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对于金属，折射进表面的光线的能量会立即被金属中的自由电子吸收，转换成电子的能量，不再可见
对于非金属（电介质或绝缘体），它们往往不是由单一成分构成，而可以认为其中包含了很多折射率不同的微粒，光线遇到这些粒子后发生反射折射，在物质内部不断传播，散射到不同方向，其中一部分会再次穿过表面被观察到,这种现象称为次表面散射(Subsurface Scattering)
根据物质属性和观察尺度的不同，次表面散射会表现出不同的效果
如果光线在物质中传播距离小于观察尺度（绿色区域，可以认为是一个像素区域），我们看到情况如下面的右上图，入射点、反射点、次表面散射的出射点看起来是同一个点。其中反射部分（图中浅棕色出射光）就是我们常说的高光（Specular Light），常聚集在一个方向周围，向这个方向观察该点会看到明显的高光，从其他方向观察该点时高光则比较微弱；次表面散射部分（图中蓝色出射光）是漫射光（Diffuse Light），光线被散射到各个方向。
如果光线在物质中的传播距离大于观察尺度，就需要使用次表面散射算法来建模
BRDF(bidirectional reflectance distribution function),是用来描述反光小平面的反射光和入射光的比例关系的函数，设一个不透明的、不发光的小平面，BRDF的定义为
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测量入射光的设备测量小平面的照度（E）最方便，不用关心方向，而测量出射光则是测量辐射率(L)最方便，用狭长的测光筒
一束光照到平面后，被平面反射到各个方向，其中一个出射方向的光通量只是整个反射光通量极小的一部分，当出射方向立体角趋于0时，
',6)),Q("mjx-container",F,[(m(),a("svg",X,T[34]||(T[34]=[t('',1)]))),T[35]||(T[35]=Q("mjx-assistive-mml",{unselectable:"on",display:"block"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML",display:"block"},[Q("munder",null,[Q("mo",{"data-mjx-texclass":"OP",movablelimits:"true"},"lim"),Q("mrow",{"data-mjx-texclass":"ORD"},[Q("msub",null,[Q("mi",null,"ω"),Q("mi",null,"o")]),Q("mo",{stretchy:"false"},"→"),Q("mn",null,"0")])]),Q("mfrac",null,[Q("mrow",null,[Q("mi",null,"d"),Q("msub",null,[Q("mi",null,"L"),Q("mi",null,"o")])]),Q("msub",null,[Q("mi",null,"L"),Q("mi",null,"i")])]),Q("mo",null,"="),Q("mn",null,"0")])],-1))]),Q("p",null,[T[42]||(T[42]=l("所以在实际计算中使用辐射率和辐射率比值是没有意义的。而如果分母改成表面上接收到的来自光源方向的微分辐照度，")),Q("mjx-container",_,[(m(),a("svg",S,T[36]||(T[36]=[t('',1)]))),T[37]||(T[37]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"d"),Q("mi",null,"E"),Q("mo",null,"="),Q("msub",null,[Q("mi",null,"L"),Q("mi",null,"i")]),Q("mo",{stretchy:"false"},"("),Q("mi",null,"l"),Q("mo",{stretchy:"false"},")"),Q("mi",null,"d"),Q("mi",null,"ω"),Q("mi",null,"cos"),Q("mo",{"data-mjx-texclass":"NONE"},""),Q("msub",null,[Q("mi",null,"θ"),Q("mi",null,"i")])])],-1))]),T[43]||(T[43]=l("，")),Q("mjx-container",E,[(m(),a("svg",G,T[38]||(T[38]=[t('',1)]))),T[39]||(T[39]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"d"),Q("mi",null,"E")])],-1))]),T[44]||(T[44]=l("会小很多，所以比值")),Q("mjx-container",J,[(m(),a("svg",N,T[40]||(T[40]=[t('',1)]))),T[41]||(T[41]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"d"),Q("msub",null,[Q("mi",null,"L"),Q("mi",null,"o")]),Q("mrow",{"data-mjx-texclass":"ORD"},[Q("mo",null,"/")]),Q("mi",null,"d"),Q("msub",null,[Q("mi",null,"E"),Q("mi",null,"i")])])],-1))]),T[45]||(T[45]=l("是有意义的"))])])}const K=d(g,[["render",z],["__file","BRDF.html.vue"]]),q=JSON.parse('{"path":"/note/graphics/illumination_model/BRDF/BRDF.html","title":"BRDF(双向反射分布函数)","lang":"zh-CN","frontmatter":{},"headers":[{"level":2,"title":"1. 物体对光的反应","slug":"_1-物体对光的反应","link":"#_1-物体对光的反应","children":[{"level":3,"title":"1.1 金属","slug":"_1-1-金属","link":"#_1-1-金属","children":[]},{"level":3,"title":"1.2 非金属","slug":"_1-2-非金属","link":"#_1-2-非金属","children":[]}]},{"level":2,"title":"2. BRDF","slug":"_2-brdf","link":"#_2-brdf","children":[{"level":3,"title":"2.1 定义","slug":"_2-1-定义","link":"#_2-1-定义","children":[]},{"level":3,"title":"2.2 特性","slug":"_2-2-特性","link":"#_2-2-特性","children":[]},{"level":3,"title":"2.3 为什么BRDF方程是L和E的比值","slug":"_2-3-为什么brdf方程是l和e的比值","link":"#_2-3-为什么brdf方程是l和e的比值","children":[]}]}],"git":{"contributors":[{"name":"thejinchao","username":"thejinchao","email":"thejinchao@gmail.com","commits":1,"url":"https://github.com/thejinchao"}]},"filePathRelative":"note/graphics/illumination_model/BRDF/BRDF.md"}');export{K as comp,q as data};