1. Illumination and Shading
Mengxia Zhu
Fall 2007 .

2. Why?
What light source is used and how the object response to the light makes difference
Lighting gives you a 3D view to an object
A unlit sphere looks no different from a 2D disk
To get realistic pictures, the color computation of pixels must include lighting calculations

3. Local and Global Illumination
Local illumination: a surface point receives light directly from all light sources in the scene. Consider in isolation
Global illumination: a point receives light after light rays interact with other objects in the scene.
Shadow
Refract through transparent object
Reflection and transmission rays

4. Illumination Vs. Shading
Illumination (lighting) model: To model the interaction of light with surfaces to determine the final color & brightness of the surface
Shading model: applies the illumination models at a set of points and colors the whole image.

5. Local illumination
Only consider the light, the observer position, and the object material properties V L

6. Types of Light Ambient Diffuse Specular
Light that’s been scattered so much by the environment that its direction is impossible to determine - it seems to come from all directions
Diffuse
Light that comes from one direction, but it gets scattered equally in all directions
Specular
Light comes from a particular direction, and its tends to bounce off the surface in a preferred direction

7. OpenGL Lighting Model
OpenGL approximates lighting as if light can be broken into red, green, and blue components
The RGB values for lights mean different than for materials
For light, the numbers correspond to a percentage of full intensity for each color
For materials, the numbers correspond to the reflected proportions of those colors

8. Ambient light (background light)
The light that is the result from the light reflecting off other surfaces in the environment
A general level of brightness for a scene that is independent of the light positions or surface directions
Has no direction
Each light source has an ambient light contribution, Ia
For a given surface, we can specify how much ambient light the surface can reflect using an ambient reflection coefficient : Ka (0 < Ka < 1)

9. Ambient Light Ambient, omni-directional light
So the amount of light that the surface reflect is therefore
Ka ambient coefficient
Iamb =

10. Diffuse Light
The illumination that a surface receives from a light source and reflects equally in all directions
This type of reflection is called Lambertian reflection
The brightness of the surface is independent of the observer position (since the light is reflected in all direction equally) L

11. Lambert’s Law
Lambert’s law: the radiant energy for a given light source is:
Id * cos(q)
Id : the intensity of the light source
is the angle between the surface
normal (N) and light vector (L)
If N and L are normalized, cos(q) =
Surface’s material property: assuming that the surface can reflect Kd (0<Kd<1), diffuse reflection coefficient) amount of diffuse light.

12. The Diffuse Component
Brightness is inversely proportional to the area of the object illuminated (dot product of light vector and surface normal)
greatest when N and L are parallel
smallest when N and L are orthogonal
In calculations, max {N.L, 0} is used to avoid negative values
The total diffuse reflection = ambient + diffuse

13. Specular Light
These are the bright spots on objects (such as polished metal, apple ...)
Light reflected from the surface unequally to all directions.

14. Phong’s Model for Specular
Brightness depends on the angle between reflection vector (R) and viewer vector (V), i.e, on direction of viewer

15. Summing the Components
Ambient + Diffuse + Specular = final

16. Phong Illumination Curves
Specular exponents are much larger than 1, and
Values of 100 are not uncommon.
The specular reflection exponent n is 1 for a slightly glossy surface and infinity for a perfect mirror.
: glossiness, rate of falloff

17. Changing Specular exponent n

18. Double length of vector
Reflected Ray N
How to calculate R? R + L = 2(N*L) N
R = 2(N*L) N - L L R f f a V f L
2N(N•L)
Double length of vector f L
R = 2N(N•L) - L
Subtract L f L
N(N•L)
Project L onto N

19. Half Vector An alternative way of computing phong lighting is:
H (halfway vector): halfway between V and L: (V+L) L N H V

20. Putting It All Together
Single Light:

21. Attenuation Attenuation factor The intensity becomes
Light attenuates with distance from the source
where d = distance between the light and object
kc = constant attenuation
A light source does not give an infinite amount of light
kl = linear term
kq = quadratic term
Models the theoretical attenuation from a point source
The intensity becomes

22. Spotlight Effect
When the vertex lies inside the cone of illumination produced by spotlight, its contribution to the light intensity is
Where D gives the spotlight’s direction. The intensity is maximum in the center of cone and is attenuated toward the edge of the cone
s is 1 if the source is not spotlight
m is exponent determining the concentration of the light
The intensity of light source is

23. Putting All Together Multiple lighting calculation in RGB mode gives
Vertex color

24. Adding Lighting to the Scene
Define normal vectors for each vertex of each object
Create, select, and position one or more light sources
Create and select a lighting model
Define material properties for the objects in the scene

25. Creating Light Sources
Properties of light sources are color, position, and direction
void glLight{if}(GLenum light, GLenum pname, TYPE param);
void glLight{if}v(GLenum light, GLenum pname, TYPE *param);
Creates the light specified by light that can be GL_LIGHT0, GL_LIGHT1, … or GL_LIGHT7
Pname specifies the characteristics of the light being set
Param indicates the values to which the pname characteristic is set
glEnable(GL_LIGHT0);

26. Color for a Light Source
GLfloat light_ambient[] = {0.0,0.0,0.0,1.0};
GLfloat light_diffuse[] = {1.0,1.0,1.0,1.0};
GLfloat light_specular[] = {1.0,1.0,1.0,1.0};
glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);
glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);

27. Position of Light Source
Positional light source
(x, y, z) values specify the location of the light
GLfloat light_position[] = {x, y, z, w};
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
Directional light source
(x, y, z) values specify the direction of the light located at the infinity
No attenuation
GLfloat light_position[] = {x, y, z, 0};

28. Attenuation Attenuation factor for a positional light
Needs to specify three coefficients
glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, 2.0);
glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 1.0);
glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 1.0);
Ambient, diffuse, and specular contributions are all attenuated

29. Spotlights The shape of the light emitted is restricted to a cone
glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 45.0);
The cutoff parameter is set to 45 degrees
GLfloat spot_direction[] = {-1.0, -1.0, 0.0];
glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, spot_direction);
specifies the spotlight’s direction which determines the axis of the cone of light
glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, 2.0);
Controls how concentrated the light is

30. Multiple Lights You can define up to eight light sources
Need to specify all the parameters defining the position and characteristics of the light
OpenGL performs calculations to determine how much light each vertex gets from each source
Increasing number of lights affects performance

31. Defining Material Properties
Specifying the ambient, diffuse, and specular colors, the shininess, and the color of any emitted light
void glMaterial{if}(GLenum face, GLenum pname, TYPE param);
void glMaterial{if}v(GLenum face, GLenum pname, TYPE *param);
Specifies a current material property for use in lighting calculations
Face can be GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK
Pname identifies the particular material property being set
Param defines the desired values for that property

32. Reflectance Diffuse and ambient reflection Gives color
GLfloat mat_amb_diff[] = {0.1, 0.5,0.8,1.0};
glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE, mat_amb_diff);
Specular reflection
Produces highlights
GLfloat mat_specular[] = {1.0,1.0,1.0,1.0);
Glfloat low_shininess[] = {5.0};
glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
glMaterialfv(GL_FRONT, GL_SHININESS, low_shininess);

33. Shading Models for Polygons
Constant Shading (flat shading)
Compute illumination at any one point on the surface. Use face or one normal from a pair of edges. Good for far away light and viewer or if facets approximate surface well.
Per-Pixel Shading
Compute illumination at every point on the surface.
Interpolated Shading
Compute illumination at vertices and interpolate color

34. Constant Shading Compute illumination only at one point on the surface
Okay to use if all of the following are true
The object is not a curved (smooth) surface (e.g. a polyhedron object)
The light source is very far away (so N.L does not change much across a polygon)
The eye is very far away (so V.R does not change much across a polygon)
The surface is quite small (close to pixel size)

35. Un-lit

36. Flat Shading

37. Mach Band ?

38. Polygon Mesh Shading
Shading each polygonal facet individually will not generate an illusion of smooth curved surface
Reason: polygons will have different colors along the boundary, unfortunately, human perception helps to even accentuate the discontinuity: mach band effect

39. Smooth Shading Need to have per-vertex normals Gouraud Shading
Interpolate color across triangles
Fast, supported by most of the graphics accelerator cards
Phong Shading
Interpolate normals across triangles
More accurate, but slow. Not widely supported by hardware

40. Gouraud Shading Normals are computed at the polygon vertices
If we only have per-face normals, the normal at each vertex is the average of the normals of its adjacent faces
Intensity interpolation: linearly interpolate the pixel intensity (color) across a polygon surface

41. Linear Interpolation If v1 and v2 are known, then
Calculate the value of a point based on
the distances to the point’s two neighbor points
If v1 and v2 are known, then
x = b/(a+b) * v1 + a/(a+b) * v2

42. Linear Interpolation in a Triangle
To determine the intensity (color) of point P in the triangle,
we will do:
determine the intensity of 4 by linearly interpolating between 1 and 2
determine the intensity of 5 by linearly interpolating between 2 and 3
determine the intensity of P by linear interpolating between 4 and 5

43. Mach Band ?

44. Image

45. Phong Shading Model
Gouraud shading does not properly handle specular highlights, specially when the n parameter is large (small highlight).
Reason: colors are interpolated.
Solution: (Phong Shading Model)
1. Compute averaged normal at vertices.
2. Interpolate normals along edges and scan-lines. (component by component)
3. Compute per-pixel illumination.

49. References
Slides from Karki at LSU
Jian Huang at UTK