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		  CS 448 - Camera technologies, April 24, 2008

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==> See also the Powerpoint slides for this class.
    On the slides, mouse over the orange "talk balloon"
    to see my speaker's notes and lists of references

==> Again, I didn't quite get to everything in these notes during the class.
    I've left them in the notes, in case you want to browse on your own.

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	  *** Components of a camera (besides lens and aperture) ***

Viewfinder:
	o 35mm format (24mm x 36mm)
		- separate viewfinder
			-> Ansel Adams, p. 12-13
		- single-lens reflex (SLR)
			-> mirror and pentaprism, Goldberg, p. 137
	o medium format (2 1/4 x 2 1/4 inch)
		- twin lens reflex
			-> Adams, p. 22-23
		- single lens reflex
			-> Adams, p. 24-25, with ground-glass screen,
		- but the image is weak, so these are no longer used
	o large format (4 x 5 inch, 5 x 7, 8 x 10, 11 x 14)
		- ground glass screen
			-> Adams, p. 30-31, i.e. a view camera,
			   as I showed in class two weeks ago
Focusing aids:
	o split wedge
		- two prisms
			-> Goldberg, p. 12
		- gather light from two regions in lens
			-> Goldberg, p. 18
		- If image is well focused, the two images coincide, else not
			-> Goldberg, p. 14-15, but this figure is wrong, see
			-> downloaded/photography/kerr-split-focusing.pdf,
			   fig. 10 (p. 11) shows how each half of circle sees
			   only half of aperture, hence half of defocus blur,
			   causing a visible offset unless there is no blur.

	o basis for computer vision algorithms
		- by placing attenuating masks in the aperture
			-> Simoncelli and Farid, Proc. ECCV 1996.
		- by rotating a lens element through several positions
			-> Subramanian et al., Proc. ICCV 2001.

Autofocus systems: (from Goldberg)
	o active methods
		- time of flight using ultrasonic chirp, or
		- triangulation using narrow beam of infrared light
		- both fail at long distances, dark or reflective objects, etc.
	o passive methods
		- these methods require a reasonably bright scene

		- two-view methods (for non-SLR cameras) compares 1D images
		  from two viewpoints using 1D CCD strip, looks for match
			-> Goldberg, p. 33
			-> Canon EOS cameras?

		- contrast method compares contrast of images at three depths,
		  if in focus, image will have high contrast, else not
			-> Goldberg, p. 36

		- phase methods compares two parts of lens at the sensor plane,
		  if in focus, entire exit pupil sees a uniform color, else not
			-> crayons_lf1-collage.jpg,
			   assumes object has diffuse BRDF

		- a variant directs two parts of lens through lenslets, if in
		  focus, left and right halves of lenslet images are identical
			-> Goldberg, p. 41

		- another variant directs two parts of lens through lenslets,
		  if in focus, distance between imagelets matches a reference
			(similar in spirit to split wedge focusing)
			-> Goldberg, p. 43

	o use depth info to drive zoom lens, to keep object fixed size in image
		(Goldberg, p. 47)
	o rubber focus: vary focus, hoping that sharp edges will dominate blurs
		(Goldberg, p. 50)
		-> Convolution of (Norwegian village) scene with 1/r^2 filter
	o could deconvolve such an image to recover the original, sharp scene,
	  which leads to...

	o wavefront coding
		-> see Powerpoint slides

Shutters:
	o leaf shutter
			-> Adams, p. 82
		- speeds down to 1/500 second
		- shutter located at aperture stop
		- center of lens (not image!) gets light for longer than edges

	o focal-plane shutter
			-> Adams, p. 83, or Goldberg, p. 70
			-> Canon has vertical-travel focal-plane shutter
		- capable of faster speeds, but
		- distorts fast-moving objects
			-> Lartigue, Grand Prix auto race, in Adams, p. 85

	o rotary shutters for movie cameras (Goldberg, p. 86)

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			*** High speed photography ***

