3ds Max 2012 Bible - Kelly L. Murdock [472]
Color bleeding
Another characteristic of global illumination is color bleeding. As a light ray strikes the surface of an object and bounces, it carries the color of the object that is struck with it to the next object. The result of this is that colors from one object bleed onto adjacent objects. You can control this effect using the Color Bleed setting. You can greatly exaggerate the amount of color bleeding by increasing the Object Multiplier along with the Color Bleed value. You also can select colors to use for a color filter and for extra ambient light.
Note
The color bleeding effect doesn't happen unless the Bounce value is set to 2 or greater. •
When using color bleeding, you also want to enable the Exposure Control to the scene. Exposure Control is found in the Environment panel (keyboard shortcut, 8), which can be opened with the Rendering⇒Environment menu command.
Tip
When changing the Exposure Control settings, you can get a quick preview of the scene by clicking the Render Preview button in the Exposure Control rollout of the Environments and Effects dialog box. •
Cross-Reference
The Exposure Control features are discussed in Chapter 23, “Rendering a Scene and Enabling Quicksilver.” •
Figure 45.3 shows an example of color bleeding with several colored cylinders projecting from a gray Box object. The Object Multiplier value was set to 4.0, and the Color Bleed was set a maximum value of 25.0 with a Bounce value of 3. Using the Exposure Control settings, you can isolate the color bleed.
FIGURE 45.3
Color bleeding spreads color about the scene. Exposure Control can highlight it with Automatic (left) and Logarithmic (right).
Quality versus speed
The big trade-off of global illumination is between quality and render time. The more rays per sample that you specify, the better the quality and the longer the render time. This is controlled with the Rays/Sample setting. The Rays/Sample setting and the number of Bounces dramatically increase the rendering time. The Ray Bias setting biases rays toward object edges versus flat areas.
Tip
If you want to see a preview of your scene using light tracing, set the Rays/Sample value to around 10 percent of its normal value and render the scene. The resulting image is grainy, but it shows a rough approximation of the scene lighting without having to change the Bounce value. •
If you don't include enough rays in the scene, then noise patterns appear within the scene. The Filter Size can help control the amount of noise that appears in the scene.
The number of Bounces value specifies the number of times the ray bounces before being dropped from the solution. A setting of 0 is the same as disabling the Light Tracer, and the maximum value of 10 requires a long time to compute. The Cone Angle defines the cone region within which the rays are projected. The Volumes option is a multiplier for the Volume Light and Volume Fog atmosphere effects.
Adaptive undersampling
With the Adaptive Undersampling option enabled, the Light Tracer focuses on the areas of most contrast, which usually occur along the edges of objects. When this option is enabled, you can specify the spacing of the samples and how finely the samples get subdivided. The Initial Sample Spacing options range from 1 × 1 to a very dense 32 × 32. The Subdivision Contrast affects the density for contrast edges between objects and shadows. This value is a minimum amount of contrast that is allowed. If the amount of contrast is greater than this value, then the area is further subdivided into more samples. These high-contrast areas use the Subdivide Down To setting. The Show Samples option displays each sample as a red dot on the rendered image.
Tutorial: Viewing color bleeding
One of the easiest effects of the Light Tracer to see is color bleeding. Although this is often undesirable, it is a telltale sign of global illumination.
To compare the differences between a regular rendering