Introduction to computer vision 5

Jean Ponce

jean.ponce@ens.fr

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Zuhaib Akhtar za2023@nyu.edu

Ayush Jain aj3152@nyu.edu

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Slides will be available after classes

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• Radiance and irradiance
• BRDF and radiosity
• Sources and shading
• Photometric stereo
• Color sources
• Trichromacy
• Color reflectance

Radiometry and color

Light and shadows

[These slides courtesy of S. Narasimhan, CMU.]

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Reflections

Reflections and refractions

Refraction

Caustics

(Source: Wikipedia)

Is glass really transparent?

Interreflections

Scattering

Light scattering

in the sky (Nasa)

The two main types of reflection: diffuse and specular

Shading and orientation (distant light sources)

Interreflections and soft shadows

Image Formation: Radiometry

What determines the brightness of an image pixel?

The light

source(s)

The surface

normal

The surface

properties

The optics

The sensor

characteristics

Photometry

• L is the radiance.

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• E is the irradiance.

δ²P = L( P, v ) δA δω

δ²P = L( P, v ) cosθ δA δω

DEFINITION: The radiance is the power traveling at some

point in a given direction per unit area perpendicular to this

direction, per unit solid angle.

DEFINITION: Angles and Solid Angles

(radians)

(steradians)

δ²P = L( P, v ) δA δω

δ²P = L( P, v ) cosθ δA δω

PROPERTY: Radiance is constant along straight lines (in vacuum).