The speed of brief events:
	o Harold Edgerton, Stopping Time
		-> water streaming from a faucet (1932), p. 45
		-> flattened tennis ball, p. 54
		-> vortices from spinning blades, p. 58-59
		-> tennis serve, p. 83
		-> bullet through apple, p. 126
		-> milk drop, p. 127
		-> atomic explosion, 1/100,000,000 second exposure, p. 145
	o other brief events
		- airbag inflation:	0.035 sec
	o motion capture for computer animation
		- Vicon 8i captures at 250 fps
		- 500 fps is necessary to avoid hand-editing

Visual perception:
	o How brief an event can we see?
	o critical fusion frequency (CFF)
		- the flicker rate above which the stimulus appears continuous
		- depends on brightness, contrast, and size of source
		- 50-60 fps
	o persistence of vision
		- an ember rotating on a wheel appears continuous at high speed
		  from which Chevalier d'Arcy (1765) estimated persistence at
		  0.1 second
	o tachistoscopic imagery
		- recognition after brief presentation varies depending on
		  duration and stimulus complexity, typical is 50 - 500 ms
		- "erased" by bright flash, so might be a "frame buffer"?!
		- use in subliminal advertising

Historical development of high-speed photography:
	o Muybridge (1870s) - 1/1000th second shutter
		-> galloping horse, Ray, p. 14
	o Ernst Mach (1880s) - spark photographs of bullets in flight
		-> drawing of apparatus, bullet in flight, Ray, p. 10
	o Lucien Bull (1904) - rotating film drum, 2000 fps x 50 frames
		-> bees in flight, Ray, p. 13
	o F.J. Tuttle (1930s) - rotating prism to aim light at moving film,
	  acheived up to 8000 fps, required very bright illumination
		-> diagram, Ray, p. 20
	o drum cameras - fixed film on rotating drum, strobed light
	  instead of prism, up to 250,000 fps, but limited duration
	o rotating mirror - rotating mirror, fixed film, multiple lenses,
	  20 million fps, limited duration, only useful for explosions?!
	o image converter - image intensifier tube (Ray, p. 136) with pulsed
	  amplification, single image with exposure of 1/600 millionth sec.
	o video systems - 1,000 fps at 1024 x 1024 pixels, can trade off
	  frame rate and spatial resolution

Unusual high-speed techniques:
	o stroboscopic illumination
		- strobe lights - down to 1 microsecond (Ray, p. 160)
		- pulsed laser - 15 ns (Ray, p. 165)
	o streak photography using a cross-slit camera
			-> Ray, p. 126
		- used at the racetrack
			-> Kingslake, Optics in Photography, p. 24
	o shadowgraphy
		- silhouette of object directly onto film (point light source)
		- shock wave refracts the air, making them visible as caustics
			-> Ray, p. 257
	o Schlieren photography (optional)
		- knife edge at object space focal distance, see my drawing
			-> Ray, p. 261
	o synchronization
		- microphones are often used

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		*** Introduction to coded aperture imaging ***

See Powerpoint slides.

The two papers we'll be discussing in class on Thursday, April 24:

	Levin, A., Fergus, R., Durand, F., Freeman, W.T.,
	Image and Depth from a Conventional Camera with a Coded Aperture,
	Proc. SIGGRAPH 2007.

	Raskar, R., Agrawal, A., Tumblin, J.,
	Coded Exposure Photography: Motion Deblurring using Fluttered Shutter,
	Proc. SIGGRAPH 2006. 

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			     *** Sources cited ***

Camera technology:

	Norman Goldberg,
	Camera technology: the dark side of the lens,
	Academic Press, 1992.

	Ansel Adams
	The Camera,
	Little, Brown, and Co., 1976.

High-speed photography:

	Edgerton, H.,
	Stopping Time,
	Abrams, 1987.

	Sidney F. Ray ed.,
	High Speed Photography,
	Focal Press, 1997.

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