannot provide matched sets of attributes, Mib_photon_basic works well for simple Global Illumination renders.

Figure 12.21. Dielectric_material and Dielectric_material_photon materials connected to a shading group node

Two additional attributes are provided by mental ray for rendering volume materials: Accuracy and Radius. These attributes are found in the Photon Volume subsection of the Caustics And Global Illumination section of the mental ray tab in the Render Settings window. You can use these attributes to control photon tracing with Mib_volume and Parti_volume materials. Descriptions of each follow:

For the Final Gather system to work, the Raytracing and Final Gathering attributes must be checked in the Secondary Effects subsection of the Rendering Features section of the mental ray tab. In addition, Final Gather has a number of unique attributes in the Final Gathering section (see Figure 12.24).

Figure 12.24. The Final Gathering section of the mental ray tab in the Render Settings window

The following attributes are found in the Final Gathering Options subsection:

Final Gather does not require lights to render a scene. The system can use irradiance alone. Technically speaking, irradiance is a measure of the rate of flow of electromagnetic energy, such as light, from a per-unit area of a surface. The Ambient Color and Incandescence attributes of standard Maya materials represent irradiance.

For example, in Figure 12.25 a scene is rendered with Final Gather. The Enable Default Light attribute is unchecked in the Render Options section of the Common tab of the Render Settings window. A Fractal texture with an orange Color Gain attribute is mapped to a Blinn's Incandescence attribute, which provides the only light for the scene. Although the ground plane is assigned to a second Blinn material with Ambient Color and Incandescence values set to 0, it reflects the orange energy.

In addition, standard Maya materials carry Irradiance and Irradiance Color attributes in the mental ray section of their Attribute Editor tab. If the Irradiance attribute is mapped, the map becomes an irradiant light source. Irradiance Color serves as a multiplier for the resulting irradiant light.

Figure 12.25. A primitive object lights a scene with orange irradiance. This scene is included on the CD as irradiance.ma.

You can view irradiant Final Gather points, as well as Final Gather points in general, through the mental ray Map Visualizer window. If a valid Final Gather map is listed in the Map File Name field, the points are automatically displayed in the workspace view as colored dots. The Point Size attribute controls the size. Search Radius Scale controls the density of displayed points; in most cases, it is not necessary to adjust this attribute.

To demonstrate Global Illumination, Final Gather, and caustics, we'll use a variation of the famous Cornell Box (created at the Cornell University Program of Computer Graphics in 1984 to test physical-based lighting techniques). This particular box contains two point lights (see Figure 12.26). The Intensity attributes of the lights are left at 1. The floating C shape is assigned to a transparent Blinn with a Refractive Index set to 1.5. The camera's Background Color attribute is set to light red.

Figure 12.26. A Cornell Box. Yellow circles indicate the positions of two point lights. This scene is included on the CD as box_start.ma.

In the first step of the process, the Render Using attribute of the Render Settings window is switched to mental ray. The Quality Presets attribute is changed to Preview: Global Illumination, which checks on the Global Illumination and Ray Tracing attributes. Emit Photons is checked for each light. The lights produce the default 10,000 photons with a default Photon Intensity of 8000. The resulting render has visible photon hits. In addition, the white walls are a dingy gray (see Figure 12.27).

Figure 12.27. The Cornell Box is rendered with preview-quality Global Illumination settings. This scene is included on the CD as box_step1.ma.

Since the scene is 10 units high, we'll change the Radius attribute (found in the Global Illumination Options subsection) to 5. (To derive an appropriate value, we'll use the formula listed in the "Adjusting Global Illumination Attributes" section earlier in this chapter.) Since the scene is a bit dim, we'll raise each point light's Intensity to 1.25. The resulting render is significantly smoother (see Figure 12.28).

Figure 12.28. The Radius attribute in the Global Illumination Options section is changed to 5. This scene is included on the CD as box_step2.ma.

