# Lesson 5:

### Contents

1. Introduction
3. Rendering Equation
4. The BRDF!
5. BRDF Approximations
6. Light Sources

# Introduction

## What is Light?

### Electro-magnetic wave propagating at speed of light

Radiometry - the study of the propagation of electromagnetic radiation in an environment

# Introduction

## What is Light?

### Ray

• Linear propagation
• Geometric optics

### Particle

• Light comes in discrete energy quanta: photons
• Quantum theory: interaction of light with matter

# Introduction

## What is Light?

### Light in Computer Graphics

• Linearity: The combined effect of two inputs to an optical system is always equal to the sum of the effects of each of the inputs individually
• Energy conservation: When light scatters from a surface or from participating media, the scattering events can never produce more energy than they started with
• No polarisation: We will ignore polarisation of the electromagnetic field; therefore, the only relevant property of light is its distribution by wavelength (or, equivalently, frequency)
• No fluorescence or phosphorescence: The behaviour of light at one wavelength is completely independent of light’s behaviour at other wavelengths or times
• Steady state: Light in the environment is assumed to have reached equilibrium, so its distribution isn’t changing over time
(this happens nearly instantaneously with light in realistic scenes, so it is not a limitation in practice)

### Ray

• Linear propagation
• Geometric optics

## Basic Quantities

There are four radiometric quantities that are central to rendering

### Intensity

They can each be derived from energy by successively taking limits over time, area, and directions

### Energy

Thus all of the radiometric quantities are in general wavelength dependent

Energy is measured in joules ($$J$$). Sources of illumination emit photons, each of which is at a particular wavelength and carries a particular amount of energy. All of the basic radiometric quantities are effectively different ways of measuring photons.

A photon at wavelength $$\lambda$$ carries energy

$$Q= \frac{hc}{\lambda},$$

where $$c \approx 3\times 10^8 m/s$$ is the speed of light and $$h\approx 6,626\times 10^{-34} m^2kg/s$$ is Planck's constant

## Basic Quantities

There are four radiometric quantities that are central to rendering

### Energy

$$Q= \frac{hc}{\lambda},$$

Radiant power (Flux) is the total amount of energy passing through a surface or region of space per unit time. Radiant power can be found by taking the limit of differential energy per differential time:

$$\Phi = \frac{dQ}{dt}$$

Its units are joules/second ($$J/s$$), or more commonly, watts ($$W$$). Total emission from light sources is generally described in terms of flux.

Note that the total amount of flux measured on either of the two spheres is the same - although less energy is passing through any local part of the large sphere than the small sphere, the greater area of the large sphere means that the total flux is the same.

Flux from a point light source measured by the total amount of energy passing through imaginary spheres around the light

## Basic Quantities

There are four radiometric quantities that are central to rendering

### Flux

$$\Phi = \frac{dQ}{dt}$$

### Energy

$$Q= \frac{hc}{\lambda},$$

Irradiance (the area density of flux arriving at a surface), or radiant exitance (the area density of flux leaving a surface) is the average density of power over the area over which photons per time is being measured:

$$E=\frac{\Phi}{A}$$

Its units are $$W/m^2$$.

Flux from a point light source measured by the total amount of energy passing through imaginary spheres around the light

E_1=\frac{\Phi}{4\pi r^{2}_{1}}
E_2=\frac{\Phi}{4\pi r^{2}_{2}}

More generally, we can define irradiance by taking the limit of differential power per differential area at a point $$p$$:

$$E(p) = \lim_{\Delta A \rightarrow 0} \frac{\Delta\Phi(p)}{\Delta A} = \frac{d\Phi(p)}{dA}$$

## Basic Quantities

There are four radiometric quantities that are central to rendering

### Flux

$$\Phi = \frac{dQ}{dt}$$

### Energy

$$Q= \frac{hc}{\lambda},$$

Lambert’s Law: Irradiance arriving at a surface varies according to the cosine of the angle of incidence of illumination, since illumination is over a larger area at larger incident angles.

