1. Rendering (彩現 渲染)

2. Content Light-Material Interaction Phong Reflection model
Gouraud vs. Phone Shading
Fall 2013

3. Rendering
The computation required to convert 3D scene to 2D display photo-realistically
Fall 2013

4. Shading
the gradation (of color) that give the 2D images the appearance of being 3D
Fall 2013

5. Light-Material Interaction
specular
diffuse
translucent
Fall 2013

6. Light point, spot, directional lights
ambient light: to account for uniform level room lighting
describe a light source through a three-component (RGB) intensity
Fall 2013

7. Phong Reflection Model
Diffuse (漫射)
Specular
Ambient
Fall 2013

8. Those Were the Days…
“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
Fall 2013

9. Lambert’s Cosine Law
The reflected luminous intensity in any direction from a perfectly diffusing surface varies as the cosine of the angle between the direction of incident light and the normal vector of the surface.
Intuitively: cross-sectional area of the “beam” intersecting an element of surface area is smaller for greater angles with the normal.
Fall 2013

10. Lambert’s Cosine Law Ideally diffuse surfaces obey cosine law.
Often called Lambertian surfaces.
Id = kd Iincident cos  = kd Iincident (N·L).
kd is the diffuse reflectance of the material. L N 
Fall 2013

11. Phong Lighting Model Phong adds specular highlights.
His original formula for the specular term:
W(i)[cos s ]n
s is the angle between the view and specular reflection directions.
“W(i) is a function which gives the ratio of the specular reflected light and the incident light as a function of the the incident angle i.”
Ranges from 10 to 80 percent.
“n is a power which models the specular reflected light for each material.”
Ranges from 1 to 10.
Fall 2013

12. Phong Lighting Model More recent formulations are slightly different.
Replace W(i) with a constant ks independent of the incident direction.
What do we lose when we do this?
Is= ks Iincident cosn = ks Iincident (V·R)n
R = 2(N·L)N – L
Fall 2013

13. Ambient Reflection
Local illumination models account for light scattered from the light source only
Light may be scattered from all surfaces in the scene. We are missing a lot of light, typically over 50%
Ambient term = a coarse approximation to this missing flux
This is a constant everywhere in the scene
Fall 2013

14. Johann Heinrich Lambert (1728 – 1777) was a Swiss mathematician, physicist and astronomer.
Diffuse Reflection
Lambertian scatters (wikipedia): the irradiance landing on the area element is proportional to the cosine of the angle between the illuminating surface and the normal.
When a Lambertian surface is viewed from any angle, it has the same radiance.
Fall 2013

15. Specular Reflection a
Effect of Shininess Coefficient a. v
Fall 2013

16. Phong Reflection Model
L: light source property (RGB)
R: material property (RGB)
ambient reflection
diffuse reflection
specular reflection
final result
To consider distance attenuation
a: shininess coefficient
Fall 2013

17. Phong Model (cont)
For multiple light sources:
Fall 2013

18. Blinn-Phong Model Popular variation of Phong model.
Uses the halfway vector, H.
Is = ks Iincident (N·H)n.
H = (L+V) / | L+V |
What are the advantages? L N  H V
Fall 2013

19. Blinn-Phong Model Popular variation of Phong model.
Uses the half vector, H.
Is = ks Iincident (N·H)n.
H = (L+V) / | L+V |
Faster to compute than reflection vector.
Still view-dependent since H depends on V.
Jim Blinn
( now) L N  H V
Fall 2013

20. Blinn-Phong Model Time-Consuming! n r l
An alternate formulation employs the half vector H
Time-Consuming!
Fall 2013 20

21. Blinn-Phong Highlights
Does using N.H vs. R.V affect highlights?
Yes, the highlights “spread” (Wikipedia)
Why?
Is this bad?
Fall 2013

22. Blinn-Phong Highlights
Does using N.H vs. R.V affect highlights?
Yes, the highlights “spread”.
Why?
Is this bad?
Not really, for two reasons.
Can always adjust the exponent.
Phong and Blinn-Phong are not physically based, so it doesn’t really matter!
Fall 2013

23. Target of Shading: Polygon, Vertex or Fragments
Recall the rendering (OpenGL) pipeline
Fall 2013

24. Shading Modes Flat vs. Smooth Local vs. Global
Flat: single color per face
Gouraud (intensity interpolation)
Phong (normal interpolation)
Local vs. Global
Fall 2013

25. Gouraud vs. Phong Most h/w implement Gouraud shading
Phong shading can better imitate specular effects (∵normals are interpolated)
Fall 2013

26. Fall 2013

27. Compare: Flat, Gouraud, Phong
Fall 2013

28. Rendering Pipeline Tutorial
No longer on line [local copy]

29. Rendering Pipeline (Foley and van Dam)
Fall 2013