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February 5, 2015 1 [email protected]
February 5, 2015 2
“In trying to improve the quality of the synthetic images, we do
not expect to be able to display the object exactly as it would
appear in reality, with texture, overcast shadows, etc. We
hope only to display an image that approximates the real
object closely enough to provide a certain degree of realism.”
Bui Tuong Phong, 1975
February 5, 2015 3
Objects are made visible by light. Our visual perception of an object is
determined by the form and quality of the illumination. Lighting defines or
influences color, texture, brightness, contrast, and even the mood of a scene.
Lighting
Light source - There are different types of sources of light, such as point sources
(e.g., a small light at a distance), extended sources (e.g., the sky on a cloudy
day), and secondary reflection (e.g., light that bounces from one surface to
another).
February 5, 2015 [email protected] 5
What are the patterns of light in this room?
Projector as light source
Light transmitted through windows
Blue light reflecting from screen
Blackboard is matte surface
Edge of screen is shiny surface
Shadows underneath the desks
February 5, 2015 6
If we don’t have lighting effects nothing looks three dimensional,to a great degree
the realism of a three-dimensional object is determined by its lighting.
Some solid objects are virtually impossible to represent without lighting effects.
Figure below shows the enhanced realism that results from lighting effects on a
solid object.
Light
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Light itself is an electromagnetic wave. Electromagnetic waves are similar to
sound waves in that they contain different frequencies, but are electromagnetic
and can propagate in vacuum. EM waves are thus a signal that's made out of
one or more frequencies, for example the EM waves used by a microwave oven
are very high frequency, while radio waves are very low frequency.
Monochromatic light is light made up of one single pure frequency (this is
certainly not the general case, most light you see is multichromatic).
Light
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modeled as
electromagnetic waves, radiation
photons
geometric rays
photons
particles
characterized by wavelength (perceived as color in the visible spectrum)
carry energy (inversely proportional to the wavelength)
travel along a straight line at the speed of light
Light
February 5, 2015 9
The Eye and Color Perception
light falls on the retina, and on the retina are 2 types of cells with photosensitive
chemicals, photoreceptors: rods and cones.
The rods only detect whether or not light is present.
To detect color, you'd need photoreceptors that are sensitive to only a certain
frequency. That's exactly what the cones do:
There are 3 types of cones, those that are sensitive to red, those sensitive to
green, and those sensitive to blue.
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there are 8 (2^3) main colors one can distinguish:
No cones excited: Black
Red cones excited, but not the Blue and Green ones: Red
Green cones excited, but not the Blue and Red ones: Green
Blue cones excited, but not the Red and Green ones: Blue
Red and Green cones excited, but no the Blue ones: Yellow
Blue and Green cones excited, but no the Red ones: Cyan
Red and Blue cones excited, but no the Green ones: Magenta
All three the cone types excited: White
Colors
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Radiometric Units
• Light is a form of energy … measured in Joules (J)
• Power: energy per unit time … measured in Joules/sec = Watts (W)
Also called Radiant Flux (Φ)
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In computer graphics, we traditionally take a simplified view of how light
propagates through space. Photons travel along straight paths until they hit a
surface boundary and are then reflected according to a reflection function of
some sort. A single photon will carry a certain amount of energy, which is
represented by its wavelength.
The relationship between its wavelength λ and the amount of energy it
carries (ΔE) is given by
λ ΔE = 1239.9,
where ΔE is measured in electron volts (eV).
Radiometric Units
February 5, 2015 14
Radiometric Quantities
radiant energy Q
photons have some radiant energy
radiant flux , radiant power P
rate of flow of radiant energy per unit time:
e. g., overall energy of photons emitted by a source per time
flux density (irradiance, radiant existence)
radiant flux per unit area:
rate at which radiation is incident on, or exiting a flat surface area dA
describes strength of radiation with respect to a surface area
no directional information
February 5, 2015 15
Radiometric Quantities Irradiance
irradiance measures the overall radiant flux (light flow, photons per unit time)
into a surface element Radiometric
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Radiometric Quantities Radiant Intensity
radiant intensity
radiant flux per unit solid angle:
radiant flux (light flow, photons per unit time) incident on, emerging
from, or passing through a point in a certain direction
February 5, 2015 17
Solid Angle
2D angle in 3D space, that an object subtends at a point.
measured in dimensionless unit called steradian (sr).
A steradian is the solid angle subtended at the center of a sphere of radius r
by a portion of the sphere surface with area A=r * r.
It is a measure of how large the object appears to an observer looking from
that point
February 5, 2015 18
Visible Spectrum
photons are characterized by a wavelength within the visible spectrum
from 390 nm to 750 nm.
light consists of a set of photons
the distribution of wavelengths within this set is referred to as the
spectrum of light.
spectra are perceived as colors.
if the spectrum consists of a dominant wavelength, humans perceive a "rainbow"
color (monochromatic)
if all wavelengths are equally distributed, humans perceive a shade of gray,
ranging from black to white (achromatic).
otherwise, colors "mixed from rainbow colors" are perceived (chromatic)
February 5, 2015 [email protected] 21
Relationship between frequency ( ) and wavelength ( )
where c is the speed of light
c
Frequency and Wavelength
February 5, 2015 22
Type of light energy Object condition
all the incident light energy is
absorbed
invisible
nearly all the incident light energy
is absorbed
appears black
only a small fraction is absorbed appears white
the incident light energy is nearly
equally reduced for all w.l.
appears gray
the incident light energy is
selectively reduced for all w.l.
appears colored
The amount of energy absorbed, transmitted or reflected depends on the
wavelength (w.l.) of the light
Frequency and Wavelength
February 5, 2015 [email protected] 26
February 5, 2015 27
There are a variety of simple light types, but the most basic ones are directional
and point lights.