Consider a light source with area $$A$$ and flux $$\Phi$$ that is illuminating a surface. If the light is shining directly down on the surface, then the area on the surface receiving light $$A_1$$ is equal to $$A$$. Irradiance at any point inside $$A_1$$ is then $$E_1=\frac{\Phi}{A}$$

However, if the light is at an angle to the surface, the area on the surface receiving light is larger. If $$A$$ is small, then the area receiving flux, $$A_2$$ , is roughly $$A/\cos\Theta$$. For points inside $$A_2$$, the irradiance is therefore $$E_2=\frac{\Phi\cos\Theta}{A}$$

Irradiance (the area density of flux arriving at a surface), or radiant exitance (the area density of flux leaving a surface) is the average density of power over the area over which photons per time is being measured:

$$E=\frac{\Phi}{A}$$

Its units are $$W/m^2$$.

## Basic Quantities

There are four radiometric quantities that are central to rendering

### Flux

$$\Phi = \frac{dQ}{dt}$$

$$E=\frac{\Phi}{A}$$

### Energy

$$Q= \frac{hc}{\lambda},$$

### Planar angle

The planar angle $$\theta$$ is the total angle subtended by some object with respect to some position.

If we project a shaded object onto a unit circle, some length of the circle $$l$$ will be covered by its projection. The arc length ($$l=\theta$$) is the angle subtended by the object.

Planar angles are measured in radians.

### Solid angle

The solid angle $$\omega$$ extends the 2D unit circle to a 3D unit sphere. The total area $$s$$ is the solid angle subtended by the object.

Solid angles are measured in steradians [$$sr$$].

The entire sphere subtends a solid angle of $$4\pi sr$$, and a hemisphere subtends $$2\pi sr$$.

## Basic Quantities

There are four radiometric quantities that are central to rendering

### Flux

$$\Phi = \frac{dQ}{dt}$$

$$E=\frac{\Phi}{A}$$

### Energy

$$Q= \frac{hc}{\lambda},$$

### Intensity | Solid Angle

Intensity is the angular density of emitted power. For an infinitesimal light source surrounded with a unit sphere, we can compute the angular density of emitted power over the entire sphere of directions:

$$I=\frac{\Phi}{4\pi}$$

Its units are $$W/sr$$. But more generally we are interested in taking the limit of a differential cone of directions:

$$I=\lim_{\Delta\omega\rightarrow 0}\frac{\Delta\Phi}{\Delta\omega}=\frac{d\Phi}{d\omega}$$

Intensity describes the directional distribution of light, but it is only meaningful for point light sources

## Basic Quantities

There are four radiometric quantities that are central to rendering

### Flux

$$\Phi = \frac{dQ}{dt}$$

$$E=\frac{\Phi}{A}, E(p)=\frac{d\Phi(p)}{dA}$$

### Energy

$$Q= \frac{hc}{\lambda},$$

### Intensity

$$I=\frac{d\Phi}{d\omega}$$

Radiance is the flux density per unit area and per unit solid angle. In terms of flux, it is defined by:

$$L=\frac{d\Phi}{d\omega dA^{\bot}},$$

where $$dA^\bot$$ is the projected area of $$dA$$ on a hypothetical surface perpendicular to $$\omega$$.

Radiance $$L$$ is defined as flux per unit solid angle $$d\omega$$ per unit projected area $$dA^\bot$$

$$L(p,\omega)=\lim_{\Delta\omega\rightarrow 0}\frac{\Delta E_\omega(p)}{\Delta\omega} = \frac{dE_\omega(p)}{d\omega},$$

where $$E_\omega$$ denotes irradiance at the surface that is perpendicular to the direction $$\omega$$.

