1 illumination and shading day 6, 2013 © jeff parker

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Illumination and Shading

Day 6, 2013

© Jeff Parker

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Outline

Gallery

Goal all Semester: Realistic images

Review lighting in the real world

Types of illumination

Shading Strategies

Shading Algorithms

Limitations of shading algorithms

Alternatives

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Gallery

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DanChris

Fixfunction perspective( fovy, aspect, near, far )

{ // Polina's fix

var f = 1.0 / Math.tan( radians(fovy) / 2 );

// var d = far - near;

var d = -(far - near);

var result = mat4();

result[0][0] = f / aspect;

result[1][1] = f;

result[2][2] = (near + far) / d;

result[2][3] = 2 * near * far / d;

result[3][2] = -1;

result[3][3] = 0.0;

return result;

}

The on-line version has been revised

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Outline

Goal in Computer Graphics is to make something good enough to use

For design work

For entertainment

For instruction

…

Nothing we have done so far looks "real"

Tonight we will take a big step forward - lighting

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Mike

8

Derrick

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Dan

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Outline

Gallery

Goal all Semester: Realistic images

Review lighting in the real world

Types of illumination

Shading Strategies

Shading Algorithms

Limitations of shading algorithms

Alternatives

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Examples

We start by looking at light in some photosWhat do you see?

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Examples

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Examples

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Examples

Luxo Jr

Pixar's first movie

http://www.youtube.com/watch?v=Hrnz2pg3YPg

Spectrum

Light is visible electromagnetic radiation

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Observations

Light is generated by a source

Sun, Light bulb, phosphorescence,…

Light travels through space, air, glass, water…

Travels in straight line – may bend at boundaries

When light strikes object some wavelengths are absorbed, and some are reflected

The reflected light travels through air, glass, …

Some of the reflected light reaches our eyes

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Lambert's Law

Different types of finish

Matt vs. glossy

Smooth, bumpy, brushed

Most surfaces are matt: many man made surfaces are smooth

Lambertian – luminance is isotropic

Looks the same from every viewpoint

Luminance depends on angle of incidence

Sunlight is strong in summer, weak in winter

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Snell's law

Snell's law decribes how light bends moving from one medium to another

€

sinθ1

sinθ2

=n1

n2

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Fresnel's law

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Milestones

Hooke (1665) – Light is rapid vibration (wave)

Newton (1666) – Light is a corpuscle (particle)

Young (1801) – Double-Slit experiment - wave

Maxwell (1860) – Electromagnetic disturbance

Einstein (1905) – Photoelectric effect - Light comes in discrete quanta (photons) – 1921 Nobel Prize

Quantum Mechanics – Light is both particle and wave

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Wave or Particle?

Experimental evidence for the two theories

Phenomenon Wave Particle

Reflection yes yes

Refraction yes yes

Interference yes no

Diffraction yes no

Polarization yes no

Photoelectric Effect no yes

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Human sensitivity

The cones sense different wavelengths

The original version of the Retinylidene protein is thought to have been tuned for green light

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Spectrum of sources

Compare incandescent to sunlight

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Halogen Bulb

Fresnell's law extends Snell's law

Refraction is often accompanied by reflection

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Blacklight

Compare incandescent to sunlight

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Fluorescent Bulb

Fresnell's law extends Snell's law

Refraction is often accompanied by reflection

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Takeaway

Different light sources emit different spectra

The same scene looks different under different light

OpenGL gives you control over your light source's spectrum

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Light Sources

As well as direct light, there are many indirect sources

We call this ambient light

Other points get direct illumination

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Point source

Some lights are from a radial point source

Light bulb: rays radiate

Some sources are directional – all the rays are ~ parallel

This is an ideal model – best approximation for distant sources

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Other Light Sources

Spot Lights

Area light source

A 2D area emits light

Soft shadows

Extended light source

Spherical Light source (3D)

Soft shadows

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Hard and Soft Shadows

Hard shadows are created by point sources

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Shadows

Shadows are important in creating atmosphere

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Putting it together

The illumination of a scene is made of two parts

The light sources

Emitted spectrum

Geometry (position and direction)

