Graphics18-Raytracing

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Course Website:http://www.comp.dit.ie/bmacnamee

Computer Graphics 18:Ray-tracing
http://www.comp.dit.ie/bmacnameehttp://www.comp.dit.ie/bmacnamee


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50Contents

Today we will have a look at ray-tracingwhich can be used to generate extremely

realistic images


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50Ray-Tracing Examples

Imagesfromt

hePOV

RAYHallofFame:hof.povray.org
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50Ray-Tracing Examples (cont)

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50Ray-Tracing Setup

Ima

gestakenfromHearn&B

aker,ComputerGraphics

withOpenGL(2004)


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50Basic Ray-Tracing

Ray tracing proceeds as follows: Fire a single ray from each pixel position into

the scene along the projection path

Determine which surfaces the ray intersects

and order these by distance from the pixel

The nearest surface to the pixel is the visiblesurface for that pixel

Reflect a ray off the visible surface along thespecular reflection angle

For transparent surfaces also send a raythrough the surface in the refraction direction

Repeat the process for these secondary rays


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50Terminating Ray-Tracing

We terminate a ray-tracing path when anyone of the following conditions is satisfied:

The ray intersects no surfaces

The ray intersects a light source that is not areflecting surface

A maximum allowable number of reflections

have taken place


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50Ray-Tracing & Illumination Models

At each surface intersection he illuminationmodel is invoked to determine the surface

intensity contribution

ImagestakenfromHearn&B

aker,ComputerGraphicswithOpenGL(2004)


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50The Shadow Ray

The path from the intersection to the lightsource is known as the shadow ray

If any object intersects the shadow ray

between the surface and the light sourcethen the surface is in shadow with respect to

that source


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50Ray-Tracing & Transparent Surfaces

For transparent surfaces we need to calculatea ray to represent the light refracted through

the material

The direction of the refracted ray is determinedby the refractive

index of the

material




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50Ray-Tracing Tree

As the rays ricochet around the scene eachintersected surface is added to a binary ray-tracing tree

The left branches in the tree are used torepresent reflection paths

The right branches in the tree are used torepresent transmission paths

The trees nodes store the intensity at thatsurface

The tree is used to keep track of allcontributions to a given pixel


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50Ray-Tracing Tree Example

S3 S4

S1 S2

Projection

Reference Point


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50Ray-Tracing Tree Example (cont)




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50Ray-Tracing Tree (cont)

After the ray-tracing tree has beencompleted for a pixel the intensitycontributions are accumulated

We start at the terminal nodes (bottom) ofthe tree

The surface intensity at each node isattenuated by the distance from the parent

surface and added to the intensity of theparent surface

The sum of the attenuated intensities at the

root node is assigned to the pixel


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50Ray Tracing Demo

Theres a very nice Java demo which allows us step through the ray-tracing

process available at:http://www.siggraph.org/education/materials/HyperGraph/raytrace/rt_java/raytrace.html
http://www.siggraph.org/education/materials/HyperGraph/raytrace/rt_java/raytrace.htmlhttp://www.siggraph.org/education/materials/HyperGraph/raytrace/rt_java/raytrace.html


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50Summary


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ImagestakenfromHearn&Baker,ComputerGraphicswithOpenGL(2004)

Graphics18-Raytracing

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