lec16

# lec16 - Announcement Photometric Stereo Recap An...

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1 CS152, Spring 2011 Intro Computer Vision Photometric Stereo Recap Introduction to Computer Vision CSE152 Lecture 16 CS152, Spring 2011 Intro Computer Vision Announcement An interesting analysis of Midterm Scores of those who attend class: 65.6% Scores of those who skip class: 42.7% • HW3 posted – Photometric stereo • Discussion Section Thursday 5/19 Time: 1-1:50 pm Location: SERF building, room 102 CS152, Spring 2011 Intro Computer Vision Photometric Stereo Rigs: One viewpoint, changing lighting Camera stays fixed, scene is static. One light turned on and first image is aquired Second light is turned on, and second image is acquired, etc. Surfac normals are estimated and then surface is integrated. CS152, Spring 2011 Intro Computer Vision An example of photometric stereo CS152, Spring 2011 Intro Computer Vision BRDF Bi-directional Reflectance Distribution Function ρ ( θ in , φ in ; θ out , φ out ) Function of – Incoming light direction: θ in , φ in – Outgoing light direction: θ out , φ out Ratio of incident irradiance to emitted radiance ^ n ( θ in , φ in ) ( θ out , φ out ) CS152, Spring 2011 Intro Computer Vision Photometric Stereo: Three problems 1. General but known reflectance function 2. Lambertian surfaces with known lighting 3. Lambertian surfaces with unknown lighting

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2 CS152, Spring 2011 Intro Computer Vision Photometric Stereo: General BRDF and Reflectance Map CS152, Spring 2011 Intro Computer Vision Coordinate system x y f(x,y) Surface: s (x,y) =(x,y, f(x,y)) Tangent vectors: Normal vector n = s x × s y = f x , f y , 1 Λ Ν Μ Ξ Π Ο CS152, Spring 2011 Intro Computer Vision Gradient Space (p,q) x y f(x,y) Normal vector Gradient Space : (p,q) n CS152, Spring 2011 Intro Computer Vision Image Formation For a given point A on the surface, the image irradiance E(x,y) is a function of 1. The BRDF at A 2. The surface normal at A 3. The direction of the light source n s . E(x,y) A CS152, Spring 2011 Intro Computer Vision Reflectance Map Let the BRDF be the same at all points on the surface, and let the light direction s be constant. 1. Then image irradiance is a function of only the direction of the surface normal. 2. In gradient space, we can write E(p,q) 3. We can measure E(p,q) by taking an image of a sphere made of a single material under distant lighting n s E(x,y) CS152, Spring 2011 Intro Computer Vision Example Reflectance Map: Lambertian surface For lighting from front E(p,q)
3 CS152, Spring 2011

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lec16 - Announcement Photometric Stereo Recap An...

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