Before I describe the raytracing process in more detail, the scanline process is worth a closer look. In general, the scanline process is as follows:

By comparison, the raytracing process fires off a virtual ray from the camera eye through each pixel of a view plane (see Figure 11.2). The number of pixels in the view plane corresponds to the number of pixels required for a particular render resolution.

The first surface the ray intersects determines the pixel's color. That is, the material qualities of the surface are used in the shading calculation of the pixel. If raytrace shadows are turned on, secondary shadow rays are fired from the point of intersection to each shadow-producing light. If a shadow ray intersects another object before reaching a particular light, then the original intersection point is shadowed by that object. If the first surface is reflective and/or refractive, additional rays are created at the original intersection point. One ray represents the reflection, and the other represents the refraction. If either ray intersects a secondary reflective and/or refractive surface, the ray-splitting process is repeated. This continues until the rays reach a predefined, maximum number of reflection and refraction intersections. When a reflection ray reaches a secondary surface, the shading model of the secondary surface is calculated and contributed to the original intersection point. Hence, the color of secondary surface appears on the original surface as a reflection. A similar process occurs with a refraction ray, whereby the secondary surface shading model is contributed to the original intersection point. However, the direction that the refraction ray travels in is influenced by the Refractive Index (which is discussed later in this chapter).

Figure 11.2. Simplified representations of the raytrace render method

Because a single camera eye ray can easily produce numerous shadow, reflection, and refraction rays, raytrace rendering is significantly more complex than the equivalent render with the scanline process. Consequently, the Maya Software renderer combines the scanline and raytrace techniques. If Raytracing is checked on but a surface possesses no reflective or refractive qualities, Maya applies the scanline process and avoids tracing rays.

Another method by which Maya reduces raytrace calculations is through the creation of voxels. Voxels are virtual cubes created from the subdivision of a scene's bounding box (which includes all objects within the scene). Maya tests for ray intersections with voxels before calculating more exact surface intersections. This effectively reduces the number of surfaces that are involved with the intersection calculations. Without voxels, Maya would have to test every surface in a scene.

The Ray Tracing subsection of the Memory And Performance Options section in the Maya Software tab controls voxel creation through Recursion Depth, Leaf Primitives, and Subdivision Power attributes.

The Raytracing Quality section of the Maya Software tab provides Reflections, Refractions, Shadows, and Bias attributes. The Reflections attribute sets the maximum number of times a camera eye ray will generate reflection rays before it is killed off. The Refractions attribute sets the maximum number of times a camera eye ray will generate refraction rays before it is killed off (see Figure 11.3). The limit for both attributes is 10, which is satisfactory for a water glass, bottle, or vase.

Figure 11.3. Rays generated by a single camera eye ray while the Reflections and Refractions attributes are set to 2

The Shadows attribute, on the other hand, sets the maximum number of times a camera eye ray can reflect and/or refract and continue to generate shadow rays. The higher the value, the more recursive the shadows; that is, shadows will appear within reflections of reflections and refractions of refractions. This attribute only has an effect if raytraced shadows are used. Depth map shadows, whether they are generated by Maya Software or mental ray, will automatically show up in all recursive reflections. If Shadows is set to 0, all raytrace shadows are turned off. A value of 10 will render shadows within nine recursive levels of reflection or refraction (see Figure 11.4). If Shadows is set to 10 and no raytrace shadows appear in reflections or refractions, increase the Ray Depth Limit attribute in the Raytrace Shadow Attributes subsection of the light's Attribute Editor tab. (See Chapter 3 for more detailed information.)

The Bias attribute serves as an adjustment for 3D motion blur in scenes with raytrace shadows. Often, raytrace shadows create dark bands around the center of rapidly moving objects. You can increase the Bias value to remove this artifact (see Figure 11.5).

Figure 11.4. (Top to bottom) Spheres rendered with the Shadows attribute set to 1, 2, and 5, respectively. This scene is included on the CD as shadows.ma.

Figure 11.5. (Left) Sphere rendered with raytrace shadows and 3D motion blur. (Right) The same render with the Bias attribute set to 0.5. This scene is included on the CD as bias.ma.

