MTOR: About Ray Tracing

The majority of ray tracing effects possible in PRMan are accessible to the RenderMan artist via MTOR and Slim. These effects range from traced reflections, to refractions, to subsurface scattering, to indirect illumination. They can be much more expensive than the traditional bag of cheats, but they may often provide the only solution. Knowing when to use ray tracing (or perhaps more appropriately, when not to use it) is key. Here we'll outline the various ways that rays may behave.

In PRMan the reyes algorithm and ray tracing live side by side in a hybrid rendering scheme. The addition of ray tracing to PRMan does not limit its fundamental performance as a scanline renderer. However, when ray tracing is desired, surface shaders can be created that know how to perform any number of trace requests. These shaders then probe the scene with rays.

To make ray tracing more feasible in production environments, the management of ray tracing calculations has been given careful thought. First, only surfaces who need to cast rays are required to perform ray tracing calculations, so you can pay for ray tracing only when needed. Secondly, there is a high level of control over how rays are cast from these surfaces. Rays can be restricted by distance, so that rays only travel a finite length, avoiding unnecessary calculations. Rays emitted from one surface can be told to only trace a select group of objects, which can be useful for special effects and/or to increase efficiency by reducing ray run time. Using these various techniques to limit the amount of ray tracing in a scene can have a substantial impact on making tracing as efficient as possible in a production environment.

These documents provide the introductory footing needed to get acquainted with ray tracing in PRMan, from reflections, to caustics, to irradiance caches.

How To's

How do I enable Ray Tracing?
Ray tracing must be enabled in the RenderMan Globals. If disabled all ray trace requests are ignored. That is controlled with the Enable Ray Tracing parameter.

How can I make cool glass?
Use the Slim Glass shader. Attach it to an object. If it has volume, enable the "double sided" parameter. Render. Tweak. Repeat.

Global Controls

Ensemble Overrides
Many of the settings found in the Rays Control Panel can be overridden on a per shader basis, via the controls found in the ensemble. These controls allow you to assign ray tracing behavior on a per-surface basis, overriding the scenes defaults and independent of other surfaces.

The ensemble's ray controls default to the global settings defined in MTOR's Rays Control panel. By explicitly setting values for these parameters the ensemble will override the global settings. This explicit control on a per-shader basis allows shaders to be configured to only perform as much work as needed, so if one shader requires a higher quality setting, like multiple ray bounces, other shaders can still use lower quality settings. In this way scenes can be managed to do only as much tracing as needed. For more information about the ensemble ray overrides refer to: The Ensemble's Ray Tracing Controls

Selective Ray Tracing

Any shader can make a ray trace request, and PRMan affords a lot of flexibility in what a shader may ask a ray to do. Rays can be sent off in arbitrary directions, along the reflection direction, along the normal, along the refraction direction, etc. Rays can be blurred and multi-sampled. Rays can be jittered around a surface point or shot from any arbitrary point in the scene. A ray can return color, surface opacity, distance, or any arbitrary attribute. Rays may also be "tagged" with a label, so that rays with different labels cause other shaders to behave differently.

Slim provides access to all of these operations. In some cases, like Swiss Army, Slim makes assumptions for you about the ray direction and origin, but in other cases, like the Ray Trace color, these may be adjusted on a per-shader basis for the instances when this level of control is required.

Multi-sampling

Trace Sets

Max Distance

Traced Output Variables

Traced Output Variables
When a ray is cast from a surface into a scene, it can be told to return any arbitrary data about any surface it hits. Usually color is returned by default, but PRMan provides a method to return other types of data, as well.

For the non-programmer, there are a few interesting alternatives to color that can be returned from a surface. However, the full potential of output variables requires custom shader and templates that have these variables built into them.

This is supported by Slim's Ray Trace Messaging function.

Message Spec
The "Message Spec" parameter defines what info a ray cast from this surface will return. Some of the more interesting options are:

ray:length

Returns the length of a ray.

 
ray:direction

Returns the direction of a ray.

surface:s

Returns the S coordinates

 
surface:t
Returns the T coordinates

Not all of these options are immediately useful, but they're available as an option. Out of them all, ray length is probably the most useful. The length of the ray can be added to shaders to create special effects.

The addition of "ray:length", causes the length of a ray to be returned.

Example: Mapping Ray Length
The length of the ray is returned in world units. So if a ray travels 15 units, its value will be 15. Often it is useful to map these values between 0 and 1. To do this use the Remap Node. The image on the right shows which parameters to adjust.

Adjust "inputmax" to the maximum distance of the rays.
Setting the "inputmax" to 10 will map all values
from "0 to 10" to "0 to 1".

Now the mirror casts rays that return ray length, not color.
Closer areas are blacker. Whiter areas are farther.
In this case the inputmax has been set to "20."

The same rays cast along the normal, instead of the reflection direction.
This setting can be adjusted with the Ray Direction Parameter.

