Monte Carlo Path Tracing

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# Monte Carlo Path Tracing - PowerPoint PPT Presentation

Monte Carlo Path Tracing. Today Path tracing Random walks and Markov chains Eye vs. light ray tracing Bidirectional ray tracing Next Irradiance caching Photon mapping. Light Path. Light Transport. Integrate over all paths Questions How to sample space of paths. Path Tracing.

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Presentation Transcript
Monte Carlo Path Tracing
• Today
• Path tracing
• Random walks and Markov chains
• Eye vs. light ray tracing
• Bidirectional ray tracing
• Next
• Photon mapping

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Light Path

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Light Transport
• Integrate over all paths
• Questions
• How to sample space of paths

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

### Path Tracing

Penumbra: Trees vs. Paths

4 eye rays per pixel

16 shadow rays per eye ray

64 eye rays per pixel

1 shadow ray per eye ray

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Path Tracing: From Camera

Step 1. Choose a camera ray r given the (x,y,u,v,t) sample

weight = 1;

Step 2. Find ray-surface intersection

Step 3.

if light

return weight * Le();

else

weight *= reflectance(r)

Choose new ray r’ ~ BRDF pdf(r)

Go to Step 2.

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

M. Fajardo Arnold Path Tracer

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Cornell Box: Path Tracing

10 rays per pixel

100 rays per pixel

From Jensen, Realistic Image Synthesis Using Photon Maps

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Path Tracing: Include Direct Lighting

Step 1. Choose a camera ray r given the (x,y,u,v,t) sample

weight = 1;

Step 2. Find ray-surface intersection

Step 3.

weight += Lr(light sources)

Choose new ray r’ ~ BRDF pdf(r)

Go to Step 2.

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

### Discrete Random Walk

Discrete Random Process

States

Creation

Termination

Transition

Assign probabilities to each process

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Discrete Random Process

States

Creation

Termination

Transition

Equilibrium number of particles in each state

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Discrete Random Walk

States

• Generate random particles from sources.
• Undertake a discrete random walk.
• Count how many terminate in state i

[von Neumann and Ulam; Forsythe and Leibler; 1950s]

Creation

Termination

Transition

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Monte Carlo Algorithm
• Define a random variable on the space of paths

Path:

Probability:

Estimator:

Expectation:

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Monte Carlo Algorithm
• Define a random variable on the space of paths

Probability:

Estimator:

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Estimator
• Count the number of particles terminating in state j

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Equilibrium Distribution of States
• Total probability of being in states P
• Note that this is the solution of the equation
• Thus, the discrete random walk is an unbiased estimate of the equilibrium number of particles in each state

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

### Light Ray Tracing

Examples

Backward ray tracing, Arvo 1986

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Path Tracing: From Lights

Step 1. Choose a light ray

Choose a light source according to the light source power distribution function.

Choose a ray from the light source radiance (area) or intensity (point) distribution function

w = 1;

Step 2. Trace ray to find surface intersect

Step 3. Interaction

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Path Tracing: From Lights

Step 1. Choose a light ray

Step 2. Find ray-surface intersection

Step 3. Interaction

u = rand()

if u < reflectance

Choose new ray r ~ BRDF

goto Step 2

else

terminate on surface; deposit energy

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

### Bidirectional Path Tracing

Bidirectional Ray Tracing

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Path Pyramid

From Veach and Guibas

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Comparison

Bidirectional path tracing

Path tracing

From Veach and Guibas

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Generating All Paths

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Symmetric Light Path

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Symmetric Light Path

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Symmetric Light Path

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Three Consequences
• Forward estimate equal backward estimate

- May use forward or backward ray tracing

- Importance sampling paths

• Solve for small subset of the answer

University of Texas at Austin CS395T - Advanced Image Synthesis Spring 2007 Don Fussell

Example: Linear Equations
• Solve a linear system
• Solve for a single xi?