## Incident and Exitant Radiance Functions

We make a distinction between radiance arriving at the point (e.g., due to illumination from a light source) and radiance leaving that point (e.g., due to reflection from a surface)

The incident radiance function $$L_i(p, \omega)$$ describes the distribution of radiance arriving at a point as a function of position and direction

The exitant radiance function $$L_o(p, \omega)$$ gives the distribution of radiance leaving the point

L_i(p, \omega)
L_o(p,\omega)
p
p

Note that for both functions, $$\omega$$ is oriented to point away from the surface, and, thus, for example, $$L_i(p,-\omega)$$ gives the radiance arriving on the other side of the surface than the one where $$\omega$$ lies

# Photometry

Photometry is the study of visible electromagnetic radiation in terms of its perception by the human visual system

All of the radiometric measurements like flux, radiance, etc. have corresponding photometric measurements

### Luminosity Function

Our visual system responds differently to different wavelengths

• Luminous efficiency function $$V(\lambda)$$ (aka spectral response curve) describes the relative sensitivity of the human eye to various wavelengths
• Represents the average human spectral response
• Separate curves exist for light and dark adaptation of the eye

Each radiometric quantity can be converted to its corresponding photometric quantity by integrating against the  luminosity function.

### Example

Radiance $$L(\lambda)$$ can be converted to luminance $$Y$$ as

$$Y = \int_\lambda L(\lambda)V(\lambda)d\lambda$$

# Photometry

Photometry is the study of visible electromagnetic radiation in terms of its perception by the human visual system

All of the radiometric measurements like flux, radiance, etc. have corresponding photometric measurements

Radiant energy joule (Q) Luminous energy talbot (T)
Radiant flux watt (W) Luminous flux lumen (lm)
Intensity W / sr Luminous flux lm / sr = candela (cd)
Irradiance W / m^2 Illuminance lm / m^2 = lux (lx)
Radiance W / (m^2 sr) Luminance lm / (m^2 sr) = cd / m^2 = nit

# The Rendering Equation

### The most important equation in Computer Graphics

p
\vec{n}
\omega_o
\omega_i
L_o(p,\omega_o)=L_{emitted}(p,\omega_o)+L_{reflected}(p, \omega_o, \omega_i)
\theta_i

In Physics called radiative transport equation and expresses energy equilibrium in scene:

Emitted radiance ($$L_e$$) describes the emissivity of the surface and is non-zero only for light sources

Reflected radiance ($$L_r$$) is in general an integral over all possible incoming directions of radiance times angle-dependent surface reflection function.

It leads to Fredholm integral equation of second kind: thus, numerical methods are necessary to compute an  approximate solution

# The Rendering Equation

### Example

Computation of irradiance at a point

Irradiance at a point $$p$$ with surface normal $$\vec{n}$$ due to radiance over a set of directions $$\Omega$$ is

$$E(p,\vec{n}) = \int_\Omega L_i(p, \omega_i)\left|\cos\theta_i\right|d\omega,$$

where $$L_i(p,\omega_i)$$ is the incident radiance function and the $$\cos\theta_i$$ term in this integral is due to the $$dA^\bot$$ term in the definition of radiance

• $$\theta_i$$ is measured as the angle between $$\omega_i$$ and the surface normal $$\vec{n}$$
• Irradiance is usually computed over the hemisphere $$H^2(\vec{n})$$ of directions about a given surface normal $$\vec{n}$$
p
\vec{n}
\omega_i
\theta_i

# The Rendering Equation

### Integrals over Spherical Coordinates

Computation of irradiance at a point

Irradiance at a point $$p$$ with surface normal $$\vec{n}$$ due to radiance over a set of directions $$\Omega$$ is

$$E(p,\vec{n}) = \int_\Omega L_i(p, \omega)\left|\cos\theta\right|d\omega$$

p
\vec{n}
\theta
dS
r
\phi
d\phi
du
dv
d\omega=\frac{dS}{r^2}
d\theta
dS=du\cdot dv
du = r\cdot d\theta
dv = r\cdot\sin\theta\cdot d\phi
=r^2\cdot\sin\theta\cdot d\theta\cdot d\phi
=\sin\theta\cdot d\theta\cdot d\phi

# The Rendering Equation

### Integrals over Spherical Coordinates

Computation of irradiance at a point

Irradiance at a point $$p$$ with surface normal $$\vec{n}$$ due to radiance over a set of directions $$\Omega$$ is

$$E(p,\vec{n}) = \int_\Omega L_i(p, \omega)\left|\cos\theta\right|d\omega$$

d\omega=\frac{dS}{r^2}
=\sin\theta\cdot d\theta\cdot d\phi

We can thus see that the irradiance integral over the hemisphere can equivalently be written as

$$E(p,\vec{n}) = \int^{2\pi}_{0}\int^{\pi/2}_{0} L_i(p, \theta,\phi)\cos\theta \sin\theta d\theta d\phi$$

