Rendering Algorithms Rollout (mental ray Renderer)

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Rendering menu > Render > Render Scene dialog > Renderer panel > Rendering Algorithms rollout

Main toolbar > Render Scene > Render Scene dialog > Renderer panel > Rendering Algorithms rollout

Note: The Renderer panel appears only when the mental ray renderer is the currently active renderer.

The controls in this rollout let you choose whether to render using ray-tracing, scanline rendering, or both. You can also choose the method used to accelerate ray-tracing.

The controls labeled Trace Depth limit the number of times each ray can be reflected, refracted, or both.

Procedure

To set trace depth for reflections and refractions:

Count the number of times you want an object to be reflected or refracted in the scene.
On the mental ray: Rendering Algorithms rollout, set Max Reflections to the number of reflections you want, and Max Refractions to the number of refractions you want.
Set Max Depth to equal the value you chose for Max Reflections plus the value you chose for Max Refractions.

The greater the number of reflections and refractions, the more slowly your scene will render. On the other hand, too low a value for Max Reflections or Max Refractions (or Max Depth, controlling both) can make your rendering look unrealistic.

Interface

By default, both Ray Trace and Scanline are enabled, which lets the mental ray renderer use a combination of ray tracing and scanline rendering to render the scene. Scanline rendering is used for direct illumination (“primary rays”) only; ray tracing is used for indirect illumination (caustics and global illumination) as well as reflections, refractions, and lens effects.

You can disable one or the other option, but not both. For example, if only ray tracing is enabled and you turn it off, Autodesk VIZ enables scanline rendering.

Scanline group

Enable—When on, the renderer can use scanline rendering. When off, the renderer uses the ray-tracing method only. Scanline rendering is faster than ray tracing, but cannot generate reflections, refractions, shadows, depth of field, or indirect lighting. Default=on.

Use Fast Rasterizer (Rapid Motion Blur)—When on, uses a fast rasterizer method to generate the first generation of rays to trace. This can improve rendering speed. Default=off.

This option works well with object motion blur, and also with scenes that have no motion blur.

The following settings are available for the fast rasterizer:

Samples per Pixel—Controls the number of samples per pixel used by the fast rasterizer method. More samples result in greater smoothness, at a cost of render time. Range=1 to 225. Default=16.
Shades per Pixel—Controls the approximate number of shading calls per pixel. Greater values result in more accurate renderings, at a cost of render time. Range=0.1 to 10000 (ten thousand). Default=1.0.

Note: mental ray provides a Time Samples setting specifically for motion blur in the fast rasterizer. When Use Fast Rasterizer is on, the Camera Effects rollout >Time Samples label changes to Time Samples (Fast Rasterizer) to indicate that this version of Time Samples is now in effect.

Ray Tracing group

Enable—When on, mental ray uses ray tracing to render reflections, refractions, lens effects (motion blur and depth of field), and indirect lighting (caustics and global illumination). When off, the renderer uses the scanline method only. Ray tracing is slower but more accurate and more realistic. Default=on.

You must turn on Ray Trace to render reflections, refractions, depth of field, and indirect lighting (caustics and global illumination).

Use Autovolume—When on, uses the mental ray autovolume mode. When Autovolume is on, you can render nested or overlapping volumes such as the intersection of two spotlight beams. Autovolume also enables a camera to move through the nested or overlapping volumes. Default=off.

To use Autovolume, Ray Trace must be turned on, Scanline must be turned off, and the shadow mode must be set to Segments. (You set the shadow mode on the Shadows And Displacement rollout.) If these conditions aren't met when you click to turn on Autovolume, an alert warns you about this, and gives you the option of making the appropriate setting changes.

Raytrace Acceleration Group

Method—The drop-down list sets which algorithm to use for raytrace acceleration. The other controls in this group box change, depending on which acceleration method you choose. These are the alternatives:

BSP (the default)

The BSP method has Size and Depth controls. See Ray-Trace Acceleration: Parameters for the BSP Methods.

This method is the fastest on a single-processor system. Use it for small-to-medium size scenes on a single processor. BSP is also the best method to use when ray tracing is turned off.
Grid

The Grid method has Size, Depth, and Resolution controls. See Ray-Trace Acceleration: Parameters for the Grid Method.

This method uses less memory than BSP. It is also faster than BSP on multiprocessor systems.

Note: If you attempt to render motion blur with the Grid method active, the mental ray renderer automatically switches to the BSP method. This happens in the MI file but is not reflected in the Autodesk VIZ interface.
Large BSP

The Large BSP method has the same controls as BSP. See Ray-Trace Acceleration: Parameters for the BSP Methods.

This method is a variant of the BSP method. Portions of the partitioning tree it uses can be swapped in and out of memory. This enables mental to render very large scenes, at a cost of ray-tracing time. Use this method for very large ray-traced scenes, and also when Use Placeholder Objects is turned on (see Translator Options Rollout (mental ray Renderer)). Use Placeholder Objects is recommended when you are doing distributed rendering.

Trace Depth group

Trace depth controls the number of times a light ray can be reflected or refracted. At 0, no reflection or refraction occurs. Increasing these values can increase the complexity and realism of a scene, at a cost of greater rendering time.

Tip: In some cases, you might want to set Max. Refractions high and Max. Reflections low. For example, you might have the camera looking through several glasses that are lined up, so they're overlapping from the camera's point of view. In this situation, you might want the light rays to refract twice for each glass (once for each layer), so you'd set Max. Refractions to 2 x [number of glasses]. However, to save rendering time, you could set Max. Reflections to 1, resulting in accurate multi-layer refraction with a relatively fast rendering time.

Max. Depth—Limits the combination of reflection and refraction. Tracing of a ray stops when the total number of reflections and refractions reaches the Max Depth. For example, if Max Depth equals 3 and the two trace depths each equal the default value of 2, a ray can be reflected twice and refracted once, or vice versa, but it can’t be reflected and refracted four times. Default=6.

Max. Reflections—Sets the number of times a ray can be reflected. At 0, no reflection occurs. At 1, the ray can be reflected once only. At 2, the ray can be reflected twice, and so on. Default=6.

Max. Refractions—Sets the number of times a ray can be refracted. At 0, no refraction occurs. At 1, the ray can be refracted once only. At 2, the ray can be refracted twice, and so on. Default=6.