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ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
ECE 516 / CS 532
Computer Vision
Pradeep Sen Advanced Graphics Lab
Class 3 January 28, 2009
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Last time
ย Finished Forsyth Ch 1 ย Started talking about radiometry
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Today
ย More on Radiometry
ย Local shading models
ย Ch. 4 & and start of ch. 5
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Radiometry
ย The science of measuring light
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Radiance
ย The power traveling at some point in a specified direction, per unit area perpendicular to the direction of travel, per unit solid angle (W/m^2 sr) ย Think of it as the power (energy per unit time) in a ray of light ย Constant along rays (in a vacuum) ย Useful property because it gives us an invariant when simulating light flow (used in both computer graphics and vision)
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Radiance
ย Radiance is constant along rays
ย Power 1->2, leaving 1:
ย Power 1->2, arriving at 2:
ย But these must be the same, so that the two radiances are equal
L x ( 1 , ฯ , ฯ)( dA 1 cos ฯ 1 )^ โ โ dA^2 cos r 2 ฯ^2 โโ
L x ( (^) 2 , ฯ , ฯ) ( dA 2 cos ฯ 2 )^ โ โ dA^1 cos r 2 ฯ^1 โโ
P 1
P 2
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Irradiance
ย Incident power per unit area (W/m^2 )
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Local reflectance model
ย The radiance leaving a point on a surface is due only to the radiance arriving at this point ย All the light leaving a surface at a given wavelength is due to the light arriving at that wavelength ย Surfaces do not generate light
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Local reflectance model
ย The radiance leaving a point on a surface is due only to the radiance arriving at this point
ย All the light leaving a surface at a given wavelength is due to the light arriving at that wavelength
ย Surfaces do not generate light
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
The BRDF
ย Bidirectional Reflectance Distribution Function ย A ratio of the radiance in the outgoing direction to the incident radiance ย Units: 1/sr
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Radiance leaving a surface
ย To compute the radiance leaving a surface point in a particular direction we multiply the irradiance from a direction by the BRDF
ย If we want to determine the outgoing radiance from all the incident irradiance, we integrate over the hemisphere of incident directions
ECE516 / CS532 Computer Vision Pradeep Sen Class 3 โ January 28, 2009
Important properties of the BRDF
ย Helmholtz reciprocity: symmetric in the incoming and outgoing direction ย Cannot be arbitrary values: the total energy in cannot be less than total energy out (no energy created) ย This is a physical restriction
