真实感图形绘制(光照模型 视无关)课件

10
800 rays used in lighting pass.
Note:
-improved caustic definition,-lighting effect of mirror, -reflection of caustic,
-shadowing due to mirror lighting.Caustics
From Alan Watt, “3D Computer Graphics”
Standard raytracer:
Diffuse table and blue ball,
mirrors left, right and back, transparent red ball
Bi-directional raytracer More rays in the light pass
Single Pass (Conventional RT)
Note : caustic due to red transparent ball
Henrik http://www.gk.dtu.dk/~hwj
Bidirectional example
200 rays used in lighting pass 400 rays used in lighting pass
Refraction caustics
Henrik http://www.gk.dtu.dk/~hwj
15Direct illumination 16
Global Illumination
17eye
diagram photograph:
18
Original sculpture by John Ferren lit by daylight from behind.
Image rendered with radiosity. note color bleeding effects.Ray traced image. A standard ray tracer cannot simulate the interreflection of light between 19
Radiosity vs. Ray Tracing
Ray-tracing
View-dependent
Specular and refraction
Radiosity
View-independent Diffuse only
20
Radiosity vs. Ray Tracing
•Ray tracing is an algorithm
–If the camera is moved, we have to start over
•Radiosity is computed in object-space
–View-independent (just don't move the light)
–Can pre-compute complex lighting to allow interactive walkthroughs
26
The Rendering Equation
x
Museum simulation. Program of Computer Graphics, Cornell University.
50,000 patches. Note indirect lighting from ceiling.
32
Radiosity Overview
在辐射度方法中，所有的景物表面都假设为理想的朗伯漫反射表面
所有的入射光在各个方向上反射的光强都一样
整个场景被划分为一系列的小区域（small areas, or patches ）设小区域i 向外辐射的所有能量的辐射度为Bi,并认为在i 内所有地方的辐射度为一个常数
单位如下，表示单位立体角单位面积的瓦特数:
Watts / steradian * meter 2
x'
ω'
x'x
x
x’
Discrete Radiosity Equation
A i
A j
•discrete representation
•iterative solution
•costly geometric/visibility calculations
patches, over which the radiosity =i B The Radiosity Matrix
求解该矩阵，就可以为每一个patch 得到一个B i ，它与视点
是无关的。

12n B B B
⋮12n E E E
⋮=
111112112212221111n n n n nn F F F F F F F ρρρρρρρ−−− −−
−−
⋯⋮⋱⋯⋯+=i i E B
37121
21112i i i i i i i i in n n n B B B E B F B E B F F B E B E ρρρ
=+
⋮⋮⋮⋮⋮⋮⋯This method is fundamentally a Gauss-Seidel relaxation （高
斯-赛德尔迭代法，松弛法）
38
(intensities) needed by the 39
Factory simulation. Program of Computer Graphics, Cornell University.
30,000 patches.
Questions?40
Example
•Color bleeding is extreme in this example
•Textures are applied after solving for illumination •Some meshing
artifacts are visible -note the banding
around the pictures
on the wall
From Alan Watt, “3D Computer Graphics”
From Alan Watt, “3D Computer Graphics”
Calculating the Form Factors
•Point-patch
•Patch-patch
53Coarse patch solution (145 patches)Improved solution (1021 subpatches) Adaptive subdivision (1306 subpatches)
54
Discontinuity Meshing
•Create mesh depending on expected shadow boundaries •Limits of umbra and penumbra
–Captures nice shadow boundaries –Complex geometric computation
–The mesh is getting complex
penumbra
umbra
source
blocker
55
Discontinuity Meshing 56
With visibility skeleton & discontinuity meshing
Hierarchical Approach
目的：减少计算形状因子数量
–根据作用的重要性，面片表
示成多个层次
–作为辐射源面片时，剖分可
粗一些，作为光能接受面片
时，剖分细一些。

•桌子和小立方体都对周围墙
面有影响，但是小立方体影
响不大，微微移动小立方
体，光影不会变化太大，桌
子可以剖分的大一些；
•但是在绘制桌子时，要绘制
局部细节，小立方体对它的
影响大，桌子需要细剖分；
57
constant
constant
proportional to the incident radiance.
constant
If the light source is a point source, a pixel
intensity will only be a function of。