As soon as the Raytracing attribute is checked, the Maya Software renderer creates reflections for all objects. The amount of reflectivity is controlled on a per-material basis by the material's Reflectivity attribute. This attribute works in conjunction with the Specular Roll Off and Specular Color attributes. If Specular Roll Off and Specular Color are set to 0, there is no reflection. A material's Eccentricity attribute, on the other hand, has no effect on the strength of a reflection and can be set to 0.

In contrast, the Reflected Color attribute found on Blinn, Phong, Phong E, and Anisotropic materials does not require raytracing. If the Reflected Color value is set to a color other than black or is mapped, a simulated reflection is applied directly to the assigned surface. If the mapped texture is an environment texture, the results are quite convincing (see Chapter 5 for a demonstration). Although a raytraced reflection is more accurate than a texture mapped to the Reflected Color attribute, raytracing is considerably less efficient. In any case, you can map the Reflected Color attribute and raytrace at the same time (see Figure 11.6). The color of light reflected in the raytrace process is multiplied by the Reflected Color attribute.

Figure 11.6. (Left) Raytraced chrome rim with the camera's Background Color set to blue. (Middle) Same rim with an environment texture mapped to the Reflected Color attribute, but without raytracing. (Right) Same rim with an environment texture and raytracing.

The Reflection Limit attribute (found in a material's Raytrace Options section) is a per-material attribute that sets the number of times a camera eye ray is allowed to reflect off the assigned surface before it is killed. Maya will compare the Reflection Limit to the Reflections attribute in the Raytracing Quality section of the Maya Software tab and use the lower value of the two.

The Reflection Specularity attribute (also found in a material's Raytrace Options section) controls the contribution of specular highlights to reflections. For example, in Figure 11.7 a car rim is assigned to two materials. The spokes are assigned to a red Blinn with its Reflection Specularity set to 0. The outer rim is assigned to a gray Blinn with its Reflection Specularity set to the default 1. Thus, the reflection of the spokes in the rim does not include the specular component. However, if the spoke's Reflection Specularity is returned to 1, the specular highlights of the spokes become visible in the rim reflection. When set to a value below 1, the Reflection Specularity attribute can help reduce anti-aliasing artifacts resulting from recursive reflections that contain a high degree of detail.

Figure 11.7. The Reflection Specularity attribute of a Blinn material determines whether specular highlights appear in reflections.

Refraction is the change in direction of a light wave due to a change of speed. When a light wave crosses the boundary between two materials with different refractive indices, its speed and direction are shifted. The human brain, unaware of this shift, assumes that all perceived light travels in a straight line. Thus, refracted light is perceived to originate from an incorrect location and objects appear bent or distorted. (For more detailed information on refractive indices, see Chapter 7.)

In Maya, refractions are defined on a per-material basis in the Raytrace Options section of the material's Attribute Editor tab. Refraction attributes include Refractive Index, Refraction Limit, Light Absorbance, Surface Thickness, Shadow Attenuation, and Chromatic Aberration (See Figure 11.8).

The mental ray Quality Presets attribute, found in the mental ray tab of the Render Settings window, supplies 15 different presets. The presets offer a quick way to set all the attributes within the Rendering Features and Anti-Aliasing Quality sections (see Figure 11.14). For instance, if you set Quality Presets to Draft, the Primary Renderer is set to Scanline and anti-aliasing is kept at a bare minimum to speed the render. If you set Quality Presets to Preview: Global Illumination, Raytracing and Global Illumination is checked for the Secondary Effects attribute; in addition, matching Global Illumination attributes found in the Caustics And Global Illumination section are set to a quality appropriate for a preview. For maximum control, you can set all the Rendering Features and Anti-Aliasing Quality attributes by hand. Descriptions of each attribute follow:

Figure 11.14. The Rendering Features section (Left) and Anti-Aliasing Quality section (Right) of the mental ray tab in the Render Settings window

You can access additional sampling attributes on a per-surface basis in the Render Stats section of a surface's Attribute Editor tab. If you check Shading Samples Override, Shading Samples and Max Shading Samples become available. These two attributes override Min Sample Level and Max Sample Level for the selected surface, but function in the same manner.

