1. Computer Graphics
Lighting

2. Outline Lighting Lighting models Ambient Diffuse Specular
Surface Rendering Methods

3. What we know
We already know how to render the world from a viewpoint.

4. “Lighting” Two components:
Lighting Model or Shading Model - how we calculate the intensity at a point on the surface
Surface Rendering Method - How we calculate the intensity at each pixel

5. Jargon
Illumination - the transport of light from a source to a point via direct and indirect paths
Lighting - computing the luminous intensity for a specified 3D point, given a viewpoint
Shading - assigning colors to pixels
Illumination Models:
Empirical - approximations to observed light properties
Physically based - applying physics properties of light and its interactions with matter

6. The lighting problem… What are we trying to solve?
Global illumination – the transport of light within a scene.
What factors play a part in how an object is “lit”?
Let’s examine different items here…

7. Two components Light Source Properties Object Properties
Color (Wavelength(s) of light)
Shape
Direction
Object Properties
Material
Geometry
Absorption

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Global Effects
shadow
multiple reflection
translucent surface
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9. Local vs Global Rendering
Correct shading requires a global calculation involving all objects and light sources
Incompatible with pipeline model which shades each polygon independently (local rendering)
However, in computer graphics, especially real time graphics, we are happy if things “look right”
Exist many techniques for approximating global effects
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10. Light Source Properties
Color
We usually assume the light has one wavelength
Shape
point light source - approximate the light source as a 3D point in space. Light rays emanate in all directions.
good for small light sources (compared to the scene)
far away light sources?

11. Distributed Lights Light Source Shape continued
distributed light source (not supported natively in OpenGL) - approximating the light source as a 3D object. Light rays usually emanate in specific directions
good for larger light sources
area light sources

12. Light Source Direction
In computer graphics, we usually treat lights as rays emanating from a source. The direction of these rays can either be:
Omni-directional (point light source)
Directional angle (spotlights)
Directional (parallel rays)

13. Light Position
We can specify the position of a light with an x, y, and z coordinate.
What are some examples?
These lights are called positional lights
Q: Are there types of lights that we can simplify?
A: Yep! Think about the sun. If a light is significantly far away, we can represent the light with only a direction vector. These are called directional lights. How does this help?

14. Contributions from lights
We will breakdown what a light does to an object into three different components. This APPROXIMATES what a light does. To actually compute the rays is too expensive to do in real-time.
Light at a pixel from a light = Ambient + Diffuse + Specular contributions.
Ilight = Iambient + Idiffuse + Ispecular

15. Ambient Term - Background Light
The ambient term is a HACK!
It represents the approximate contribution of the light to the general scene, regardless of location of light and object
Indirect reflections that are too complex to completely and accurately compute
Iambient = color

16. Diffuse Term
Contribution that a light has on the surface, regardless of viewing direction.
Diffuse surfaces, on a microscopic level, are very rough. This means that a ray of light coming in has an equal chance of being reflected in any direction.
What are some ideal diffuse surfaces?

17. Lambert’s Cosine Law Diffuse surfaces follow Lambert’s Cosine Law
Lambert’s Cosine Law - reflected energy from a small surface area in a particular direction is proportional to the cosine of the angle between that direction and the surface normal.
Think about surface area and # of rays

18. Diffuse Term
To determine how much of a diffuse contribution a light supplies to the surface, we need the surface normal and the direction on the incoming ray
What is the angle between these two vectors?
Idiffuse = kdIlightcos = kdIlight(N . L)
Ilight = diffuse (intensity) of light
kd [0..1] = surface diffuse reflectivity
What CS are L and N in?
How expensive is it?

19. Example
What are the possible values for theta (and thus the dot product?)

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Normal for Triangle n
plane n ·(p - p0 ) = 0 p1
n = (p1 - p0 ) × (p2 - p0 ) p p0 p2
normalize n  n/ |n| X
Note that right-hand rule determines outward face
(programatically: ‘winding’ or order of vertices)
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21. Specular Reflection
Specular contribution can be thought of as the “shiny highlight” of a plastic object.
On a microscopic level, the surface is very smooth. Almost all light is reflected.
What is an ideal purely specular reflector?
What does this term depend on?
Viewing Direction
Normal of the Surface

22. Snell’s Law
Specular reflection applies Snell’s Law.
We assume l = r

23. Snell’s Law is for IDEAL surfaces
Most surfaces are not ideal.
Think about the amount of light reflected at different angles. N R L V 

24. Different for shiny vs. dull objects

25. Snell’s Law is for IDEAL surfaces
Think about the amount of light reflected at different angles. N R L V   

26. Phong Model Phong Reflection Model
An approximation: set the intensity of specular reflection proportional to (cos )shininess
What are the possible values of cos ?
What does the value of shininess mean?
How do we represent shinny or dull surfaces using the Phong model?
Ispecular = ksIlight (cos )shininess = ksIlight (V.R)shininess

27. The Shininess Coefficient
Values of a between 100 and 200 correspond to metals
Values between 5 and 10 give surface that look like plastic
cosa f
-90 f 90
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28. How do we compute R? N*(N.L) R+L=2N(N.L) R = 2N(N.L)-L L N R N(N.L) V L 

29. Simplify this Instead of R, we compute halfway between L and V.
We call this vector the halfway vector, H. H N R V  L 

30. Combining the terms
Ambient - the combination of light reflections from various surfaces to produce a uniform illumination. Background light.
Diffuse - uniform light scattering of light rays on a surface. Proportional to the “amount of light” that hits the surface. Depends on the surface normal and light vector.
Sepecular - light that gets reflected. Depends on the light ray, the viewing angle, and the surface normal.

