lecture 8c

8/11/2019 Lecture 8c

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Ray Tracing and other

Surface Rendering Methods

CMSC 161: Interactive Computer

Graphics


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Global Illumination Algorithms

Global illuminationindirectly reflected andtransmitted light

In contrast, local illuminationis light that comes directly from

light sources to the point being shaded

Much of the light in real-world environments does not comefrom direct light sources

So far we have modeled global illumination by an ambient

illumination term (constant)

Two different classes of algorithms have been used togenerate pictures that emphasize its contribution:

Recursive ray tracing

Radiosity methods


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Light Transport in Brief

Light comes fromsources, reflects off

surfaces, and gets to

the eye

We are interested inlight that gets to a

particular pixel

What parts of the

scene couldcontribute?


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Light Transport and Ray Tracing

Ray tracing idea: answer the question What light arriveshere? by analyzing where it might have come from and

looking there

First level: look along ray to camera

Second level: think of how light could get to the surface Directly from sources

Indirectly via reflection

Indirectly via transmission

Higher level: recursive


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Ray Tracing

Determines the visibility of surfaces by tracing imaginaryrays of light from the viewers eye to the objects in the

scene

Think of it as a variation of the depth-buffer algorithm


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Ray Tracing

Ray Tracing vs.Ray Casting Is based on geometric optics methods, which trace the

paths of light rays

In ray tracing, a ray of light is traced in a backwards direction

(i.e. from the eye or camera through a pixel in the image plane)

Ray tracing handles shadows, multiple specular

reflections and texture mapping in a straightforward

manner


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Ray Tracing

Center of projection and a window onan arbitrary view plane are selected

For each pixel in the window, an eye

rayis fired from the center of projection

through the pixels center into thescreen

The pixels color is set to that of the

object at the closest intersection

If the ray misses all objects, then thatpixel is shaded the background color
http://en.wikipedia.org/wiki/File:Ray_trace_diagram.svghttp://en.wikipedia.org/wiki/File:Ray_trace_diagram.svg


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Ray Tracing

We then reflect the ray off the visible surface along aspecular path

If the surface is transparent, we also send a ray through

the surface in the refraction direction

Reflection and refraction rays (in addition to shadow rays) arereferred to as secondary rays

This procedure is repeated for each secondary ray


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Ray-Tracing Tree

As the rays from a pixel ricochet through space, eachsuccessively intersected surface is added to a binary

ray-tracing tree

Left branches represent reflection paths while right branches

represent transmission paths


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Ray-Tracing Tree

Maximum depth of ray-tracing trees can be set as a useroption, or by availability of storage

A path is terminated if it reaches the preset maximum or if the

ray strikes a light source

The intensity assigned to a pixel is then determined byaccumulating the intensity contributions, starting at the

bottom of its ray-tracing tree

Involves attenuation before adding intensities


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Brief History

Was first developed by Appel (used a sparse grid of raysto determine shading) and by Goldstein and Nagel (to

simulate trajectories of ballistic projectiles and nuclear

particles)

Appel was the first to ray trace shadows Goldstein and Nagel pioneered the use of ray tracing to

evaluate Boolean set operations

Whitted and Kay extended ray tracing to handle specular

reflection and refraction


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Ray Tracing Pseudocode


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Ray Tracing Effects

Basic ray tracer: mirror reflections, sharp shadows


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Ray Tracing Procedure

If a ray hits an object, we want to know if that point onthe object is in a shadow. So, when the ray hits an

object, a secondary ray (called a shadow ray) is shot

towards the light sources


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Ray Tracing Procedure

How much light do I see if I look along this ray?


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Distributed Ray Tracing

Randomly distribute rays according to variousparameters in the illumination model:

Pixel area

Reflection and refraction directions

Camera lens area Time

The idea is based in the theory of oversampling:

Instead of approximating an integral by a single scalar value,

the function is point sampled and these samples are used todefine a more accurate scalar value


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Soft and Hard Shadows

Distribute a number of shadow rays over the area of thelight source


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Glossy Reflections and Translucency

Refraction Reflection

Caustics


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Motion Blur

Is the streaked or blurred appearance that movingobjects have because a cameras shutter is open for a

finite amount of time

Achieved by distributing rays over time


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Camera Lens Depth of Field

Objects appear to be more or less in focus depending ontheir distance from the lens, an effect known as depth of

field

Objects near the focal plane are projected as sharp images

Objects in front or in back of the focal plane are blurred

Images (slides 14 to 17) from www-csl.csres.utexas.edu

Efficiency Considerations for Visible-


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Efficiency Considerations for Visible

Surface Ray Tracing

Ray tracing is so time-consuming because of theintersection calculations

Since each ray must be checked against all objects, for a

naive ray tracer (with no speedup techniques) the time is

proportional to the product of the number of rays and thenumber of objects in the scene

Some techniques:

Optimizing intersection calculations

Hierarchies Spatial partitioning

O ti i i I t ti C l l ti


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Optimizing Intersection Calculations

Bounding volumes provide a particularly attractive way todecrease the amount of time spent on intersection

calculations

The object does not need to be tested if the ray fails to

intersect with its bounding volume

Hi hi


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Hierarchies

Although bounding volumes do not by themselvesdetermine the order of frequency of intersection tests,

bounding volumes may be organized in nested

hierarchieswith objects at the leaves and internal nodes

that bound their children Child volume is guaranteed not to intersect with a ray if

its parent does not

Thus, if intersection tests begin at the root, many branches of

the hierarchy (and hence many objects) may be triviallyrejected

S ti l P titi i


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Spatial Partitioning

Subdivides space top-down Bounding box of the scene is calculated first, then

divided into a regular grid of equal-sized extents

Partitions can be examined in the order in which the ray

passes through them; thus as soon as a partition isfound in which there is an intersection, no more partitions

need to be inspected

R di it Li hti M d l


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Radiosity Lighting Model

Acurrate model of diffuse reflections from a surface byconsidering energy transfers between surfaces, subject

to conservation of energy laws

Unlike conventional rendering algorithms, radiosity

methods first determine all the light interactions in anenvironment in a view-independent way

Based on thermal-engineering models

Adding S rface Detail


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Adding Surface Detail

Modeling Surface Detail with Polygons Model structure and patterns with polygons facets

For example, squares on a checkerboard, tile patterns in a

linoleum floor, panels in a door

Texture Mapping Map texture patterns onto the surfaces of objects

The texture pattern may either be defined

In a rectangular array

Or as a procedure that modifies surface intensity values

Compute texture coordinates at the intersection point

Texture Mapping


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Texture Mapping

This process is akin to applying patterned paper on aplain white box

Mip Mapping


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Mip Mapping

MIP mapsare precalculated, optimized collections ofimages that accompany a main texture

To increase rendering speed

Reduce aliasing artifacts and achieve illusion of depth

Need more space in memory Switch to a suitable mipmap image when the texture is viewed

from a distance or at a small size

MIP means multum in parvo, meaning much in a small

space

Bump Mapping


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Bump Mapping

Although texture mapping can be used to add finesurface detail, it is not a good method for modeling the

surface roughness that appears on some objects

The illumination detail in a texture pattern usually does not

correspond to the illumination direction in the scene

Bump mapping applies a perturbation function to the

surface to the surface normal and then uses the

perturbed normal in the illumination-model calculations

Silhouettes and shadows remain unaffected

lecture 8c

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