Download - Lecture 4a: Cameras
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Lecture 4a: Cameras
CS6670: Computer VisionNoah Snavely
Source: S. Lazebnik
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Reading
• Szeliski chapter 2.2.3, 2.3
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Image formation
• Let’s design a camera– Idea 1: put a piece of film in front of an object– Do we get a reasonable image?
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Pinhole camera
• Add a barrier to block off most of the rays– This reduces blurring– The opening known as the aperture– How does this transform the image?
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Camera Obscura
• Basic principle known to Mozi (470-390 BC), Aristotle (384-322 BC)
• Drawing aid for artists: described by Leonardo da Vinci (1452-1519)
Gemma Frisius, 1558
Source: A. Efros
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Camera Obscura
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Home-made pinhole camera
http://www.debevec.org/Pinhole/
Why soblurry?
Slide by A. Efros
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Shrinking the aperture
• Why not make the aperture as small as possible?• Less light gets through• Diffraction effects...
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Shrinking the aperture
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Adding a lens
• A lens focuses light onto the film– There is a specific distance at which objects are “in
focus”• other points project to a “circle of confusion” in the image
– Changing the shape of the lens changes this distance
“circle of confusion”
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Lenses
• A lens focuses parallel rays onto a single focal point– focal point at a distance f beyond the plane of the lens
(the focal length)• f is a function of the shape and index of refraction of the lens
– Aperture restricts the range of rays• aperture may be on either side of the lens
– Lenses are typically spherical (easier to produce)
focal point
F
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Thin lenses
• Thin lens equation:
– Any object point satisfying this equation is in focus– What is the shape of the focus region?– How can we change the focus region?– Thin lens applet: http://www.phy.ntnu.edu.tw/java/Lens/lens_e.html (by Fu-Kwun Hwang )
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Depth of Field
• Changing the aperture size affects depth of field– A smaller aperture increases the range in which the
object is approximately in focus
f / 5.6
f / 32
Flower images from Wikipedia http://en.wikipedia.org/wiki/Depth_of_field
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Depth of Field
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The eye
• The human eye is a camera– Iris - colored annulus with radial muscles
– Pupil - the hole (aperture) whose size is controlled by the iris– What’s the “film”?
– photoreceptor cells (rods and cones) in the retina
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Eyes in nature: eyespots to pinhole camera
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Eyes in nature
Source: Animal Eyes, Land & Nilsson
(polychaete fan worm)
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Lens Based Camera Obscura, 1568
Before Film was invented
Srinivasa Narasimhan’s slide
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Still Life, Louis Jaques Mande Daguerre, 1837
Film camera
Srinivasa Narasimhan’s slide
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Silicon Image Detector, 1970
Silicon Image Detector
Shree Nayar’s slide
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Digital camera
• A digital camera replaces film with a sensor array– Each cell in the array is a Charge Coupled Device
• light-sensitive diode that converts photons to electrons• other variants exist: CMOS is becoming more popular• http://electronics.howstuffworks.com/digital-camera.htm
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Color
• So far, we’ve talked about grayscale images• What about color?• Most digital images are comprised of three color
channels – red, green, and, blue – which combine to create most of the colors we can see
• Why are there three?
=
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Color perception
• Three types of cones– Each is sensitive in a different region of the spectrum
• but regions overlap• Short (S) corresponds to blue• Medium (M) corresponds to green• Long (L) corresponds to red
– Different sensitivities: we are more sensitive to green than red• varies from person to person (and with age)
– Colorblindness—deficiency in at least one type of cone
L response curve
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Field sequential
YungYu Chuang’s slide
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Field sequential
YungYu Chuang’s slide
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Field sequential
YungYu Chuang’s slide
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Prokudin-Gorskii (early 1900’s)
Lantern projector
http://www.loc.gov/exhibits/empire/YungYu Chuang’s slide
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Prokudin-Gorskii (early 1990’s)
YungYu Chuang’s slide
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Color sensing in camera: Prism• Requires three chips and precise alignment• More expensive
CCD(B)
CCD(G)
CCD(R)
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Color filter array
Source: Steve Seitz
Estimate missing components from neighboring values(demosaicing)
Why more green?
Bayer grid
Human Luminance Sensitivity Function
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Bayer’s pattern
YungYu Chuang’s slide
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Foveon X3 sensor• Light penetrates to different depths for
different wavelengths• Multilayer CMOS sensor gets 3 different
spectral sensitivities
YungYu Chuang’s slide
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Color filter array
red green blue outputYungYu Chuang’s slide
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X3 technology
red green blue outputYungYu Chuang’s slide
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Foveon X3 sensor
X3 sensorBayer CFA
YungYu Chuang’s slide
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Historical context• Pinhole model: Mozi (470-390 BC),
Aristotle (384-322 BC)• Principles of optics (including lenses):
Alhacen (965-1039) • Camera obscura: Leonardo da Vinci
(1452-1519), Johann Zahn (1631-1707)• First photo: Joseph Nicephore Niepce (1822)• Daguerréotypes (1839)• Photographic film (Eastman, 1889)• Cinema (Lumière Brothers, 1895)• Color Photography (Lumière Brothers, 1908)• Television (Baird, Farnsworth, Zworykin, 1920s)• First consumer camera with CCD:
Sony Mavica (1981)• First fully digital camera: Kodak DCS100 (1990)
Niepce, “La Table Servie,” 1822
CCD chip
Alhacen’s notes