Rendering ( 彩現 渲染 )

Content

• Light-Material Interaction• Phong Reflection model• Gouraud vs. Phone Shading

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Rendering

• The computation required to convert 3D scene to 2D display photo-realistically

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Shading

• the gradation (of color) that give the 2D images the appearance of being 3D

Light-Material Interaction

specular diffuse translucent

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Light

• point, spot, directional lights

• ambient light: to account for uniform level room lighting

• describe a light source through a three-component (RGB) intensity

• Specular

Phong Reflection Model

• Diffuse ( 漫射 ) • Ambient

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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

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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.

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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

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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.

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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

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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.

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Johann Heinrich Lambert (1728 – 1777) was a Swiss mathematician, physicist and astronomer.

Specular Reflection

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Effect of Shininess Coefficient .

v

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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

Phong Model (cont)

• For multiple light sources:

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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

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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)

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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,

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Blinn-Phong Highlights

• Does using N.H vs. R.V affect highlights?– Yes, the highlights “spread” (Wikipedia)– Why?

• Is this bad?

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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!

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Target of Shading: Polygon, Vertex or Fragments

Recall the rendering (OpenGL) pipeline

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Shading Modes

• Flat vs. Smooth– Flat: single color per

face– Gouraud (intensity

interpolation)– Phong (normal

interpolation)

• Local vs. Global

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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)

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Compare: Flat, Gouraud, Phong

Rendering Pipeline Tutorial

No longer on line [local copy]

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Rendering Pipeline (Foley and van Dam)