Attenuation – energy decreases with distance

Surface

Reflectance spectrum

Geometry

Absorption

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Shading options

We will look at alternative forms of shading

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Shading options

We will look at alternative forms of shading

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Shading options

The Quest for Realism (Jim Blinn, 1980)

A brief history of lighting in Computer Graphics

http://www.youtube.com/watch?v=pbSBUEH0PYo

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Reflectance Models

We will break down light into three components

Ambient Light

Diffuse light – point light source, matt finish

Specular – point light source, glossy finish

Each component will have three sub-components

Red, Green, Blue – 9 values in all

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Diffuse Light: Matt surface

A Lambertian surface (e.g. unfinished wood) has the same brightness, whatever the viewer's angle.

Not all patches of a matt surface have the same brightness:

it depends upon position of light source

The Diffuse Intensity (ID) is equal to the cosine of the angle between the Normal vector N and the vector towards the light, L, multiplied by the Intensity of the light, IL.

L and N are assumed to be of unit length (normalized)

€

ID = L ⋅NIL

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Diffuse Light: Matt surface

This is a step forward – can tell where the light is

But we see the edges of the patches

€

ID = L ⋅NIL

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GouraudHenri Gouraud, 1971, PhD University of Utah

Interpolate shading over a polygon

Good results without requiring small polygons

Start with normal vector for each polygon

At each vertex, average incident normals to get per-vertex "normal"

Use "vertex normals" to compute color for each vertex

Interpolate colors across each face

Sylvie Gouraud

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PhongBui Tuong Phong, 1973, PhD University of Utah

Interpolate shading over a polygon

Start with normal vector for each polygon

At each vertex, average incident normals to get per-vertex "normal"

Interpolate normals across faces

Use the interpolated normal to compute the color

Gouraud shading: Compute normals at each vertex, compute color

Then interpolate color for each fragment

Phong shading: have a unique normal at each fragment

Compute normal at each vertex, as before

Interpolate normals across face

Fresh computation of color from normal at each fragment

Thus a per fragment computation

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Specular Reflections

Bui Tuong Phong, 1973, PhD University of Utah

Approximate the specular highlights on a surface

Bi-Directional Reflectance Distribution Function (BRDF)

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BRDF sampling

Special purpose hardware to sample BRDF

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BRDF sampling

Sample values

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LightPosition

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LightMaterial

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Specular Highlights

For specular component, we use 4 vectors at each point

l Light: points towards light

v Viewer: points towards viewer

n Normal to the point

r Reflector: Where light would bounce

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Specular Highlight

Phong added a specular term

Strong as vector v approaches reflected vector r, drops off as they differ (angle phi)

Ir = Reflected Intensity

ks = Absorption coef

I = Incoming Intensity

alpha = shininess coef

€

Ir ≈ ks I cosα φ

Ir ≈ ks I(v ⋅r)α φ

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Computing Reflector r

The angle of reflection must equal the angle of incidence

The three vectors should be co-planar

€

r + l

2= (l ⋅n)n

r + l = 2(l ⋅n)n

r = 2(l ⋅n)n − l

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Shininess Coef

Metals have small specular reflection – large alpha (100-200)

Plastics have wide specular reflection – smaller alpha (5-10)

€

Ir ≈ ks I(v ⋅r)α φ

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Blinn-Phong

Jim Blinn proposed alternative to Phong model

Approximate the angle phi

Uses h, the "halfway" vector midway between l and v

The new angle psi is half phi if all vectors are coplanar.

Given alpha, we find matching power beta.

OpenGL uses Flat or Gouraud with Blinn-Phong shading

€

h =(l + v)

| l + v |

€

Ir ≈ ks I(h ⋅n)β φ

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Why is this any better?

If we are using Phong shading, the normal n varies as we interpolate over each polygon.

As n changes, we must recompute the vector r

Given direct light and a distant eye, l and v do not change, or do not change as quickly.

