Computer Vision, Image Processing and Computational ... · Shutter speed and focal length Because...
Transcript of Computer Vision, Image Processing and Computational ... · Shutter speed and focal length Because...
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Computer Vision, Image Processing
and Computational Photography
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Course Staff
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Teacher :
Ofir Pele
TA:
Oded Horowits
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Communications
3
WWW: moodel
Forums:
Q & A – ask the staff about material.
Exercises forums.
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Course
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• programming exercises in Matlab,
It might be possible to use c++ with
ceemple
• Some of the exercises will be checked
also in a frontal test
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Books & Resources
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Book:
Richard Szeliski:
“Computer Vision Algorithms and Applications”
http://szeliski.org/Book
Additional papers are in the syllabus
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What are we going to learn ?
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Topics
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• Computer Vision: Image Model
• Image Processing: Image Image
• Computational Photography: Image Image
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Topics
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• Computer Vision: Image Model
• Image Processing: Image Image
• Computational Photography: Image Image
Different techniques from Image Processing,
Can use Computer Vision as an ingredient.
Can use modify the camera.
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Isn’t it easy ?
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Computer vision seems easy, but it is not
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• We see “effortlessly”:
• “Seeing” seems simpler than thinking.
• Everyone (including toddlers) can “see” but only few
can perform really “hard” stuff like playing Chess or
solving a mathematical problem.
• We use 50%-70% of our brain in order to “see” !
• We presume that our “seeing” reflects the physical
world, but it is not always the case.
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Examples
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Photo Tourism overview
Scene reconstruction
Photo Explorer Input photographs
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Photo Tourism overview
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Optical character recognition (OCR)
Digit recognition, AT&T labs
/http://www.research.att.com/~yann
Technology to convert scanned docs to text
License plate readers http://en.wikipedia.org/wiki/Automatic_number_plate_recognition
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Face detection
Many new digital cameras now detect faces
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Smile detection?
Digital Still Camera 70 shot® T-Sony Cyber
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Face recognition
Who is she?
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Vision-based biometrics
story ” Read the How the Afghan Girl was Identified by Her Iris Patterns“
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Pirates of the Caribbean, Industrial Light and Magic
Click here for interactive demo
Special effects: motion capture
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Pirates of the Caribbean, Industrial Light and Magic
Click here for interactive demo
Special effects: motion capture
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Smart cars
Mobileye
Vision systems currently in high-end BMW, GM, Volvo models
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Smart cars
Mobileye
Vision systems currently in high-end BMW, GM, Volvo models
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OrCam – Helping the Visually impaired
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Vision in space
Vision systems (JPL) used for several tasks • Panorama stitching
• 3D terrain modeling
• Obstacle detection, position tracking
• For more, read “Computer Vision on Mars” by Matthies et al.
NASA'S Mars Exploration Rover Spirit captured this westward view from atop
a low plateau where Spirit spent the closing months of 2007.
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Robotics
http://www.robocup.org/
NASA’s Mars Spirit Rover
http://en.wikipedia.org/wiki/Spirit_rover
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Medical imaging
Image guided surgery
Grimson et al., MIT 3D imaging
MRI, CT
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BriefCam
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High Dynamic Range
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High Dynamic Range
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Human Vision
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Bees and humming birds see differently
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Bees and humming birds see differently
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Mantis Shrimp even more
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The Human Eye
(A) Iris – changes the amount of light entering the eye
(B) Lens – can change shape to focus
(C) Retina – where an image is formed
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Light hits the retina which contains photosensitive cells:
Rods (~ 120M) and Cones (~7M)
The Human Eye – Color Perception
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Why more green ?
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Why more green ?
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Optical Illusions
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Measuring light vs.
measuring scene properties
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Measuring light vs.
measuring scene properties
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Assumptions can be wrong
Ames room
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Camera - Image Formation
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Let’s design a camera –
Idea 1: Put film in front of an object
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Let’s design a camera –
Idea 2: “pinhole camera”
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Pinhole camera
If we treat pinhole as a point, only one ray
from any given point can enter the camera.
Virtual
image
pinhole
Image
plane
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Effect of pinhole size - apperture
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Pinhole sizes –
Too big OK Too small=
Diffraction
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lens sensor
Point in focus
Lens – gather more light
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lens sensor
Point in focus
Lens – gather more light
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SLR Lens –
contains several “lens elements”
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Focal length: pinhole optics
pinhole
Film/
sensor
pinhole
Film/
sensor scene
f d
s
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Lenses 24mm
50mm
135mm
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Focal length: pinhole optics
What happens when the focal length is doubled?
Projected object size
Amount of light gathered
pinhole
Film/
sensor scene
f d
s
2f
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Focal length: pinhole optics
What happens when the focal length is doubled?
