10 数字图像处理_光度立体视觉.ppt

光度立体视觉 Photometric Stereo 信息视觉处理 Materials from: /afs/cs/academic/class/15385-s06/lectures/ppts/ 双向反射分布函数（BRDF） 定义：输出方向的辐射度与输入方向的辐照度的比率； BRDF可以从零（在该方向没有反射光）变化至无穷大（输出方向的单位辐射度来自于输入方向任意小的辐射度）； BRDF在输入和输出方向是对称的。（著名的Helmholtz互易原理）； 尽管对于某些输入与输出的角度，BDRF可以较大，但对大多数来说它不能大。事实上平均值必须相当小。 Mechanisms of Reflection Example Surfaces Diffuse Reflection and Lambertian BRDF Diffuse Reflection and Lambertian BRDF White-out: Snow and Overcast Skies Diffuse Reflection from Uniform Sky Specular Reflection and Mirror BRDF Combing Specular and Diffuse: Dichromatic Reflection Diffuse and Specular Reflection Image Intensity and 3D Geometry Shading as a cue for shape reconstruction What is the relation between intensity and shape? Reflectance Map Surface Normal Surface Normal Gradient Space Reflectance Map Relates image irradiance I(x,y) to surface orientation (p,q) for given source direction and surface reflectance Lambertian case: Reflectance Map Lambertian case Reflectance Map Lambertian case Reflectance Map Glossy surfaces (Torrance-Sparrow reflectance model) Shape from a Single Image? Given a single image of an object with known surface reflectance taken under a known light source, can we recover the shape of the object? Given R(p,q) ( (pS,qS) and surface reflectance) can we determine (p,q) uniquely for each image point? Solution Take more images Photometric stereo Add more constraints Shape-from-shading Photometric Stereo Photometric Stereo Solving the Equations More than Three Light Sources Get better results by using more lights Color Images The case of RGB images get three sets of equations, one per color channel: Simple solution: first solve for n using one channel Then substitute known n into above equations to get Or combine three channels and solve for n Computing light source directions Trick: place a chrome sphere in the scene the location of the highlight tells you the source direction Specular Reflection - Recap Computing the Light Source Directi