Two forms of motion blur are employed by the mental ray renderer: No Deformation and Full (respectively named Linear and Exact in previous versions). No Deformation motion blur is equivalent to Maya Software's 2D motion blur, in that it is unable to accurately portray rapid changes in direction or surface deformation. Full motion blur, on the other hand, plots the motion vectors of each surface vertex and is thus sensitive to deformation. (Unfortunately, Full motion blur handles rapid changes in direction in a fashion similar to No Deformation motion blur.) Attributes for mental ray motion blur are located in the Motion Blur section of the mental ray tab and include Shutter Open, Shutter Close, Motion Back Offset, and Static Object Offset, Motion Blur By, and Motion Steps (see Figure 11.16).

Figure 11.16. The Motion Blur section of the mental ray tab in the Render Settings window

By default, mental ray motion blur appears extremely grainy. You can remedy this, however, by adjusting the Time Samples attribute (see Figure 11.17).

Figure 11.17. Motion blurred stick with two different Time Samples settings. This scene is included on the CD as mental_motion.ma.

In actuality, Time Samples is a "dummy" attribute that remotely controls the Time Contrast R, Time Contrast B, and Time Contrast G attributes found in the Time Contrast subsection (these are simply labeled Red, Green, and Blue in Maya 2008). Color Contrast, found in the same subsection, also serves as a dummy remote for Time Contrast R, Time Contrast B, and Time Contrast G. Ultimately, the Time Contrast attributes determine the number of temporal samples taken per spatial sample. For example, if the Time Contrast attributes are each set to 0.5, then 2 temporal samples are taken per spatial sample per frame. This equates to the formula 1 / Time Contrast.

Shadows created by mental ray are defined in two areas—the Shadows section of the mental ray tab in the Render Settings window and the shadow-specific sections of each light's Attribute Editor tab.

A light's Use Ray Trace Shadows attribute functions in the same manner for the mental ray renderer as it does for the Maya Software renderer. (Maya Software shadows are discussed in Chapter 3.) However, mental ray supplies separate attributes in the Shadows section of the mental ray tab in the Render Settings window (see Figure 11.18). The Shadow Method attribute controls the method of shadow calculation and has four options: Disabled, Simple, Sorted, Segments. The Simple method creates fast, efficient shadows and is appropriate for most animation. The Sorted method determines shadow order if multiple objects obscure the rendered point from the point-of-view of the shadowing light. With the Simple and Sorted method, shadow rays are generated by the light from the light's origin. Unless a custom mental ray shader is used, Sorted offers no advantage over Simple. The Segments method provides a more sophisticated model whereby shadow rays are traced from the rendered point back to the shadowing light. When the shadow ray strikes an obscuring object, it is terminated and a new shadow ray is born at the intersection. The new shadow ray continues toward the shadowing light (unless it too strikes an object). Each shadow ray, referred to as a shadow "segment," invokes a shadow shader. The Segments method is necessary if software-rendered particles or other volume effects require shadows. The Disabled option turns off shadows.

Figure 11.18. The Shadows section of the mental ray tab in the Render Settings window

Raytrace shadows, by their very nature, understand object transparency. However, if depth map shadows are used with the mental ray renderer, object transparency information is ignored. This holds true for standard Maya depth maps, controlled by the Depth Map Shadow Attributes section of a light's Attribute Editor tab, as well as mental ray shadow maps, controlled by the mental ray section of a light's Attribute Editor tab. (For more information on depth map shadows, see Chapter 3.) Nevertheless, you can impart transparency to the depth map shadows by adjusting the Format attribute in the Shadows section of the mental ray tab in the Render Settings window.

If the Format attribute is set to Detail, mental ray takes into account surface properties, such as transparency, when building the shadows (see Figure 11.19). In addition, the Detail option provides superior depth map shadows for motion-blurred objects. If Use Ray Trace Shadows is checked in a light's Attribute Editor tab, the Format option is ignored for that light.

Figure 11.19. A mental ray shadow map with and without the Detail option. This scene is included on the CD as mental_detail.ma.

You can also access the Detail option through the mental ray section of a spot, point, or directional light's Attribute Editor tab. If you switch the Shadow Map Format attribute from Regular Shadow Map to Detail Shadow Map, the light overrides the Format attribute in the Shadows section of the mental ray tab of the Render Settings window. When selected, the Detail Shadow Map option opens three additional attributes in the Detail Shadow Maps Attributes subsection of the light's Attribute Editor tab. The Samples attribute controls the number of pixel samples taken at the intersection points of shadow-casting objects. The Accuracy attribute controls the quality of the shadow. Low Accuracy values increase quality but slow the render. An Accuracy value of 0 will allow Maya to choose the best solution for the scene. The Alpha attribute, when checked, forces a scalar (grayscale) shadow.

