Lighting and Shading

Properties of Light
Light Sources
Phong Illumination Model
Normal Vectors
[Angel, Ch. 6.1-6.4]

Announcements
• Written assignment #1 due Thursday
– Handin at beginning of class
• Programming assignment #2 out Thursday

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The Rendering Equation
• (Angel, Ch 13)

Outline
• Lighting models (OpenGL oriented)
• Reflection models (Phong shading)
• Normals
• Color

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Common Types of Light Sources
• Ambient light: no identifiable source or direction
• Point source: given only by point
• Distant light: given only by direction
• Spotlight: from source in direction
– Cut-off angle defines a cone of light
– Attenuation function (brighter in center)
• Light source described by a luminance
– Each color is described separately
– I = [Ir Ig Ib]T (I for intensity)
– Sometimes calculate generically (applies to r, g, b)

Ambient Light
• Intensity is the same at all points
• This light does not have a direction (or .. it is
the same in all directions)

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Point Source
• Given by a point p0
• Light emitted from that point equally in all
directions

• Intensity decreases with square of distance

One Limitation of Point Sources

• Shading and shadows inaccurate
• Example: penumbra (partial “soft” shadow)

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Distant Light Source
• Given by a vector v
• Intensity does not vary with distance (all
distances are the same .. infinite!)

Spotlight
• Most complex light source in OpenGL
• Light still emanates from point
• Cut-off by cone determined by angle T

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Spotlight Attenuation
• Spotlight is brightest along ls
• Vector v with angle I from p to point on surface
• Intensity determined by cos I
• Corresponds to projection of v onto Is
• Spotlight exponent e determines rate

for e = 1

for e > 1
curve narrows

For any of these light sources, it is

easy to compute illumination arriving
at a point (e.g., a vertex)

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Now we can think about how the
incoming light is reflected by a surface

Surface Reflection
• When light hits an opaque surface some is absorbed, the rest is
reflected (some can be transmitted too--but never mind for now)
• The reflected light is what we see
• Reflection is not simple and varies with material
– the surface’s micro structure define the details of reflection
– variations produce anything from bright specular reflection
(mirrors) to dull matte finish (chalk)

Incident Light Camera

Reflected Light

Surface

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Phong Illumination Model
• Calculate color for arbitrary point on surface
• Basic inputs are material properties and l, n, v:

l = vector to light source

n = surface normal
v = vector to viewer
r = reflection of l at p
(determined by l and n)

Basic Calculation
• Calculate each primary color separately
• Start with global ambient light
• Add reflections from each light source
• Clamp to [0, 1]
• Reflection decomposed into
– Ambient reflection
– Diffuse reflection
– Specular reflection
• Based on ambient, diffuse, and specular
lighting and material properties

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Ambient Reflection
• Intensity of ambient light uniform at every point
• Ambient reflection coefficient ka, 0 w ka w 1
• May be different for every surface and r,g,b
• Determines reflected fraction of ambient light
• La = ambient component of light source
• Ambient intensity Ia = ka La
• Note: La is not a physically meaningful quantity

Diffuse Reflection
• Diffuse reflector scatters light
• Assume equally all direction
• Called Lambertian surface
• Diffuse reflection coefficient kd, 0 w kd w 1
• Angle of incoming light still critical

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Lambert’s Law
• Intensity depends on angle of incoming light
• Recall
l = unit vector to light
n = unit surface normal
T = angle to normal
• cos T = l d n
• Id = kn (l d n) Ld
• With attenuation:
q = distance to light source,
Ld = diffuse component of light

Specular Reflection
• Specular reflection coefficient ks, 0 w ks w 1
• Shiny surfaces have high specular coefficient
• Used to model specular highlights
• Do not get mirror effect (need other techniques)

specular reflection specular highlights

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Shininess Coefficient
• Ls is specular component of light
• r is vector of perfect reflection of l about n
• v is vector to viewer
• I is angle between v and r
• Is = ks Ls cosD I
• D is shininess coefficient
• Compute cos I = r d v
• Requires |r| = |v| = 1
• Multiply distance term Higher D is narrower

Summary of Phong Model

• Light components for each color:
– Ambient (L_a), diffuse (L_d), specular (L_s)
• Material coefficients for each color:
– Ambient (k_a), diffuse (k_d), specular (k_s)
• Distance q for surface point from light source

l = vector from light r = l reflected about n

n = surface normal v = vector to viewer

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Comparison of Phong model to the
rendering equation

Raytracing Example

Martin Moeck,
Siemens Lighting

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Radiosity Example

Restaurant Interior. Guillermo Leal, Evolucion Visual

BRDF
• Bidirectional Reflection Distribution Function
• Measure for
materials
• Isotropic vs.
anisotropic
• Mathematically
complex

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Accurate normal vectors are important
for modeling surface reflection

Normal Vectors
• Summarize Phong

• Surface normal n is critical

– Calculate l d n
– Calculate r and then r d v

• Must calculate and specify the normal vector

– Even in OpenGL!
• Two examples: plane and sphere

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Normals of a Plane, Method I
• Method I: given by ax + by + cz + d = 0
• Let p0 be a known point on the plane
• Let p be an arbitrary point on the plane
• Recall: u d v = 0 iff u orthogonal v
• n d (p – p0) = nd p – nd p0 = 0
• We know that [a b c 0] Td p0 = -d (because p0
satisfies the plane equation)
• Consequently n0 must be [a b c 0]T
• Normalize to n = n0/|n0|