Note: if the radiance is the same from all directions, the equation simplifies to

$$E=\pi L_i$$

# The Rendering Equation

### Integrals over Area

Computation of irradiance at a point

Irradiance at a point $$p$$ with surface normal $$\vec{n}$$ due to radiance over a set of directions $$\Omega$$ is

$$E(p,\vec{n}) = \int_\Omega L_i(p, \omega)\left|\cos\theta\right|d\omega$$

d\omega=\frac{dS\cdot\cos\theta}{r^2}

# The Rendering Equation

### Integrals over Area

Computation of irradiance at a point

Irradiance at a point $$p$$ with surface normal $$\vec{n}$$ due to radiance over a set of directions $$\Omega$$ is

$$E(p,\vec{n}) = \int_\Omega L_i(p, \omega)\left|\cos\theta\right|d\omega$$

d\omega=\frac{dS\cdot\cos\theta}{r^2}

Therefore, we can write the irradiance integral for the quadrilateral source as

$$E(p,\vec{n}) = \int_S L\cos\theta_i\frac{\cos\theta_o}{r^2}dS$$

p
\vec{n}
\theta_i
\theta_o
d\omega
dS

To compute irradiance at a point $$p$$ from a quadrilateral source, it’s easier to integrate over the surface area of the source than to integrate over the irregular set of directions that it subtends. Thus $$L$$ here is the emitted radiance from the surface of the quadrilateral source.

# The Rendering Equation

We would like to know how much radiance is leaving the surface in the direction $$\omega_o$$ toward the camera, $$L_o(p,\omega_o)$$, as a result of incident radiance along the direction $$\omega_i$$, $$L_i(p,\omega_i)$$.

p
\vec{n}
\omega_o
\omega_i
\theta_i
L_o(p,\omega_o)=L_e(p,\omega_o)+L_r(p, \omega_o, \omega_i)

Because of the linearity assumption from geometric optics, the reflected differential radiance is proportional to the irradiance:

$$dL_r(p,\omega_o)\propto dE(p,\omega_i)$$

or

$$dL_r(p,\omega_o) = k\cdot dE(p,\omega_i)$$

or

$$dL_r(p,\omega_o) = f_r(p,\omega_o,\omega_i)\cdot dE(p,\omega_i)$$

Recall: Irradiance at a point $$p$$ with surface normal $$\vec{n}$$ due to radiance over a set of directions $$\Omega$$ is

$$E(p,\omega_i) = \int_\Omega L_i(p, \omega_i)\left|\cos\theta_i\right|d\omega$$

# The Rendering Equation

We would like to know how much radiance is leaving the surface in the direction $$\omega_o$$ toward the camera, $$L_o(p,\omega_o)$$, as a result of incident radiance along the direction $$\omega_i$$, $$L_i(p,\omega_i)$$.

p
\vec{n}
\omega_o
\omega_i
\theta_i
L_o(p,\omega_o)=L_e(p,\omega_o)+L_r(p, \omega_o, \omega_i)

Because of the linearity assumption from geometric optics, the reflected differential radiance is proportional to the irradiance:

$$dL_r(p,\omega_o)=f_r(p,\omega_o,\omega_i) L_i(p,\omega_i)\cos\theta_id\omega$$

Recall: Irradiance at a point $$p$$ with surface normal $$\vec{n}$$ due to radiance over a set of directions $$\Omega$$ is

$$E(p,\omega_i) = \int_\Omega L_i(p, \omega_i)\left|\cos\theta_i\right|d\omega$$

The rendering Equation:

$$L_o(p,\omega_o) = L_e(p,\omega_o) + \int_{H^2(\vec{n})} f_r(p,\omega_o,\omega_i)L_i(p,\omega_i)\cos\theta_i d\omega_i$$