The same rays, this time cast from an arbitrary direction using the Ray Direction Parameter.

Example Shader
Here's a Slim shader that uses Traced Output Variables: Advanced Ray Tracing: The Jam Shader

The Jam Shader

Ray Labels

Ray Traced Shadows

Pseudo-Area Lights

Ray Tracing FAQ

Should I pay attention to surface normals?
Surface normals help determine how a ray bounces when it hits a surface. If surface normals of an object are facing the wrong direction, reflections will not appear correctly. Ensure that traced surfaces have outward facing normals.

Ray Origin Ray origin determines when a surface point is hit, where the ray will be fired from. Generally, CurrentPoint is the preferred selection.	Ray Origin
Types of Origins Any type of manifold can be used as an origin for a ray trace call. In all but the most eccentric cases, however, you'll want to shoot rays from the current surface point, and so CurrentPoint is generally the best bet.	Types of Origins
Ray Direction Ray direction is highly useful. This determines the difference between reflection and refraction, by the direction the ray bounces off an object. In some Slim templates assumptions are made about this, but others allow you to tweak the ray direction yourself.	Ray Direction
Ray Direction Types A shader that makes a ray trace request can send a ray off along any of these vectors, which includes arbitrary direction.	Types of Directions

Samples By increasing the ray samples, we increase the amount of rays cast from any point. For some effects a sample of "1" may be sufficient. However, other times casting multi-sample allows for greater image quality (using multi-sampling for anti-aliasing) and special effects (like blurry reflections). Higher samples will also dramatically add more time to rendering and should be increased with care.	Typical "Sample" Parameter, increase for higher fidelity and expect longer render times.
Sample Base This parameter allows rays to be jittered from any given point, as measured in micro-polygons. The direction of the ray doesn't change, only the position from where it is cast. This allows for a broader sampling when creating multi-sampled anti-aliased-type effects.	"Sample Base" Parameter
In the first image on the right we have a ray tracing "window," a rectangle in the middle which rays are shot into the scene. The first image is generated with one sample. Look closely and you can see a certain amount of "jaggies" along the edges. By increasing the samples to ten, the jaggy artifacts are reduced. In this case each of the twelve rays is slightly jittered, effectively anti-aliasing the samples.	1 Sample 10 Samples
Blur Increasing the value of blur increases size of the ray's sample cone. This allows the creation of blurry reflections, refractions, subsurface scattering, etc. Blurring traces also requires a greater number of samples to be cast from any given point in order to reduce splotchy artifacts.	"Blur" Parameter. Measured in Radians
In the images on the right, the number of samples has a large effect on the quality of the resulting image. In this case, twelve samples might not be enough for a high quality render, but twelve produces a much more acceptable effect than one single sample.	1 Sample "0.25" Blur	12 Samples "0.25" Blur
Here the reflection blur has been increased to "1.0" Notice how the blur of the reflection increases as it recedes from the base of the cylinder. Again samples could be increased to a value higher than 12 for a final render.	1 Sample "1.0" Blur	12 Samples "1.0" Blur

Trace Sets Trace sets provide a method to define which objects a particular shader will trace. Objects can be defined as trace sets (which are simple Maya sets with an added attribute). These sets can be referenced in shaders in the "Trace Set" parameter. The visibility of objects to tracing can still be enabled and disabled on a global level (using Maya's Attribute Spread Sheet). The Trace Set brings an added level of control where objects can be visible to tracing on a per-shader basis. The effective use of trace sets can limit the total amount of ray tracing calculations which are performed in a scene, significantly reducing memory requirements.
Referencing a Trace Set To reference a trace set from a shader performing trace calculations, add its name to the "Trace Set" field.	Define which trace sets are rendered here. When left empty every object visible to tracing is rendered.
Creating a Trace Set To create a trace set, select all of the objects in Maya to be included in the set and pick "Create Trace Set" from the RenderMan menu. Trace sets can be managed and edited using the Maya Set Editor.	Create a trace set from the RenderMan Menu
Naming the Trace Set The trace set can be quickly named by selecting the Option Box.	Name the trace set via the Option Box
The Trace Set Attribute A Trace Set is a simple Maya set with an added attribute called "RMan Trace Set."	The "RMan Trace Set" Attribute is automatically attached to the trace set.
Example #1
Reflections without trace sets
The yellow and blue cylinder are added to a trace set called, "two_cylinders"
The reflection is limited to the trace set "two_cylinders" and rendered. Note neither the sky or the middle cylinder are included in the reflection.