Returning to the Shadows section of the mental ray tab, the Shadow Maps Disabled option of the Format attribute overrides light shadow settings and turns off every shadow map in the scene. The Regular option turns on standard shadow maps (unless overridden by the Detail Shadow Map option of the light). The Regular (OpenGL Accelerated) option attempts to use OpenGL acceleration to speed up shadow calculations (if available through the system graphics card).

When the Format attribute is switched to Regular, Regular (OpenGL Accelerated), or Detail, the Rebuild Mode attribute becomes available in the same section. The Reuse Existing Maps option retrieves previously rendered shadow maps where feasible. The Rebuild All And Overwrite option creates shadow maps from scratch with each render. The Rebuild All And Merge option retrieves previously rendered shadow maps and updates the maps wherever and visible points have shifted toward the shadowing light.

All mental ray depth map shadow types create motion-blurred shadows when Motion Blur is set to No Deformation or Full and Motion Blur Shadow Maps, in the Shadows section, is checked. Reflected and refracted shadows are equally subject to mental ray motion blur.

By default, mental ray's Ray Tracing attribute is checked. The Ray Tracing attribute is listed twice in the mental ray tab—once in the Rendering Features section and once in the Raytracing section (see Figure 11.20). The two check boxes are linked.

Figure 11.20. The Raytracing section of the mental ray tab in the Render Settings window

The following attributes, found in the Raytracing section, control the rays used for the render:

Water, in its liquid form, is either calm or turbulent. When calm, large bodies of water exposed to an intense light source are very reflective. For example, in Figure 11.22 a calm tide pool reflects the sky. Where the water is fairly shallow and is at a perpendicular angle to the viewer, however, the underlying rocks and sand can be seen. This effect is known as a Fresnel reflection and is a result of light moving between two mediums (air and water) with two different refractive indices. (For more information on refractive indices, see Chapter 7.) As such, transparency decreases and reflectivity increases with the viewing angle (the angle between the incoming light ray and the corresponding ray reflected toward the viewer). In addition, the objects underneath the surface are darker than normal since the light received is reduced by sediment within the water.

Figure 11.22. A calm tide pool

In Maya, you can use the Sampler Info utility to determine the angle of a water surface to the camera. A simpler solution, however, involves the application of Ramp textures. For example, in Figure 11.23 a primitive NURBS plane replicates calm water. The outAlpha of a ramp texture (named RampReflectivity) is connected to the reflectivity of a blinn material node (named CalmWater). The outColor of a second ramp texture (named RampTransparency) is connected to the transparency of CalmWater. Since the ramps contain only black and white handles, the reflectivity is the strongest at the top of the plane and the transparency is strongest at the bottom of the plane. The camera's Background Color attribute provides the sky color. The scene works with either the Maya Software or mental ray renderer.

Figure 11.23. Two Ramp textures control the transparency and reflectivity of a NURBS surface. This scene is included on the CD as calm_water.ma.

Creating Water Droplets

Individual water droplets, a minute form of calm water, reveal a strong tendency to refract. For example, in Figure 11.24 the weave of the cloth is magnified by each drop. The degree of refraction is defined by a refractive index. Although air has a refractive index slightly above 1.0, water has a refractive index of 1.33. When the water's surface is curved by surface tension, the perceived refraction is stronger than the equivalent refraction provided by a flat surface. The surface curvature acts as a convex lens, causing light rays reflected off the cloth to diverge as they are refracted through the water toward the viewer. In addition, the refraction process creates "hot spots" within the drops. These spots are found in caustic regions. Caustic regions are areas in which light rays are focused by materials such as water or glass. In other words, light rays entering the water drop converge toward a focal point.

Figure 11.24. Water droplets refract the cloth they sit on.