# The Rendering Equation

p
\vec{n}
\omega_o
\omega_i
\theta_i
L_o(p,\omega_o) = L_e(p,\omega_o) + \int_{H^2(\vec{n})} f_r(p,\omega_o,\omega_i)L_i(p,\omega_i)\cos\theta_i d\omega_i

The bidirectional reflectance distribution function (BRDF) gives a formalism for describing reflection from a surface

$$f_r(p,\omega_o,\omega_i) = \frac{dL_o(p,\omega_o)}{dE(p,\omega_i)}$$

## BRDF

BRDF
is the key term of the rendering equation

BRDF
is the core of every software shader class

# The Rendering Equation

The bidirectional reflectance distribution function (BRDF) gives a formalism for describing reflection from a surface

$$f_r(p,\omega_o,\omega_i) = \frac{dL_o(p,\omega_o)}{dE(p,\omega_i)}$$

## BRDF Properties

### Reciprocity

Helmholtz reciprocity principle: BRDF remains unchanged if incident and reflected directions are interchanged

$$f_r(p,\omega_o,\omega_i) = f_r(p,\omega_i,\omega_o)$$

### Energy Conservation

The total energy of light reflected is less than or equal to the energy of incident light. For all directions $$\omega_o$$

$$\int_{H^2(\vec{n})} f_r(p,\omega_o,\omega_i) \cos\theta_i d\omega_i \le 1$$

## BRDF Measurement

A devise to measure BRDF is called Gonioreflectometer

• Point light source position $$(\theta_i,\phi_i)$$
• Light detector position $$(\theta_o,\phi_o)$$

4 directional degrees of freedom

BRDF representation

• $$m$$ incident direction samples $$(\theta_i,\phi_i)$$
• $$n$$ outgoing direction samples $$(\theta_o,\phi_o)$$
• $$m\times n$$ reflectance values (large!!!)

## Rendering from Measured BRDF

### Linearity, superposition principle

• Complex illumination: integrating light distribution against BRDF
• Sampled BRDF: superimposed point light sources

### Interpolation

• Look-up during rendering
• Sampled BRDF must be filtered

### BRDF Modeling

• Fit parameterised BRDF model to measured data
• Continuous function
• No interpolation
• Fast evaluation

## BRDF of various materials

### Reflectance may vary with

• Illumination angle
• Viewing angle
• Wavelength
• Polarisation
• ...

### Variations due to

• Absorption
• Surface micro-geometry
• Index of refraction / dielectric constant
• Scattering

Aluminium; $$\lambda=2.0\mu m$$

Aluminium; $$\lambda=0.5\mu m$$

Magnesium; $$\lambda=0.5\mu m$$

# BRDF Approximation

In case when BRDF measurement is impossible, an approximated model may be used

### Mirror

• Reflection law
• Ideal specular reflection

### Diffuse

• Lambert's law
• Ideal diffuse reflection
• Matte surfaces

### Glossy

• Directional diffuse
• Shiny surfaces

### BRDF

L_o(p,\omega_o)=L_e(p,\omega_o)+L_{r,d}(p, \omega_o, \omega_i)+L_{r,m}(p, \omega_o, \omega_i)+L_{r,s}(p, \omega_o, \omega_i)

# BRDF Approximation

## Ideal Diffuse Reflection

Light equally likely to be reflected in any output direction (independent of input direction)

### Constant BRDF

$$f_r(p,\omega_o,\omega_i)=k_d=const$$

\vec{n}
\vec{I}

### Rendering Equation

$$L_{r,d}(p,\omega_o) = \int_{H^2(\vec{n})} k_dL_i(p,\cdot)\cos\theta_i d\omega_i = k_d L_i\cos\theta_i\int_{H^2(\vec{n})} d\omega_i = 2\pi k_d L_i \cos\theta_i \propto k_dL_i(\vec{n}\cdot\vec{I})$$

# BRDF Approximation

## Lambertian Diffuse Reflection

Light equally likely to be reflected in any output direction (independent of input direction)

