rendering ( 彩現 渲染 ). content light-material interaction phong reflection model gouraud vs....
Post on 19-Dec-2015
252 Views
Preview:
TRANSCRIPT
Fall 2013 3
Rendering
• The computation required to convert 3D scene to 2D display photo-realistically
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
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 elementof 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 reflectanceof 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
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
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 14
Johann Heinrich Lambert (1728 – 1777) was a Swiss mathematician, physicist and astronomer.
Fall 2013 16
Phong Reflection Model
• L: light source property (RGB) • R: material property (RGB)• ambient reflection
• diffuse reflection
• specular reflection
• final result
: shininess coefficient
To consider distance attenuation
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?
LN
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.
LN
H
V
Jim Blinn (1949 - now)
Fall 2013
20Fall 2013 20
Blinn-Phong Model
Time-Consuming!
lnln
nlnlnlnr
2
nln
nlnl
n
l r
vrLkI sss
An alternate formulation employs the half vector H
hnLkI
h
hhlvh
sss
ˆ,
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
Fall 2013 24
Shading Modes
• Flat vs. Smooth– 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)
• Most h/w implement Gouraud shading• Phong shading can better imitate specular effects (∵normals are interpolated)
Rendering Pipeline Tutorial
No longer on line [local copy]
top related