Although the mental ray Caustics attribute is able to produce caustics (see Chapter 12), Maya Software is not. Nevertheless, you can simulate caustics by mapping the material's Incandescence attribute. For example, in Figure 11.25 each water drop is constructed from a NURBS half sphere, which is assigned to a blinn material node (named Water). The outColor of a ramp texture node (named RampIncandescence) is connected to the incandescence of Water. The facingRatio of a samplerInfo utility node is connected to the vCoord of RampIncandescence. The ramp transitions from black to dark blue. Because the center of each drop faces the camera, it receives the dark blue from the top of the ramp, which equates to a slight incandescent glow. The Refractions attribute of Water is also checked and the Refractive Index attribute is set to 3. Although a value of 3 does not correspond directly to real-world water, it produces a satisfactory look. In addition, the facingRatio of the samplerInfo node is connected to the vCoord's two additional ramps (RampColor and RampTransparency). The outColor of RampColor is connected to the color of Water, thus changing the drop color along its edge. The outColor of RampTransparency is connected to the transparency of Water, thus making the drops more transparent in the center.

Figure 11.25. (Top) Water drops rendered with Maya Software. (Bottom) The shading network for the drops. This scene is included on the CD as drops.ma.

Creating Turbulent Water

Water that tumbles, crashes, or boils creates a great deal of foam. Foam is composed of numerous bubbles. The white quality of foam (assuming the water is relatively clean) is generated by specular highlights appearing on all the bubbles. For example, in Figure 11.26 a wave hitting a rock appears very white. Yet when a small portion of the water is examined, it becomes clear that the white is composed of numerous specular "hot spots" on countless individual bubbles. Although these spots are actually reflections of a bright sky, they are too small and too dispersed to form a reflection that is coherent. By comparison, calm water produces a single large specular highlight in the form of a large, continuous reflection.

Figure 11.26. Water crashes against a rock, producing foam.

The white quality of foam is often exaggerated by rapidly moving water or a lengthy shutter speed of a still camera. If the water is exposed for 1 second, for example, the water has a chance to move a significant distance and thereby create a strong motion blur. For example, in Figure 11.27 the exposure of a waterfall is long enough to allow the foam to blur into monolithic sheets.

Figure 11.27. A long exposure blurs the foam of a waterfall.

Maya's Fluid Effects system is designed to create turbulent lakes and oceans and comes equipped with foam attributes. If the Maya Ocean system is unavailable, bump and displacement mapping on NURBS or polygon surfaces can emulate the look of foam to some degree. On the other hand, Cloud particles, once flowing through a dynamic simulation, can re-create fast-flowing water. For example, in Figure 11.28 a volume particle emitter generates 3,000 particles per second. The Particle Render Type is set to Cloud and the Radius is set to 0.25. Gravity and Turbulence fields pull the particles downward and randomize their direction respectively. The particles are rendered with mental ray. The Motion Blur attribute is set to Full (No Deformation motion blur is unable to blur software-rendered particles). The length of each particle's blue is exaggerated by setting Motion Blur By to 10. Time Samples is set to 10 to help smooth the result.

Figure 11.28. Cloud particles replicate fast-moving water. This scene is included on the CD as waterfall.ma.

When water freezes, it gains new, distinct qualities. For instance, ice cubes, icicles, and other water that is rapidly frozen contain numerous imperfections throughout their mass. Trapped air bubbles, small cracks, boundaries in the crystalline structure, and particulate matter interfere with refractions and reflections. In addition, the convoluted internal structure creates numerous, tiny specular highlights. The end result is a familiar white roughness (see Figure 11.29). The white of the specular highlight is derived from a bright sky or intense camera flash.

You can reproduce the white specular quality of ice in Maya with Water, Noise, and Ramp textures. For example, in Figure 11.30 icicles are re-created with primitive cones hung upside down. Each icicle is composed of an inner and an outer surface. The outer surfaces are assigned to a blinn material node named OuterIce. The outColor of two ramp textures are connected to the specularColor and transparency of OuterIce. The facingRatio of a samplerInfo utility node is connected to the vCoord of the ramp that controls OuterIce's transparency. This causes the icicles to be more transparent along their cores. In addition, a water texture node is connected to OuterIce's Bump Mapping attribute via a bump2d utility node. The Bump Depth value is set to 0.5, thereby reducing the regularity of the specular highlights. OuterIce's Refractive Index attribute is set to 1.1, which is less than the 1.3 assigned to real-world ice.

Figure 11.29. Icicles display distorted reflections, refractions, and specular highlights.