Normals of a Plane, Method II

• Method II: plane given by p0, p1, p2
• Points must not be collinear
• Recall: u e v orthogonal to u and v
• n0 = (p1 – p0) e (p2 – p0)
• Order of cross product determines orientation
• Normalize to n = n0/|n0|

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Normals of Sphere
• Implicit Equation f(x, y, z) = x2 + y2 + z2 –1 = 0
• Vector form: f(p) = p d p – 1 = 0
• Normal given by gradient vector

• Normalize n0/|n0| = 2p/2 = p

Angle of Reflection
• Perfect reflection: angle of incident equals
angle of reflection
• Also: l, n, and r lie in the same plane
• Assume |l| = |n| = 1, guarantee |r| = 1

Solution: D = -1 and
E = 2 (l d n)

Perhaps easier geometrically

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Summary: Normal Vectors
• Critical for Phong model (diffuse and specular)
• Must calculate accurately (even in OpenGL)
• Pitfalls
– Not unit length
– How to set at surface boundary?

Color .. Why RGB?

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Physics of Light and Color
• It’s all electromagnetic (EM) radiation
– Different colors correspond to different wavelengths O
– Intensity of each wavelength specified by amplitude
– Frequency Q SO
• long wavelength is low frequency
• short wavelength is high frequency

We perceive EM
UDGLDWLRQZLWK LQWKH
400-700 nm range

Color: What’s There vs. What We See

• Human eyes respond to “visible light”
– tiny piece of spectrum between infra-red and ultraviolet
• Color defined by emission spectrum of light source
– amplitude vs wavelength (or frequency) plot

Amplitude

Wavelength O

UV Visible IR

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The Eye

• The image is formed on the retina

• Retina contains two types of cells: rods and cones
• Cones measure color (red, green, blue)
• Rods responsible for monochrome night-vision

The Fovea

• Three types of cones: S,M,L

•Corresponds to 3 visual pigments
• Roughly speaking:
- S responds to blue
- M responds to green
- L responds to red
Cones are most densely • Note that these are not uniform
packed within a region of the - more sensitive to green than red
retina called the fovea • Colorblindness
- deficiency of one cone/pigment type

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Color Filters
• Rods and cones can be thought of as filters
– Cones detect red, green or blue parts of spectrum
– Rods detect average intensity across spectrum
• To get the output of a filter
– Multiply its response curve by the spectrum, integrate over all wavelengths
• A physical spectrum is a complex function of wavelength
– But what we see can be described by just 3 numbers—the color filter outputs
– How can we encode a whole function with just 3 numbers?
• A: we can’t! We can’t distinguish certain colors--metamers
G R
Amplitude

Wavelength

Color Models
• Okay, so our visual system is quite limited
• But maybe this is good news. . .
• We can avoid computing and reproducing the full color
spectrum since people only have 3 color channels
–TV would be much more complex if we perceived the
full spectrum
• transmission would require much higher bandwidths
• display would require much more complex methods
–real-time color 3D graphics is feasible
–any scheme for describing color requires only three
values
–lots of different color spaces--related by 3x3 matrix
transformations

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Color Spaces
• There are many ways to describe color
–Spectrum
• allows any radiation (visible or invisible) to be described
• usually unnecessary and impractical
–RGB
• convenient for display (CRT uses red, green, and blue
phosphors)
• not very intuitive
–HSV
• an intuitive color space
• H is hue - what color is it? S is saturation or purity - how non-
gray is it? V is value - how bright is it?
• H is cyclic therefore it is a non-linear transformation of RGB
–CIE XYZ
• a linear transform of RGB used by color scientists

HSV

From mathworks

B G V
G
R
R
B
G H
R B S

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Additive vs. Subtractive Color
• Working with light: additive primaries
– Red, green and blue components are added by the superposition
property of electromagnetism
– Conceptually: start with black, primaries add light
• Working with pigments: subtractive primaries
– Typical inks (CMYK): cyan, magenta, yellow, black
– Conceptually: start with white, pigments filter out light
– The pigments remove parts of the spectrum
dye color absorbs reflects
cyan red blue and green
magenta green blue and red
yellow blue red and green
black all none

– Inks interact in nonlinear ways--makes converting from monitor

color to printer color a challenging problem
– Black ink (K) used to ensure a high quality black can be printed

The Meaning of “Color”

• What’s an image?
– Irradiance: each pixel measures the incident light at a point on the film
– Proportional to integral of scene radiance hitting that point

• What’s Color?
– Refers to radiance or irradiance measured at 3 wavelengths
– Scene color: radiance coming off of surface (for illumination)
– Image color: irradiance (for rendering)
– These quantities have different units and should not be confused

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Units of Light and Color
Quantity Dimension Units .

solid angle solid angle [steradian]

a two-dimensional angle (proportional to area on a sphere)

power energy/time [watt]=[joule/sec]

photons per second; radiance integrated over incoming directions, over a finite area.

irradiance (intensity) power/area [watt/m2]

Brightness of light hitting the surface (or image) at this point (incident light)

radiance (intensity) power/(area*solid angle) [watt/(m2*steradian)]

Brightness of light reflected at this point along this direction (reflected light)

reflectance unitless [1]

what fraction of the light is reflected by a material? typically between 0 and 1.

Announcements
• Written assignment #1 due Thursday
– Handin at beginning of class
• Programming assignment #2 out Thursday

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