We can quickly compute a dot product on the fly as n varies

€

Ir ≈ ks I(h ⋅n)β φ

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Distance Term

Light from distant source is inversely proportional to the distance squared

We can add a term of the form 1/(a + bd + cd2)

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Light Sources

We add together the effects of the three forms of light (Ambient, Diffuse, Specular) over the three colors

This gives us 9 terms {d, s, a} x {r, g, b}

€

Idr , Idg , Idb, I sr , I sg , I sb , Iar , Iag , Iab

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Material Properties

Each material can be configured to react distinctly to each type of light, and to each color

Gives us 9 absorption coefficients and one shineness coefficient alpha

€

kdr , kdg , kdb, ksr , ksg , ksb , kar , kag , kab

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Phong Illumination Model

For each light source, and for each color component, the Phong model gives the following equation

For each color, we add up the contributions from all lights

€

I = ka Ia + kd Idl ⋅n + ks I s (v ⋅r)α

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Compare

Phong vs Blinn-Phong

Example

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teapots.c

from The Redbook

Single light

variety of material parameters

Example

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material.c

from The Redbook

Single light

variety of material parameters

See next page…

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* The spheres in the first row have materials with no ambient reflection.

* The second row has materials with significant ambient reflection.

* The third row has materials with colored ambient reflection.

* The first column has materials with blue, diffuse reflection only.

* The second column has blue diffuse reflection, as well as specular

* reflection with a low shininess exponent.

* The third column has blue diffuse reflection, as well as specular

* reflection with a high shininess exponent (a more concentrated highlight).

* The fourth column has materials which also include an emissive component.

* glTranslatef() is used to move spheres to their appropriate locations.

Light

// Initialize shader lighting parameters in .js

var lightPosition = vec4(1.0, 1.0, 1.0, 0.0 );

var lightAmbient = vec4(0.2, 0.2, 0.2, 1.0 );

var lightDiffuse = vec4( 1.0, 1.0, 1.0, 1.0 );

var lightSpecular = vec4( 1.0, 1.0, 1.0, 1.0 );

var materialAmbient = vec4( 1.0, 0.0, 1.0, 1.0 );

var materialDiffuse = vec4( 1.0, 0.8, 0.0, 1.0 );

var materialSpecular = vec4( 1.0, 0.8, 0.0, 1.0 );

var materialShininess = 100.0;

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Gloss

// Point source: RHS is a point


// Directional source: RHS is a vector


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LightambientProduct = mult(lightAmbient, materialAmbient);

diffuseProduct = mult(lightDiffuse, materialDiffuse);

specularProduct = mult(lightSpecular, materialSpecular);

gl.uniform4fv(gl.getUniformLocation(program, "ambientProduct"),flatten(ambientProduct));

gl.uniform4fv(gl.getUniformLocation(program, "diffuseProduct"),flatten(diffuseProduct) );

gl.uniform4fv(gl.getUniformLocation(program, "specularProduct"),flatten(specularProduct) );

gl.uniform4fv(gl.getUniformLocation(program, "lightPosition"),flatten(lightPosition) );

gl.uniform1f(gl.getUniformLocation(program, "shininess"), materialShininess);

gl.uniformMatrix4fv( gl.getUniformLocation(program, "projectionMatrix"), false, flatten(projection));

Vertex Shader for Sphere 1<script id="vertex-shader" type="x-shader/x-vertex">

attribute vec4 vPosition;

attribute vec4 vNormal;

varying vec4 fColor;

uniform vec4 ambientProduct, diffuseProduct, specularProduct;

uniform mat4 modelViewMatrix;

uniform mat4 projectionMatrix;

uniform vec4 lightPosition;

uniform float shininess;

void main()

{

vec3 pos = -(modelViewMatrix * vPosition).xyz;

vec3 light = lightPosition.xyz;

vec3 L = normalize( light - pos );

vec3 E = normalize( -pos );

vec3 H = normalize( L + E );

// Transform vertex normal into eye coordinates

vec3 N = normalize( (modelViewMatrix*vNormal).xyz);

