Distributed Ray Tracing

Robert L. CookThomas PorterLoren Carpenter

Computer DivisionLucasfilm Ltd.

SIGGRAPH 1984

Gloss/Translucency • Perturb directions reflection/transmission, with

distribution based on angle from ideal ray

Depth of field • Perturb eye position on lens

Soft shadow • Perturb illumination rays across area light

Motion blur • Perturb eye ray samples in time

Distributed Ray Tracing Cook-Porter-Carpenter (1984)

Apply distribution-based sampling to many parts of the ray-tracing algorithmRays can also be stochastically distributed in object space to simulate

Gloss• rays are distributed reflections around

reflection direction to simulate non-smooth surfaces

Depth of field • origin of rays is distributed with respect

to lens to get out-of-focus effect

Soft shadow • rays are distributed around light direction

to simulate area light sources

Motion blur • rays are distributed in time to simulate

object movement

Distributed Ray TracingApply distribution-based sampling to many parts of the ray-tracing algorithmRays can also be stochastically distributed in object space to simulate

Distributed Ray Tracing

Gloss • rays are distributed reflections around

reflection direction to simulate non-smooth surfaces

Depth of field• origin of rays is distributed with respect

to lens to get out-of-focus effect

Soft shadow • rays are distributed around light direction

to simulate area light sources

Motion blur • rays are distributed in time to simulate

object movement

Distributed Ray TracingRays can also be stochastically distributed in object space to simulate

DRT: Gloss/Translucency

• Blurry reflections and refractions are produced by randomly perturbing the reflection and refraction rays from their "true" directions.

Distributed Ray Tracing

4 rays 64 rays

Reflection

4 rays 16 rays

Transparency

Gloss • rays are distributed reflections around

reflection direction to simulate non-smooth surfaces

Depth of field• origin of rays is distributed with respect

to lens to get out-of-focus effect

Soft shadow • rays are distributed around light direction

to simulate area light sources

Motion blur • rays are distributed in time to simulate

object movement

Distributed Ray TracingRays can also be stochastically distributed in object space to simulate

Depth of Field

• The area in front of your camera where everything looks sharp and in focus.

– objects falling within that area will be acceptably-sharp and in focus;

– objects falling outside the area will be soft and out of focus.

CG Camera Models

• Pinhole – ideal camera

• All rays go through single point

• Everything in focus -- unrealistic

More Realistic Model

• Lenses with spherical surfaces• Depth of field control

DRT: Depth of Field

• Each point in the scene appears as a circle on the image plane.

DRT: Depth of field

• The lens has a diameter of F/n .The lens is focused at a distance P so that the image plane is at a distance Vp

• Points on the plane that is a distance D from the lens will focus at

DRT: Depth of field

DRT: Depth of field

For a point I on the image plane, the rays we trace lie inside the cone whose radius at D is

Depth of Field

Gloss • rays are distributed reflections around

reflection direction to simulate non-smooth surfaces

Depth-of-view • origin of rays is distributed with respect

to lens to get out-of-focus effect

Soft shadow • rays are distributed around light direction

to simulate area light sources

Motion blur • rays are distributed in time to simulate

object movement

Distributed Ray TracingRays can also be stochastically distributed in object space to simulate

Soft Shadows

• Consider the light source to be an area, not a point

• Trace rays to random areas on the surface of the light source distribute rays according to areas of varying intensity of light source (if any)

• Use the fraction of the light intensity equal to the fraction of rays which indicate an unobscured light source

Cook (1986)

Gloss • rays are distributed reflections around

reflection direction to simulate non-smooth surfaces

Depth-of-view • origin of rays is distributed with respect

to lens to get out-of-focus effect

Soft shadow • rays are distributed around light direction

to simulate area light sources

Motion blur • rays are distributed in time to simulate

object movement

Distributed Ray TracingRays can also be stochastically distributed in object space to simulate

Motion Blur

• Two objects moving so that one always obscures the other– Can’t render and blur objects separately

• A spinning top with texture blurred but highlights sharp– Can’t post-process blur a rendered object

• The blades of a fan creating a blurred shadow– Must consider the movement of other objects time

Post-process blurring can get some effects, but consider:

Temporal Jittering Sampling

Motion Blur

Cook (1986)

Cook (1986)

Distributed Ray Tracing(Summary)