cs4495/6495 introduction to computer vision€¦ · first known photograph – heliograph view from...
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3A-L1 Cameras and images
CS4495/6495 Introduction to Computer Vision
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What is an image?
• Up until now: a function – a 2D pattern of intensity values
• Today: a 2D projection of 3D points
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First Known Photograph – Heliograph
Reproduction, 1952 View from the Window at le Gras,
Joseph Nicéphore Niépce 1826
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Projection
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Projection
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Image formation – (bad) method
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Pinhole camera
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Camera Obscura (Latin: Darkened Room)
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Home-made pinhole camera
http://www.debevec.org/Pinhole/
Why so
blurry?
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Shrinking the aperture
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Shrinking the aperture
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A little bit of computational photography
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Adding a lens – and concept of focus
“circle of
confusion”
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focal point
F
Lenses
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Thin lenses
Slide by Steve Seitz
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The thin lens
Computer Vision - A Modern Approach
Slides by D.A. Forsyth
|| ' ||z || ||z
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The thin lens
|| ' ||
|| ||
y z
y z
|| ' ||z || ||z
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The thin lens
|| ' ||y z f
y f
|| ' || || ' ||
|| ||
z z f
z f
|| ' ||z || ||z
|| ' ||
|| ||
y z
y z
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The thin lens equation
1 1 1
|| || || ||z f z
1 1 1
|| || || ||z z f
|| ' ||z || ||z
|| ' || || ' ||
|| ||
z z f
z f
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The thin lens equation
1 1 1
|| || || ||z z f
Any object point satisfying this equation is in focus.
|| ' ||z || ||z
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Thin lenses
http://www.phy.ntnu.edu.tw/java/Lens/lens_e.html
(by Fu-Kwun Hwang )
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Varying Focus
Ren Ng
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Depth of field
f / 32 = small aperture
f / 5.6 = large aperture
Flower images from Wikipedia http://en.wikipedia.org/wiki/Depth_of_field
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Nice Depth of Field effect
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Field of View (Zoom)
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Field of View (Zoom)
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FOV depends on Focal Length
Larger Focal Length => Smaller FOV
1/ 2
2 tand
f
d is the “retina” or
sensor size f
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Zooming and Moving are not the same…
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Field of View / Focal Length
Large FOV, small f
Camera close to car
Small FOV, large f
Camera far from the car
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Perspective and Portraits
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Perspective and Portraits
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From Zisserman & Hartley
Effect of focal length on perspective effect
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Dolly Zoom
Pioneered by Hitchcock in Vertigo (1958)
Original(YouTube link) (2:07) Widely used (YouTube link)
Jim Rehg
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But reality can be a problem…
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Geometric Distortion
No distortion Pin cushion Barrel
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Correcting radial distortion
from Helmut Dersch
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Chromatic Aberration
Rays of different wavelength focus in different planes
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Chromatic Aberration
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Vignetting
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Lens systems Nikon 24-70mm zoom