### Constant BRDF

$$f_r(p,\omega_o,\omega_i)=k_d=const$$

### Rendering Equation

$$L_{r,d}(p,\omega_o) = k_dL_i(\vec{n}\cdot\vec{I})$$

\vec{n}
\vec{I}
\vec{n}
\vec{I}
\vec{n}
\vec{I}

### Self-Luminous spherical Lambertian Light Source

$$E\propto L_o\cdot d\omega$$

### Eyelight Illuminated Spherical Lambertian Reflector

$$E\propto L_o\cdot\cos\theta\cdot d\omega$$

# Lambertian Objects

CShaderFlat
CShaderEyelight
\vec{n}
d\omega
\theta
d\omega

## The Sun

• Absorption in photosphere

• Path length through photosphere longer from the Sun’s rim

## The Moon

• Surface covered with fine dust

• Dust on TV visible best from slanted viewing angle

# Lambertian Objects

Neither the Sun nor the Moon are Lambertian

# BRDF Approximation

## Ideal Specular Reflection

Angle of reflectance $$\theta_o$$ is equal to angle of incidence $$\theta_i$$

\vec{n}
\vec{I}
\vec{r}

### Auxiliary Variables:

\theta_i
\theta_o
\vec{n}'
\vec{n}'=\vec{n}\cos\theta_i=\vec{n}\left(\vec{n}\cdot\vec{I} \right)
\vec{a}=\vec{n}'-\vec{I}

### Reflected Ray:

\vec{r} = ?
=\vec{I}+2\vec{a}
\vec{a}
=\vec{I}+2\vec{n}'-2\vec{I}
=-\vec{I} + 2\vec{n}\left(\vec{n}\cdot\vec{I} \right)

# BRDF Approximation

## Ideal Specular Reflection

Angle of reflectance $$\theta_o$$ is equal to angle of incidence $$\theta_i$$

\vec{n}
\vec{I}
\vec{r}
\vec{n}'

### Reflected Ray:

\vec{r} = ?
\vec{a}
=-\vec{I}+2\vec{n}\left(\vec{n}\cdot\vec{I} \right)

### Reflected Ray:

\vec{r}=\vec{d}-2\vec{n}\left(\vec{n}\cdot\vec{d} \right)
\vec{n}
\vec{d}
\vec{r}
\vec{n}'
\vec{a}
\theta_i
\theta_o
\theta_i
\theta_o

# BRDF Approximation

## Ideal Specular Reflection

Dirac delta function $$\delta(x)$$ equal to zero everywhere except at $$x=0$$

### Mirror BRDF

$$f_r(p,\omega_o,\omega_i)=\frac{\delta(\cos\theta_i-\cos\theta_o)}{\cos\theta_i}\cdot\delta(\phi_i-\phi_o\pm\pi)$$

### Rendering Equation

$$L_{r,m}(p,\omega_o) = \int_{H^2(\vec{n})} \frac{\delta(\cos\theta_i-\cos\theta_o)}{\cos\theta_i}\delta(\phi_i-\phi_o\pm\pi)L_i(p,\theta_i,\phi_i)\cos\theta_i d\omega_i = L_i(p,\theta_o,\phi_o\pm\pi)$$

\vec{n}
\vec{I}
\vec{r}
\theta_i
\theta_o

side view

top view

\phi_i
\phi_o
\vec{r}
\vec{I}

# BRDF Approximation

## Directional Diffuse Reflection

### (Glossy Reflection)

Glossy reflection appears due to surface roughness

# BRDF Approximation

## Phong

### BRDF

$$f_r(p,\omega_o,\omega_i)=k_s\cos\theta^{k_e}_{\vec{r}\vec{v}}=k_s\left(\vec{r}\cdot\vec{v} \right)^{k_e}$$

### Rendering Equation

$$L_{r,s}(p,\omega_o) = k_sL_i\left(\vec{r}\cdot\vec{v}\right)^{k_e}$$

\vec{n}
\vec{I}
\vec{r}
\vec{v}
\theta_{\vec{r}\vec{v}}

# BRDF Approximation

## Phong

### Rendering Equation

$$L_{r,s}(p,\omega_o) = k_sL_i\left(\vec{r}\cdot\vec{v}\right)^{k_e}$$

\vec{n}
p
\vec{I}
\vec{r}
k_e
\cos\theta
\cos^2\theta
\cos^8\theta
\cos^{64}\theta

# BRDF Approximation

## Blinn-Phong

Make use of a halfway vector $$\vec{h}=(\vec{I}+\vec{v})/|\vec{I}+\vec{v}|$$ in order to approximate Phong reflectiion model.