Projected object size - Bigger
Amount of light gathered - Smaller
pinhole
Film/
sensor scene
f d
s
2f
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Focal length: pinhole optics
What happens when the scene is twice as far?
Is getting closer and zooming in equivalent ?
pinhole
Film/
sensor scene
f d
s
2f 2d
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Perspective vs. viewpoint
Focal lens does NOT ONLY change subject size
Same size by moving the viewpoint
Different perspective (e.g. background)
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Perspective vs. viewpoint
Telephoto makes it easier to select background (a small change in viewpoint is a big change in background.
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Perspective vs. viewpoint
Martin Scorcese, Good Fellas
Moves camera as you zoom in
Better known as the Hitchcock Vertigo effect
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Perspective vs. viewpoint
Portrait: distortion with wide angle
Why?
Wide angle Standard Telephoto
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Sensor / film size
What happens when the film is half the size?
pinhole
Film/
sensor scene
f d
½ s
2f
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Sensor / film size
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Shutter – sets the time the sensor “sees”
• Most of the time, the film/sensor is protected from light.
• When we take a picture, the shutter opens and closes, thereby exposing the film.
• Exposure is proportional to the time the shutter is open.
• Expressed in fraction of a second (1/60s, 1/125s, 1/250s, 1/500s, etc.).
shutter Two types of shutter
The two-blind system (right) is most common
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Shutter distortion
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Effect of shutter speed
Longer shutter speed
more light & motion blur
Faster shutter speed
freezes motion
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Effect of shutter speed
Longer shutter speed
more light & motion blur Faster shutter speed
freezes motion
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Freezing motion –
how fast should the shutter be ?
1/125 1/250 1/500 1/1000
Walking people Running people Car Fast train
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Shutter speed and focal length
Because telephoto “magnifies”
It also magnifies your hand shaking
Telephotos therefore require faster shutter speed
Rule of thumb:
The slowest shutter speed where normal human can hand-
hold and get a sharp picture is 1/f
E.g., a 500mm requires 1/500 s or higher.
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Solutions for shaky hands blur
• Image stabilization:
mechanically compensates for vibration
Can gain 2 or 3 shutter speeds
(1/125 or 1/60 for a 500mm)
• Put the camera on something solid
(table, floor, tripod, …)
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Aperture
Diameter of the lens opening (controlled by diaphragm)
Expressed as a fraction of focal length, in f-number
f/2.0 on a 50mm means that the aperture is 25mm
f/2.0 on a 100mm means that the aperture is 50mm
Small f number = big aperture
What happens to the area of the aperture when going
from f/2.0 to f/4.0?
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Aperture
Diameter of the lens opening (controlled by diaphragm)
Expressed as a fraction of focal length, in f-number
f/2.0 on a 50mm means that the aperture is 25mm
f/2.0 on a 100mm means that the aperture is 50mm
Small f number = big aperture
What happens to the area of the aperture when going
from f/2.0 to f/4.0?
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Aperture – real lens
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Sensitivity to light
• Sensor’s ISO sensitivity to light
Higher = more sensitive, also more noise
• Double the ISO
camera needs half as much light for the same exposure
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Sensitivity to light
• Higher ISO and same light with shutter speed / aperture
More noise (there might be movement blur or out-of-
focus-bur on the other hand)
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Sensitivity to light
Higher ISO,
same aperture and shutter speed.
different cameras,
different amount of noise – why ?
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Sensitivity to light
Higher ISO,
same aperture and shutter speed.
different cameras,
different amount of noise – why ? Mostly because of
sensor size
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Depth of Field
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Depth of Field
Changing the aperture of a camera also changes
the amount of the image that is in focus – this
amount is called the depth of field
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DoF depends on aperture
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DoF depends on aperture
90
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DoF depends on focusing distance
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DoF depends on focal length
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Only light at the focus depth is
exactly in focus,
the rest has varying circles of confusion.
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Only light at the focus depth is
exactly in focus,
the rest has varying circles of confusion.
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Depth of Field – shallow and deep
Large aperture=
small f numbers
Shallow depths of field
Small aperture=
Large f number
Deep depths of field
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Depth of Field –
can be changed by focal length
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Depth of Field –
can be changed by focus
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Creative Use of DOF
Separate your subject
from the background
Give a feeling of space
or distance to a scene
Highlight a area of the
image
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Creative Use of DOF
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Perspective effects
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Perspective effects
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Perspective effects
Far away objects appear smaller
Forsyth and Ponce
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Perspective effects
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Perspective effects Parallel lines in the scene intersect in the image
Converge in image on horizon line
Image
plane
(virtual)
Scene
pinhole
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Perspective effects
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Perspective effects
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Perspective effects
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Perspective effects
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Beyond Pinholes: Radial Distortion
Corrected Barrel Distortion