31. Ambient + Diffuse + Specular

32. Lighting Equation N R L V 
Ilambient = light source l’s ambient component
Ildiffuse = light source l’s diffuse component
Ilspecular = light source l’s specular component
kambient = surface material ambient reflectivity
kdiffuse = surface material diffuse reflectivity
kspecular = surface material specular reflectivity
shininess = specular reflection parameter (1 -> dull, > very shiny) N R L V 

33. Attenuation
One factor we have yet to take into account is that a light source contributes a higher incident intensity to closer surfaces.
What happens if we don’t do this?

34. Subtleties
What’s wrong with:
What’s a good fix?

35. Full Illumination Model
Run demo

36. Steps in OpenGL lighting
Enable lighting and select model
Specify normals
Specify material properties
Specify lights
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Normal for Triangle n
plane n ·(p - p0 ) = 0 p1
n = (p1 - p0 ) × (p2 - p0 ) p p0 p2
normalize n  n/ |n| X
Note that right-hand rule determines outward face
(programatically: ‘winding’ or order of vertices)
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Normals
In OpenGL the normal vector is part of the state
Set by glNormal*()
glNormal3f(x, y, z);
glNormal3fv(p);
Usually we want to set the normal to have unit length so cosine calculations are correct
Length can be affected by transformations
Note that scaling does not preserved length
glEnable(GL_NORMALIZE) allows for autonormalization at a performance penalty
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39. Shading Shading is how we “color” a triangle. Constant Shading
Gouraud Shading

40. Constant Shading Constant Intensity or Flat Shading
One color for the entire triangle
Fast
Good for some objects
What happens if triangles are small?
Sudden intensity changes at borders

41. Gouraud Shading Intensity Interpolation Shading
Calculate lighting at the vertices. Then interpolate the colors

42. Gouraud Shading Relatively fast, only do three calculations
No sudden intensity changes
What can it not do?
What are some approaches to fix this?
Question, what is the normal at a vertex?

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Enabling Shading
Shading calculations are enabled by
glEnable(GL_LIGHTING)
Once lighting is enabled, glColor() ignored
Must enable each light source individually
glEnable(GL_LIGHTi) i=0,1…..
Can choose light model parameters
glLightModeli(parameter, GL_TRUE)
GL_LIGHT_MODEL_LOCAL_VIEWER do not use simplifying distant viewer assumption in calculation
GL_LIGHT_MODEL_TWO_SIDED shades both sides of polygons independently
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44. Defining a Point Light Source
For each light source, we can set an RGBA for the diffuse, specular, and ambient components, and for the position
GL float diffuse0[]={1.0, 0.0, 0.0, 1.0};
GL float ambient0[]={1.0, 0.0, 0.0, 1.0};
GL float specular0[]={1.0, 0.0, 0.0, 1.0};
Glfloat light0_pos[]={1.0, 2.0, 3,0, 1.0};
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glLightv(GL_LIGHT0, GL_POSITION, light0_pos);
glLightv(GL_LIGHT0, GL_AMBIENT, ambient0);
glLightv(GL_LIGHT0, GL_DIFFUSE, diffuse0);
glLightv(GL_LIGHT0, GL_SPECULAR, specular0);
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45. Distance and Direction
The source colors are specified in RGBA
The position is given in homogeneous coordinates
If w =1.0, we are specifying a finite location
If w =0.0, we are specifying a parallel source with the given direction vector
The coefficients in the distance terms are by default a=1.0 (constant terms), b=c=0.0 (linear and quadratic terms). Change by
a= 0.80;
glLightf(GL_LIGHT0, GLCONSTANT_ATTENUATION, a);
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Spotlights
Use glLightv to set
Direction GL_SPOT_DIRECTION
Cutoff GL_SPOT_CUTOFF
Attenuation GL_SPOT_EXPONENT
Proportional to cosaf f -q q
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Global Ambient Light
Ambient light depends on color of light sources
A red light in a white room will cause a red ambient term that disappears when the light is turned off
OpenGL also allows a global ambient term that is often helpful for testing
glLightModelfv(GL_LIGHT_MODEL_AMBIENT, global_ambient)
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Moving Light Sources
Light sources are geometric objects whose positions or directions are affected by the model-view matrix
Depending on where we place the position (direction) setting function, we can
Move the light source(s) with the object(s)
Fix the object(s) and move the light source(s)
Fix the light source(s) and move the object(s)
Move the light source(s) and object(s) independently
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Material Properties
Material properties are also part of the OpenGL state and match the terms in the modified Phong model
Set by glMaterialv()
GLfloat ambient[] = {0.2, 0.2, 0.2, 1.0};
GLfloat diffuse[] = {1.0, 0.8, 0.0, 1.0};
GLfloat specular[] = {1.0, 1.0, 1.0, 1.0};
GLfloat shine = 100.0
glMaterialf(GL_FRONT, GL_AMBIENT, ambient);
glMaterialf(GL_FRONT, GL_DIFFUSE, diffuse);
glMaterialf(GL_FRONT, GL_SPECULAR, specular);
glMaterialf(GL_FRONT, GL_SHININESS, shine);
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Front and Back Faces
The default is shade only front faces which works correctly for convex objects
If we set two sided lighting, OpenGL will shade both sides of a surface
Each side can have its own properties which are set by using GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK in glMaterialf
back faces not visible
back faces visible
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Emissive Term
We can simulate a light source in OpenGL by giving a material an emissive component
This component is unaffected by any sources or transformations
GLfloat emission[] = 0.0, 0.3, 0.3, 1.0);
glMaterialf(GL_FRONT, GL_EMISSION, emission);
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