### BRDF

$$f_r(p,\omega_o,\omega_i)=k_s\cos\theta^{k_e}_{\vec{h}\vec{n}}=k_s\left(\vec{h}\cdot\vec{n} \right)^{k_e}$$

### Rendering Equation

$$L_{r,s}(p,\omega_o) = k_sL_i\left(\vec{h}\cdot\vec{n}\right)^{k_e}$$

\vec{n}
\vec{I}
\vec{r}
\vec{v}
\theta_{\vec{r}\vec{v}}
\theta_{\vec{h}\vec{n}}
\vec{h}

# BRDF Approximation

### Wrap-up

We have considered 3 elemental components that can be used to model a variety of light-surface interactions

### BRDF

L_o=L_e+L_{r,a}+L_{r,d}+L_{r,s}+(L_{r,m}+L_{r,t})

### Ambient $$L_{r,a}$$

(approximate indirect illumination)

### Perfect Transmission $$L_{r,t}$$

(only possible with recursive raytracing)

# BRDF Approximation

## Phong

$$L_r=k_aL_{i,a}+k_d\sum_lL_i(\vec{I}_l\cdot\vec{n})+k_s\sum_lL_i(\vec{r}(\vec{I}_l)\cdot\vec{v})^{k_e}$$

## Blinn-Phong

$$L_r=k_aL_{i,a}+k_d\sum_lL_i(\vec{I}_l\cdot\vec{n})+k_s\sum_lL_i(\vec{h}\cdot\vec{n})^{k_e}$$

Here index $$l$$ represents point light sources and the color of specular reflection equals to the light source

### Both Phong and Blinn-Phong reflection models are empirical (heuristic) models

• Purely local illumination
• Only direct light from the light sources
• No further reflection on other surfaces
• Constant ambient term
• Often light sources and viewer assumed to be far away

# BRDF Approximation

## Microfacet Model

### Idea

• Rough surfaces can be modelled as a collection of small microfacets
• The distribution of faces may be described statistically

### Microfacet surface models are often described by a function that gives the distribution of microfacet normals with respect to the surface normal

The greater the variation of microfacet normals, the rougher the surface is

Smooth surfaces have relatively little variation of microfacet normals

\vec{n}
\vec{n}
\vec{n}_f
\vec{n}_f

# BRDF Approximation

## Microfacet Model

### Inter-reflection

Light bounces among the microfacets before reaching the viewer

Analogously, light does not reach the microfacet

The microfacet of interest is not visible to the viewer due to occlusion by another microfacet

# BRDF Approximation

## Torrance-Sparrow

### Idea:

For directions $$\omega_i$$ and $$\omega_o$$, only microfacets with normal $$\omega_h$$ reflect light (i.e. $$\vec{n}_f\equiv\vec{h}$$)

### BRDF

$$f_r(p,\omega_o,\omega_i)=\frac{D(\omega_h)G(\omega_i, \omega_o)F_r(\omega_o)}{4\cos\theta_i\cos\theta_i}$$

\vec{n}
\vec{n}_f
\vec{I}
\vec{v}
\theta_i
\theta_o
\omega_o
\omega_i
\omega_h
• $$D(\omega_h)$$: statistical microfacet distribution function, that gives the probability that a microfacet has orientation $$\omega_h$$

• $$G(\omega_i, \omega_o)$$: geometric attenuation term, which describes the fraction of microfacets that are masked or shadowed

• $$F_r(\omega_o)$$: Fresnel term, computed by Fresnel equation and relates incident light to reflected light for each planar microfacet
\theta_h

# BRDF Approximation

## Torrance-Sparrow

### Statistical microfacet distribution function

As an approximation we may assume isotropic microfacet distributions: $$D(\omega_h)\equiv D(\theta_h)$$, where $$\theta_h=\vec{n}\cdot\vec{h}$$