To increase the realism of the glass C-shape object, we'll adjust the Raytracing section of the mental ray tab. We'll change Reflections to 4, Refractions to 4, and Max Trace Depth to 6; this will allow light to bounce around the scene for a greater length of time. To create a caustic hot spot beside the C-shape, we'll check the Caustics attribute in the Caustics And Global Illumination section of the mental ray tab.

To create a more believable connection between the blue abstract shape and the floor, we'll check the Use Ray Trace Shadows attribute for each light. Although many Cornell Box simulations rely on indirect lighting to create dark areas, raytraced shadows adds an extra level of realism with minimal effort. To make the shadows acceptably soft, we'll set the lights' Light Radius to 2, Shadow Rays to 40, and the Ray Depth Limit to 10. In the resulting render, the blue shape gains a solid contact shadow (see Figure 12.29). A caustic hot spot also appears below the C-shape; unfortunately, individual caustic photons hits are visible.

Figure 12.29. The Cornell box receives caustics and raytraced shadows. This scene is included on the CD as box_step3.ma.

To improve the overall quality of the Global Illumination, we'll raise the Global Illum Photons of each light to 25,000. Since there are more photons in the scene, we'll reduce the Radius (in the Global Illumination Options subsection of the Render Settings window) by half (giving us a value of 2.5). We'll also raise the Caustic Photons of each light to 25,000. We'll change the Radius (in the Caustics Options subsection) to 2.5, thus matching the Global Illum Photons. As for other Render Settings window attributes, we'll switch Caustic Filter Type to Cone, Accuracy (directly below the Global Illumination check box) to 1000, and Accuracy (directly below the Caustics check box) to 500. The resulting render shows a significant improvement in the quality of the caustic. However, there are still a few errant caustic photon hits the near the C-shape (see Figure 12.30).

To smooth out the few remaining photon hits, we'll check Final Gathering in the Secondary Effects subsection of the Rendering Features section of the mental ray tab and leave the Final Gathering attributes at the default values. The resulting render is now clean enough to call final (see Figure 12.31).

The Final Gather process thoroughly blends the photon hits. In some situations, Final Gather can make the color bleed extremely subtle. For example, in Figure 12.31 the red and green bleed on the white wall is so faint that it can barely be detected. Nevertheless, the result, particularly around the blue shape, is convincing.

Figure 12.30. The overall accuracy is improved by increasing the number of photons. Nevertheless, a few caustic photon hits are faintly visible, as indicated by the yellow circles. This scene is included on the CD as box_step4.ma.

Figure 12.31. The final render with Final Gather. This scene is included on the CD as box_final.ma.

You can replicate indirect lighting and the mental ray Global Illumination system with Maya volume and ambient lights. Although the result is not perfect, the render is often close enough to meet the aesthetic demand of a project that is on a tight deadline. For example, in Figure 12.32 the Cornell Box is rendered with the Maya Software renderer with Raytracing checked. The overall lighting is similar to the one rendered with Global Illumination and Final Gather (see Figure 12.31).

To achieve this, five volume lights and one ambient light are placed in the scene (see Figure 12.33). Two large volume lights are placed next to the wall lamps. Their Intensity is set to 2. Two smaller volume lights are placed near the ceiling. Their Intensity is set to 0.2 and their Color attributes are set to red and green. These lights create a false color bleed. One last volume light is placed in the center of the box with an Intensity of 0.2. This volume light creates a soft fill. Volume lights, by their very nature, have a built-in falloff, which is easily adjusted by scaling the light shape up or down. To fill in the underside of the blue shape, an ambient light is placed near the floor with an Intensity of 0.175.

Figure 12.32. The Cornell Box rendered with the Maya Software renderer

The one area in which this technique most noticeably fails is the caustic of the C-shape. The shape's shadow against the left wall is particularly inaccurate. Nevertheless, if caustics are not a critical part of a scene, you can use a similar setup to achieve refined results.

Figure 12.33. Volume and ambient lights are placed in the Cornell Box. This scene is included on the CD as maya_box.ma.