Illumination Model Illumination Model & &

Surface-rendering Surface-rendering MethodMethod

2001.07.25박 경 와

ContentsContents

ILLUMINATION MODELS– Ambient light, Diffuse reflection, Specular reflection– Illumination in the Phong model

POLYGON-RENDERING METHODS– Flat shading– Gouraud shading– Phong shading– Comparision each methods– Ray Tracing

Basic Algorithm Methods for getting better quality

BASIC ILLUMINATION BASIC ILLUMINATION MODELSMODELS

Ambient LightAmbient Light

Color does not depend on the position, only on the object

I=IaKa ( Ia : ambient light intensity, Ka: ambient reflection coefficient)

Very Crude Model – Object shape is in invisible– But user nevertheless to hide other models artifacts

Ambient LightAmbient Light

Example

Increasing Ka

Diffuse ReflectionDiffuse Reflection

Light from the light source is sent in every direction Object aspect independent from viewer position Only depends on relative position of light source

I = Ip Kd cos Ø (Ip : point light source intensity

Kd : Diffuse reflection coeffcient)

Diffuse ReflectionDiffuse Reflection

Example

Increasing Kd ( Ka=0)

Diffuse + AmbientDiffuse + Ambient

Increasing Kd

Increasing ka

Specular ReflectionSpecular Reflection Light reaching the object is reflected in the

direction having the same angle With point light source, effect is visible only at

the one point on the surface Useful for indirect illumination (reflection and shadows)

Specular ReflectionSpecular Reflection In the Phong model

– Imperfect specular reflector I = IpKs(cosα)n

α : angle between reflection and view point

Figure. Left and right

Imperfect Specular reflector

Phong ModelPhong Model Treats point light sources only Models three types of reflected light

– Ambient + diffuse + imperfect specular reflector– I = IaKa + Ip {Kdcosθ + Ks(cosα)n}

No physical meaning model

Phong ModelPhong Model

Increasing n

Ks

POLYGON-RENDERINGPOLYGON-RENDERINGMETHODSMETHODS

Constant-Intensity Constant-Intensity ShadingShading

Flat Shading– A fast and simple method– Assign all pixels inside each polygon same color

VN4

N3N2

N1

Figure.

The normal vector at vertex V calculated as the average of the surface normals for each polygon sharing that vertex

Constant-Intensity Constant-Intensity ShadingShading

Example 1)

Image with flat shading

Gouraud ShadingGouraud Shading Take the colors at the vertices Interpolate these colors across the edges and

across the scan lines Typically linear interpolation

RGB 1

RGB 2

RGB 3

J K Scan lineInterpolated

colors

Gouraud ShadingGouraud Shading

Example 2)

Image with Gouraud shading and specular highlights.

Phong ShadingPhong Shading Take the normals at the vertices Interpolate these normals across the edges and Acros

s the scan lines

normal 1

normal 2

normal 3

J K Scan lineInterpolated

nomals

Phong ShadingPhong Shading

Example 3)

Image with Phong shading and specular highlights.

Comparision Comparision Flat shading

– The simplest shading method Difference of two shading models

– Phong shading is more accurate way of shading a polygon since the illumination model is applied to every point

– More computationally intensive than the Gouraud Illumination model is applied more often Interpolated normals need to be normalized

Comparision Comparision

a) Flat shading b) Gouraud shading c) Phong shading

RAY TRACING METHODRAY TRACING METHOD

Ray TracingRay Tracing

One of the shading method To create several kinds of effects

– Very difficult or even impossible to do with other methods

Include three items – Reflection – Transparency – Shadow

For each pixel ray– Test each surface if it is intersected– Intersected

Calculated the distance from the pixel to the surface intersection point

The smallest value is visible surface for that pixel

– Reflection ray Secondary ray Along specular path

– Transparent Send a ray through the surface in the refraction direction

Basic Ray-Tracing Basic Ray-Tracing AlgorithmAlgorithm

Figure. Ray Tracing

Each secondary ray (reflection or refraction ray)– Repeated the same procedure

Objects are tested for intersection The nearest surface along secondary ray path is used

to recursively production the next generation of reflection and refraction path

– Ray tracing tree Each successively intersected surface is added to a binary ray- tracing tree

Basic Ray-Tracing Basic Ray-Tracing AlgorithmAlgorithm

Figure. Ray Tracing

Left branch Reflection Right branch Transmission Terminated

– Reach the preset maximum– Strike a light source

Pixel intensity – Sum of intensities at root node– Start at terminal node– Background intensity

If tree is empty

Ray-Tracing TreeRay-Tracing Tree

Figure. Ray Trace and Ray-Tracing tree

Reducing Object-Reducing Object-Intersection CalculationIntersection Calculation

Ray surface intersection calculation– 95 percent of the processing time in a ray tracer– Spent most of processing time checking objects that

are not visible along the ray path Enclose groups of adjacent objects within a

bounding volume Check larger boundary volume and ,if

necessary, smaller boundary volume; and so on.

Space-Subdivision MethodSpace-Subdivision Method

The other way to reduce intersection calculation Enclose a scene within a cube Uniform subdivision – (a)

– Subdivided the cube into eight equal-size octants at each step

Adaptive subdivision – (b)– Only subdivided cube containing objects E

xample - a

Exam

ple - b

Anti-aliased Ray TracingAnti-aliased Ray Tracing Two basic techniques

– Supersampling The pixel is treated as a finite square area instead of a single p

oint

– Adaptive sampling Uses unevenly spaced rays in some reason of the pixel area Ex. More rays can be used near object edges to obtains a better

estimate of the pixel intensities

Intensity FunctionIntensity Function

i in

iSiiDALAE IRVKILNKIKII ))()((

• IE : Emitted Intensity

KA, KD, Ks : Ambient /Diffuse /Specular reflection coefficient

IAL : Ambient-light Intensity

N : Unit normal vector

Li : Unit direction vector to the I-th point light source from a position on the surface

Ii : the intensity of the I-th point light source

V : Unit viewing direction vector

R : Specular-reflection direction vectorP14 P16