Directional Light
When light is coming from a distant source (like the sun), the light rays are parallel
and assumed to be of uniform intensity distributed over a large area.
Point Lights
For closer light sources, such as light bulbs, we can’t simply use a direction.
A simple way to model a local light source is as a point light that radiates light in
all directions equally.
Light Types
February 5, 2015 28
There are two types of illumination — direct and indirect, The illumination that
an object receives from a light-emitting source is direct. The illumination received
from a light-reflecting source is indirect. Figure below shows a polished sphere
illuminated by direct and indirect lighting, and by a combination of both.
Types of Illumination
February 5, 2015 [email protected] 29
Global illumination algorithms Both
kinds of sources are considered
Local illumination algorithms Only
direct lights are taken into account
According to how they handle these sources, algorithms can be grouped into:
Local and Global illumination
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Local illumination refers to direct interaction between one light source and
one object surface.
Global illumination refers to the interaction of light between all surfaces in a
scene.
Responsible for shading
Reflection between surfaces
Refraction of surfaces
Local and Global illumination
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Operation
Formula
Effect
Negative
255-C
Returns the opposite color, for example white becomes black,
red becomes cyan, ...
Darken
C/p or C-p
Divide the color though some constant (larger than 1), or
subtract a constant from it, to make it darker.
Brighten
C*p or C+p
Multiply the color by some constant (larger than 1), or add a
constant to it, to make it brighter.
Greyscale
(R+G+B)/3
Calculate the average of the 3 channels to get a gray color with
the same brightness.
Remove Channel
R=0, G=0 and/or B=0
By setting one or more channels to 0, you completely remove
that color component from the picture.
Swap Channels
R=G, G=R, ...
Swap the values of two color channels to get an image with a
completely different color.
RGB Arithmetic
February 5, 2015 35
Two models are usually mentioned in the context of lighting: the illumination
model and the reflection model. The illumination model refers to the nature
of light and its intensity distribution. The reflection model describes the
interaction of light within a surface. Both are important in developing light-
rendering algorithms.
A- Illumination Model
The simplest illumination model is one in which each polygon that forms the
object is displayed in a single shade of its own color. The result is a flat
monochromatic rendering in the circular disk on the left-side of Figure, is an
example of rendering without lighting effects.
Light models
February 5, 2015 [email protected] 36
B- Reflection Model
Most of the lighting effects result from reflection. Ambient illumination is
light that has been scattered to such a degree that it is no longer
possible to determine its direction. the case in the right-hand sphere
in Figure below. Ambient light and matte surfaces produce diffuse
reflection. Point sources and polished surfaces produce specular
reflection.
Light models
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Lighting vs. Shading
Commonly misused terms.
What’s the difference?
Lighting designates the interaction between materials and light sources.
Shading is the process of determining the color of a pixel (i.e. Computer Graphics).
Usually determined by lighting.
Could use other methods: random color, NPR, etc.
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The character of the light reflected or transmitted... depends on:
• The surface orientation
• Composition of the light source
• Geometry of the light source
• The surface properties of the object
• Direction of the light source
Reflected Lights
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1- ambient light 2- point sources 3- spotlights 4- distant light
1-Ambient Light
A surface that is not exposed to direct light may still be lit up by reflections
from other nearby objects.
A much simpler way to model all of the background reflected light is to
assume that it is just some constant color shining from every direction
equally ,This is referred to as ambient
2- Point Sources
intensity depends on the distance from the light source
Light Sources
Light is spreading steadily and with equal
intensity in all directions
It is emitting light in all directions equally. You have
to define a position and some attenuation
parameters which will affect the intensity of light
depending on the distance to the lit object.
February 5, 2015 42
3- Spotlight
To turn a point light source into a spotlight we simply add a vector
direction and an angular limit
The spot light is the most
expensive light source to
compute. It is similar to a point
light in that it has colors,
attenuation and a position, but
spot lights have an additional
direction and angle which will
define the area that is lit.
Light Sources
February 5, 2015 43
4- Distant Light Sources
infinite distance to light source, light rays considered parallel, like sun
light on earth.
Light Sources
February 5, 2015 44
Each light in the scene needs to have
- its type specified
- other relevant properties (color, position, direction…).
- Geometric properties are usually specified in world space, getting
transformed to camera space or depending on the implementation
- a set of materials define properties such as: diffuse color, specular color,
and shininess
- a bunch of triangles, Each triangle has a material assigned to it, can also
specify a normal for each vertex
February 5, 2015 45
Objects have an inherent material color which is the color that the object
reflects, The material gets its color because the reflectivity varies with the
wavelength of light.
In CG, we usually don’t represent color as a continuous spectrum. Instead,
we just represent it is a combination of red, green, and blue, Light-Material
Interactions as follow
Material Colors
February 5, 2015 46 [email protected]
February 5, 2015 48
The concept of normal is essential to lighting, in computer graphics, it is most
common to specify normal and perform lighting at the vertices. This gives us a
method of modeling smooth surfaces as a mesh of triangles with ‘shared’ normal at
the vertices. It is often very convenient if normal are unit length
Normal Transformations
Lighting requires accurate measurement of distances and angles, so we want
to compute lighting in a regular 3D space this leaves object space, world
space, or camera space as our most natural options.
To light in object space, we would have to transform the lights from world
space into each object’s space
Lighting in world space require transforming the object into world space.
To light in camera space, as we will probably want to perform clipping & some
culling in this space as well, A normal transforms as a direction, not a
position.
Normal
February 5, 2015 49 [email protected]