### BRDF

$$f_r(p,\omega_o,\omega_i)=\frac{D(\omega_h)G(\omega_i, \omega_o)F_r(\omega_o)}{4\cos\theta_i\cos\theta_i}$$

\vec{n}
\vec{h}
\vec{I}
\vec{v}
\theta_i
\theta_o
\omega_o
\omega_i
\omega_h

Blinn: $$D(\theta_h)=\frac{e+2}{2\pi}\cos^e\theta_h$$

Torrance - Sparrow:
$$D(\theta_h)=e^{-\left(\frac{\theta_h}{\sqrt{2}\alpha}\right)^2}$$

Beckmann - Spizzichino:
$$D(\theta_h)=\frac{1}{4\sigma^2\cos^4\theta_h}e^{-\tan^2\theta_h/\sigma^2}$$

\theta_h

# BRDF Approximation

## Torrance-Sparrow

### Geometric Attenuation Factor

\vec{n}
\vec{h}
\vec{I}
\vec{v}
\theta_i
\theta_o
\omega_o
\omega_i
\omega_h

Fully illuminated and visible: $$G(\omega_i,\omega_o)=1$$

$$G(\omega_i,\omega_o)=\frac{2\cos\theta_h\cos\theta_o}{(\vec{v}\cdot\vec{h})}$$

$$G(\omega_i,\omega_o)=\frac{2\cos\theta_h\cos\theta_i}{(\vec{v}\cdot\vec{h})}$$

\theta_h

The resulting geometric attenuation factor:

$$G(\omega_i,\omega_o)=\min\left(1,\frac{2(\vec{n}\cdot\vec{h})(\vec{n}\cdot\vec{v})}{(\vec{v}\cdot\vec{h})},\frac{2(\vec{n}\cdot\vec{h})(\vec{n}\cdot\vec{I})}{(\vec{v}\cdot\vec{h})}\right)$$

# BRDF Approximation

## Phong VS Torrance-Sparrow

### Torrance-Sparrow:

Directional light

• Spot-lights
• Projectors
• Distant sources

Diffuse emitters

• Torchieres
• Frosted glass lamps

Ambient light

• “Photons everywhere”

### Emitting area

Volume

• Sodium vapor lamps

Area

• CRT, LCD display
• (Overcast) sky

Line

• Clear light bulb, filament

“Point”

• Xenon lamp
• Arc lamp
• Laser diode

# Light Sources

## Light Source Classification

Power (total flux)

• Emitted energy / time

Active emission size

• Point, line, area, volume

Spectral distribution

• Thermal, line spectrum

Directional distribution

• Goniometric diagram

# Light Sources

## Light Source Specifications

### Point light with isotropic radiance

Power (total flux) of a point light source

$$\Phi$$

# Light Sources

## Point Light Source

p
\theta
d\omega
dS

Intensity of a light source (radiance cannot be defined, no area)

$$I=\frac{\Phi}{4\pi}$$

Irradiance on a sphere with radius $$r$$ around light source:

$$E=\frac{\Phi}{4\pi r^2}$$

Irradiance on some other surface $$S$$

$$E(p)=\frac{d\Phi}{dS}=I\frac{d\omega}{dS}=\frac{\Phi}{4\pi}\frac{dS\cos\theta}{r^2dS}=\frac{\Phi\cos\theta}{4\pi r^2}$$

### Irradiance $$E$$: power per $$m^2$$

• Illuminating quantity

### Distance-dependent

• Double distance from emitter: area of sphere is four times bigger

### Irradiance falls off with inverse of squared distance

• Only for point light sources

# Light Sources

## Point Light Source

### Inverse Square Law

Flux from a point light source measured by the total amount of energy passing through imaginary spheres around the light

E_1=\frac{\Phi}{4\pi r^{2}_{1}}
E_2=\frac{\Phi}{4\pi r^{2}_{2}}
\frac{E_1}{E_2}=\frac{r^{2}_{2}}{r^{2}_{1}}