// Compute terms in the illumination equation

vec4 ambient = ambientProduct;

float Kd = max( dot(L, N), 0.0 );

vec4 diffuse = Kd*diffuseProduct;

float Ks = pow( max(dot(N, H), 0.0), shininess );

vec4 specular = Ks * specularProduct;

if( dot(L, N) < 0.0 ) {

specular = vec4(0.0, 0.0, 0.0, 1.0);

}

gl_Position = projectionMatrix * modelViewMatrix * vPosition;

fColor = ambient + diffuse +specular;

fColor.a = 1.0;

}

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Fragment Shader for Sphere 1

<script id="fragment-shader" type="x-shader/x-fragment">

precision mediump float;


void main()

{

gl_FragColor = fColor;

}

</script>

Vertex Shader for Sphere 2

<script id="vertex-shader" type="x-shader/x-vertex">



varying vec3 N, L, E;




void main()

{



L = normalize( light - pos );

E = -pos;

N = normalize( (modelViewMatrix*vNormal).xyz);


}

</script>

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Fragment Shader for Sphere 2<script id="fragment-shader" type="x-shader/x-fragment">


uniform vec4 ambientProduct;

uniform vec4 diffuseProduct;

uniform vec4 specularProduct;



void main()

{

vec4 fColor;







if( dot(L, N) < 0.0 ) {

specular = vec4(0.0, 0.0, 0.0, 1.0);

}

fColor = ambient + diffuse +specular;

fColor.a = 1.0;


}

</script>

Vertex Shader for 1





uniform vec4 ambientProduct, diffuseProduct, specularProduct;





Vertex Shader for 1

void main()

{



vec3 L = normalize( light - pos );

vec3 E = normalize( -pos ); // Eye


// Transform vertex normal into eye coordinates

vec3 N = normalize( (modelViewMatrix*vNormal).xyz);

// Compute terms in the illumination equation


Vertex Shader for 1





if ( dot(L, N) < 0.0 ) {

specular = vec4(0.0, 0.0, 0.0, 1.0);

}


fColor = ambient + diffuse + specular;

fColor.a = 1.0;

}

Fragment Shader for Sphere 1




void main()

{


}

</script>

Vertex Shader for Sphere 2


...

void main()

{



L = normalize( light - pos );

E = -pos;

N = normalize( (modelViewMatrix*vNormal).xyz);


}

</script>

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Fragment Shader for 2



uniform vec4 ambientProduct;

uniform vec4 diffuseProduct;

uniform vec4 specularProduct;



void main() {

vec4 fColor;



Fragment Shader for 2 float Kd = max( dot(L, N), 0.0 );




if ( dot(L, N) < 0.0 ) {

specular = vec4(0.0, 0.0, 0.0, 1.0);

}

fColor = ambient + diffuse + specular;

fColor.a = 1.0;


}

</script>

Difference in JavaScript 1 & 2

function triangle(a, b, c)

{

normalsArray.push(a);

normalsArray.push(b);

normalsArray.push(c);

pointsArray.push(a);

pointsArray.push(b);

pointsArray.push(c);

index += 3;

}

The essential difference is how we compute normals

special case: normal @ p for sphere at origin is p

Difference in JavaScript 3 & 4

function triangle(a, b, c)

{

var t1 = subtract(b, a);

var t2 = subtract(c, a);

var normal = normalize(cross(t1, t2));

normal = vec4(normal);

normalsArray.push(normal);



... p0

p1

p2

n

p

Early Fragment Test

Don't perform full computation for fragments we will cull

Early Fragment Test

Don't perform full computation for fragments we will cull

Two passes over fragments

First pass computes the depth

Second pass checks the depth: if this fragment is live, it computes the color

Don't need to write to depth buffer in second pass

Homework

Cube Rotations

Lost World – check out Karl Sim's Evolved Virtual Life

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Conclusion

Lighting is difficult

There are techniques that model reality

The implementations often simplify reality to achieve speed

We have looked at the most common model

Purely local: for example, does not create shadows, refraction, reflections, caustics, …

For other effects, need fancier shaders or Ray Tracing

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1 illumination and shading day 6, 2013 © jeff parker

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