The inner surfaces are assigned to a second blinn material node named InnerIce. Two ramp textures are connected to the specularColor and transparency of InnerIce. The facingRatio of a second samplerInfo node is connected to the vCoord of both ramps. This causes the icicles to be transparent and dark along their cores. In addition, a noise texture node is connected to InnerIce's Bump Mapping attribute via a bump2d utility node. The Bump Depth value is set to 36. The Repeat UV of the noise texture's place2dTexture node is set to 10, 4. This creates the crinkled patterns in the center of each icicle. The outColor of the noise texture is also connected to the colorOffset of the ramp controlling InnerIce's transparency. This "dirties" the ramp and makes the transparency inconsistent. The Refractive Index attribute of InnerIce is set to 1.1, causing background icicles to become distorted behind the foreground icicles. The Light Absorbance attribute of InnerIce is set to 20, thus darkening any resulting refraction. Two directional lights strike the surfaces from each side of the screen, highlighting the edges.

Figure 11.30. (Top) Icicles rendered with Maya Software. (Bottom) The shading network for the inner surface of each icicle. This scene is included on the CD as icicles.ma.

Aside from the reproduction of various glasslike traits, limitations and quirks of the Maya Software renderer pose additional challenges. As an example, the glass pictured in Figure 11.34 is reproduced in 3D.

When comparing the Maya glass to the real glass, you'll see a number of characteristics are equivalent:

The Maya glass is constructed from a single NURBS surface. (The outline was laid out with a NURBS curve and the Revolve tool applied.) The glass is assigned to an Anisotropic material named GlassColor. The Anisotropic material offers the advantage of elongated specular highlights that can be controlled with additional attributes. The Specular Shading section of the material has the following settings:

Reflected Color is mapped with an Env Cube environment texture. The six walls of the Env Cube are mapped with a photo of the real-world from the "point-of-view" of the glass. That is, photos were taken of what was behind, in front of, beside, below, and above the glass. Since the Reflectivity attribute is set to a low 0.1, the cube provides a subtle hint of the surrounding environment. As for the Raytrace Options section, the following attributes are adjusted:

When you're replicating the refraction of a real-world glass, it is important to match the geometry as closely as possible. If the glass walls are a different thickness or the rim has a slightly different shape, the resulting refraction may be significantly different. For example, in Figure 11.35 the vertices of the NURBS rim are adjusted slightly, producing a refraction that does not match the photo. In fact, if the glass featured in Figure 11.34 were constructed with an even higher degree of accuracy, a more appropriate Refractive Index of 1.45 would work. Ultimately, creating the correct look of a refraction in 3D may require the application of a nonrealistic Refractive Index value.

Figure 11.35. When the vertices of the glass surface are adjusted slightly, a significantly different refraction is produced.

Returning to the scene used for Figure 11.34, Light Absorbance is left at the default 0. Although you can raise the Light Absorbance value to darken the glass, the darkening occurs equally over the surface. In this re-creation, the Transparency attribute is instead mapped with a Ramp texture, allowing the base of the glass to appear darker. The Ramp itself has four handles that create a white band in the center of a gray field. This forces the base to be more opaque and the rim more transparent.

Last, the Maya Software renderer has the following settings:

If Refractions is not set to the maximum value, some parts of the glass turn into "black pits." For example, in Figure 11.36 Refractions is set to 2. The base of the glass becomes solid black because the refraction rays are killed off before they have a chance to intersect all the walls of the glass and reach the table top. The color black is provided by the Background Color of the rendering camera. In Maya, Background Color is the color of empty space.

Figure 11.36. When Refractions is set to 2, the base becomes a large "black pit."

In some cases, black pits are much more subtle. Even when Refractions is set to 10, they can occur with intricate geometry or with surfaces that possess complex angles. In fact, you can see black pits in the upper-left corner of the icicle render in Figure 11.30. In a separate example, a clear glass produces small black pits at its base (see Figure 11.37). The quickest solution is to change the Background Color to something other than black. With Figure 11.37, a brown Background Color helps solve the problem.

Figure 11.37. Black pits are disguised with Background Color set to brown.

When you compare the Maya glass featured in Figure 11.34 to the real glass in Figure 11.33, you'll see that several characteristics are not equivalent: