Chapter8PPT课件

3
n s
(x,y,z)
➢ s is the direction of the light source at (x, y, z).
s changes from one location to another, eg,
s1 s2 . If the light source is close to a surface, the change of s is large.
n s
cos
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Attenuation
➢ When a light is moving away from a surface, the intensity of light received decreases
➢ For a point light source, the attenuation factor,
1
Chapter 8
Shading
To create photo-realistic graphics scenes from geometric models, we need to compute light intensities of the surfaces based on the optical properties of materials, light positions and the view point, according to the laws in physics.
Id = Rd Ii cos = Rd Ii (s • n)
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Pure diffuse reflection from directional source pointing (1,1,1)
Specular Reflection
➢ A shiny surface reflects light mainly in perfect reflection direction (r)
n s1 n s2
4
Directional sources
➢ When the light source is farther away, the change of s becomes less significant. When the distance becomes very large, s remains unchanged across a surface, e.g., the sun. This is called a directional light source (or parallel source, or distributed source.)
n s
➢ Rd ,the diffuse reflection coefficient,
0 Rd 1
➢ Id is independent of the view point
➢ cos is the projection of s on n.
If s and n are normalized,
cos = s • n (the dotproduct)
s
cos
➢ When = 0, the incident light is
perpendicular to the surface, the surface 1.2
receives maximum amount of light,
1
e.g, light from the sun at noon
0.8
➢ For a directional light source, Af = 1
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Overall
➢ Diffuse reflection from a point source
Id
Rd Ii cos
a bd cd 2
Rd Ii a bd cd 2
(s • n)
➢ Diffuse reflection from a directional source
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Contents
➢ Light Sources
➢Point sources ➢Directional sources
➢ Reflections
➢Diffuse reflection ➢Specular reflection ➢Ambient reflection
➢ Gouraud Shading ➢ Phong Shading ➢ Shadows
Point sources
➢ n is the normal of the surface. n remains the same at various locations on a plane
➢ (x, y, z) are the coordinates of a point on the surface
Af
1 d2
( in theory )
Af
1
a bd cd 2
( in practice )
d is the distance from the point light source. a, b, and c are heuristic parameters chosen empirically.
cos 0.6
0.4
➢ When = 90o, it receives no light. ➢ When > 90o , the light source is on
0.2
0
0
0.5
1
1.5
-0.2
the other side of the surface, it receives no light.
n
n
s
s
5
➢ Let Ii be the intensity of the inc the angle between s and n , 0o 90o.
➢ Ii cos is the amount of light received
in a unit area of the surface
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Diffuse Reflection
➢ A perfect rough surface reflects
light evenly in all directions
➢ Lambert cosine law
The amount of light reflected is Id = Rd Ii cos