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Ray-tracing

1 Ray-tracing. 2 Overview n Recursive Ray Tracing n Shadow Feelers n Snell’s Law for Refraction n When to stop!

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Ray-tracing

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Overview

Recursive Ray Tracing Shadow Feelers Snell’s Law for Refraction When to stop!

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Recap: Light Transport

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Recap: Local Illumination

Ambient, diffuse & specular components The sum is over the specular and diffuse

components for each light

M

j

m

jsjdjiar nhklnkIkI1 ,Ia

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Recap: Result of Ray Casting

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Correcting for Non-Visible Lights

EN H

L

surface

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Where Sj is the result of intersecting the ray L with the scene objects

Note consider your intersection points along the ray L carefully• Hint – they might be beyond the light!

M

j

mjsjdjijaar nhklnkISIkI

1 ,

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Result of Shadow Feeler

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Recursive Ray-Tracing

We can simulate specular-specular transmission elegantly by recursing and casting secondary rays from the intersection points

We must obviously chose a termination depth to cope with multiple reflections

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Introducing Reflection

Where

EN H

L

surface

R

NENER 2

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Computing the reflection direction

Ideal reflection = • R same plane as V and N

R = aE + bN

• E.N = N.R = a(N.E)+b• If a = -1, b = 2N.E

R = -E + 2(N.E)N

N

R

E

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Computing Reflectance

'rrlocalr IkII Where Ilocal is computed as before Ray r' is formed from intersection point

and the direction R and is cast into the scene as before

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Recursive Ray Tracing

L1

L2

p

p’

p’’

R1R2

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Pseudo Code

Color RayTrace(Point p, Vector direction, int depth) {

Point pd /* Intersection point */

Boolean intersection

if (depth > MAX) return Black

intersect(p,direction, &pd, &intersection)

if (!intersection) return Background

Ilocal = kaIa + Ip.v.(kd(n.l) + ks.(h.n)m)

return Ilocal + kr*RayTrace(pd, R, depth+1)

}

Normally kr = ks

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Result of Recursion

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Perfect Specular Transmission

N H

L

E

T

R

Snell’s Law

1

2

sin

sin

is index ofrefraction

1

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Using Snell’s Law

Using this law it is possible to show that:

Note that if the root is negative then total internal reflection has occurred and you just reflect the vector as normal

211

2

sin

sin

1cos1cos 22121212 NET

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Recursive Ray Tracing Including Transparent Objects

L1

L2

p

p’

p’’

R1R2

T2

T1

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New Pseudo Code

Color RayTrace(Point p, Vector D, int depth) { Point pd /* Intersection point */ Boolean intersection if (depth > MAX) return Black intersect(p,direction, &pd, &intersection) if (!intersection) return Background

Ilocal = kaIa + Ip.v.(kd(n.l) + ks.(h.n)m)

return Ilocal + kr*RayTrace(pd, R, depth+1) + kt*RayTrace(pd, T, depth+1)

}

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Direct Specular Transmission

A transparent surface can be illuminated from behind and this should be calculated in Ilocal

NH'

L

E

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Calculating H'

Use H‘ instead of H in specular term

1'

1

2

1

2

LE

H

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Putting Everything Together

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Discussion – What Can’t We Simulate?

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Remark

Specular and transmission only• What should be added to consider diffuse reflection?

Why it’s expensive• Intersection of rays with polygons (90%)

How to reduce the cost?• Reduce the number of rays• Reduce the cost on each ray

– First check with bounding box of the object– Methods to sort the scene and make it faster

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Summary

Recursive ray tracing is a good simulation of specular reflections

We’ve seen how the ray-casting can be extended to include shadows, reflections and transparent surfaces

However this is a very slow process and still misses some types of effect!