Max Distance The distance that rays travel from surfaces can be limited so that the rays are cast a fixed distance. This allows rays to sample only objects which are within a set proximity, saving the added cost of probing a scene, hitting and dicing geometry, and running shaders of remote objects that have little bearing on the effect. Max distance is especially useful when used when indirect illumination, or other effects which require many blurry samples when occlusion culling of nearby objects is most important, and far away objects have less relevance. If a ray travels the entire max distance without hitting any objects, it expires; the ray misses. When a ray misses it returns black by default, which is not always useful. Many shaders allow the specification of an environment map when a ray misses, which significantly increases the usefulness of max distance.
Setting Max Distance The control for max distance can be adjusted using the "Max Distance" parameter. Setting the value to "-1.0" causes the shader parameter to inherit the global default defined in the RenderMan Globals.. The value of Max Distance is set in world units.	"-1.0" sets the max distance to the global default in the RenderMan Controls
Max distance = infinity The entire scene is reflected in the wall.
Max distance = 12 The rays stop shortly after reaching the third barrel
Max Distance = 4 The rays stop after reaching the first barrel
Max Distance = 4, with blue environment map The environment map is used when a ray "misses"; when it travels its max distance without hitting anything. The environment map was specified in the shader, in this case Swiss Army.
Max Distance = 4, with blue environment map and indirect illumination Here's a scene where max depth comes in handy: Some color bleeding is wanted along the bottom of the wall, only objects close to the wall will have any effect. So Max Distance is used to limit the rays to only those objects close to the wall.

Ray Traced Shadows Light sources can trace shadows. Traced shadows have several features that traditional shadow maps lack. Traced shadows can be used to create soft shadows, colored shadows, and motion blurred shadows. They are also independent of resolution, unlike shadow maps. (Note that Deep Shadows can be used to create many of these same effects more efficiently than traced shadows.)
Using Traced Shadows To create traced shadows, connect a "Ray Traced Shadows" function into a light shader's "Shadow" parameter.
Colored Shadows With ray traced shadows it is possible for rays to absorb the color of semi-transparent objects, creating the effect of colored shadows. The transmission of color is automatic, but there are a couple guidelines for proper set-up: 1) You'll want to color the opacity parameter of the object. The rays absorb color from opacity, not surface color. A surface with a colorful image map but a gray opacity will produce a gray shadow. Include the color in the opacity. 2) The shadow rays actually absorb the inverse color of the opacity. So invert the opacity's color with the Invert Color function. For proper conversion the invert color function should be set to only invert the hue and not the value (which would invert the opacity). 3) Ensembles should be used with objects that cast semi-transparent shadows. The "Cast Shadows" parameter of the ensemble must be set to "Shade" to evaluate the opacity of the shader. Here's more technical information about Transmission.	Semi-Transparent Shadows
Blurred Shadows When blurred, ray traced shadows can create Soft Shadows. To create soft shadows just increase the amount of "blur." You'll also want to increase the shadow "samples" for higher quality shadows. For a final image, if you see little dots, increase the samples. Since increasing samples can add a lot of computation time, during iterative renderings keeping samples lower will speed things up. Samples The number of rays cast. More samples allow you to achieve higher quality shadows (see images below). More samples also means slower rendering. Sample Base Specifies a multiplier on the jitter area of the ray origin. The default value is 1 which means that the origin is jittered within an area which is the size of the micropolygon. Blur Controls the blurriness of your shadows by simulating an area light. Increase the number of samples to reduce noise. Bias A shadow bias value for ray traced shadow probes. Adjusting this parameter affects shadow creep. If it is too low the shadows will creep onto the front of objects. If it is too high, the shadows will creep away from the bases. The size of objects, in world coordinates, affects the ideal value for this setting and must be adjusted on a per scene basis. Trace Set Trace Sets can be used to define what objects a shader will consider for ray tracing calculations. Careful use of Trace Sets can not only be extremely efficient, it can also be used for special effects, by specifying which objects are visible to any given collection of objects. Manage Trace Sets using the familiar Maya sets, with an added Trace Set attribute.	Traced Shadow Parameters
Shadow Blur 0.25 ~ Samples 1	Shadow Blur 0.25 ~ Samples 24
Shadow Blur 1.0 ~ Samples 1	Shadow Blur 1.0 ~ Samples 24
Hybrid Shadows: Traced Shadow Maps Traced shadows and shadow maps can be used in combination to create efficient hybrid shadows. The shadow map is generated during a pre-pass and then during the final pass rays are shot along the edges of shadows, in areas of high variation, to fill in any details which the shadow map lacked due to pixel resolution. Hybrid shadows work well in cases where entire scenes with many small objects are all shadowed by the same light, like the sun. The shadow map does the bulk of the work, and the traced shadow rays fill in the missing details.
Hybrid Shadows: Knobs and Buttons To create hybrid shadows: 1) Enable shadow map generation under "Make Shadow Map" 2) Under the Ray Accelerator, reference the shadow map (as shown with "[shdmap $OBJNAME]"). 3) Adjust the blur and the samples to suit the requirements of your scene. Increasing the blur, increases the area of the shadow map into which rays are shot. Increasing the samples increasing the quality of the shadows (less dottiness) and also requires more time.	Controls for hybrid shadow maps

MTOR Ray Tracing Overview

Table of Contents

Introduction