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Cameras



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· · · ! (-)

· MATLAB
· MATLAB ·


Let's design a camera

Idea 1: put a piece of film in front of an object Do we get a reasonable image?
Slide by Steve Seitz


Pinhole camera

Add a barrier to block off most of the rays
· This reduces blurring · The opening is known as the aperture

Slide by Steve Seitz


Pinhole camera model ()

Pinhole model:
· Captures pencil of rays ­ all rays through a single point · The point is called Center of Projection (focal point) · The image is formed on the Image Plane

Slide by Steve Seitz


Perspective distortion
· What does a sphere project to?

Image source: F. Durand


Perspective distortion
· What does a sphere project to?


Perspective distortion
· The exterior columns appear bigger · The distortion is not due to lens flaws · Problem pointed out by Da Vinci

Slide by F. Durand


Perspective distortion: People


Perspective distortion
· Problem for architectural photography: converging verticals

Source: F. Durand


Perspective distortion
· Problem for architectural photography: converging verticals

Tilting the camera upwards results in converging verticals

Keeping the camera level, with an ordinary lens, captures only the bottom portion of the building

Shifting the lens upwards results in a picture of the entire subject

· Solution: view camera (lens shifted w.r.t. film)

http://en.wikipedia.org/wiki/Perspective_correction_lens

Source: F. Durand


Perspective distortion
· Problem for architectural photography: converging verticals · Result:

Source: F. Durand


Building a real camera


Camera Obscura ( )

· Basic principle known to Mozi (470-390 BCE), Aristotle (384-322 BCE) · Drawing aid for artists: described by Leonardo da Vinci (1452-1519)
Gemma Frisius, 1558

Source: A. Efros


Home-made pinhole camera

Why so blurry?

Slide by A. Efros

http://www.debevec.org/Pinhole/


Shrinking the aperture

Why not make the aperture as small as possible?
· Less light gets through · Diffraction effects...
Slide by Steve Seitz


Shrinking the aperture


Adding a lens

A lens focuses light onto the film
· Thin lens model:
­ Rays passing through the center are not deviated (pinhole projection model still holds)
Slide by Steve Seitz


Adding a lens

focal point

f

A lens focuses light onto the film
· Thin lens model:
­ Rays passing through the center are not deviated (pinhole projection model still holds) ­ All parallel rays converge to one point on a plane located at the focal length f

Slide by Steve Seitz


Adding a lens

"circle of confusion"

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

Slide by Steve Seitz


Real lenses


Lens Flaws: Chromatic Aberration
Lens has different refractive indices for different wavelengths: causes color fringing

Near Lens Center

Near Lens Outer Edge


Lens flaws: Spherical aberration
Spherical lenses don't focus light perfectly Rays farther from the optical axis focus closer


Lens flaws: Vignetting


Radial Distortion
· Caused by imperfect lenses · Deviations are most noticeable near the edge of the lens

No distortion

Pin cushion

Barrel


Color photography

Nikolaevskii Cathedral from southwest in Mozhaisk (1911)


Short history of color photography
· 1611, Marco Antonio de Dominis, synthesis of all colors by mixing red, green, violet · 1666, Newton, splitting light into spectrum · 1801, Thomas Young, three types of colorsensetive nerves (Young-Helmholtz theory) · 1861, James Clark Maxwell, first color photo demonstration through three color-filtered projectors · 1907, Lumiere brothers, first commercial color film, the Autochrome plates


Prokudin-Gorsky
1901 ­ Adolf Miethe constructed camera for color photography

Lantern projector 1902 ­ Prokudin-Gorsky working in Adolf Miethe laboratory improving sensibilizers 1903 ­ special color projector built


Prokudin-Gorsky
1905 ­ started photographic expeditions 1909 ­ expeditions supported by Nikolai II 1909-1916 ­ about 2 thousands of photos taken

L.N. Tolstoy, may 1908

2 expeditions along Volga river, 1910


Prokudin-Gorsky technique
- (1863-1944)

!
http://en.wikipedia.org/wiki/Sergei_Mikhailovich_Prokudin-Gorskii http://www.loc.gov/exhibits/empire/


Color film photography
· Development of layered photographic film · Kodak three-layer film

film development

· Detailed history of color processes: http://www.iml.unibas.ch/SKRIPTEN/ScriptCo lor/color_photography_history.pdf


Digital camera

A digital camera replaces film with a sensor array
· · Each cell in the array is light-sensitive diode that converts photons to electrons Two common types

·

­ Charge Coupled Device (CCD) ­ Complementary metal oxide semiconductor (CMOS) http://electronics.howstuffworks.com/digital-camera.htm
Slide by Steve Seitz


Color sensing in camera: Color filter array
Bayer grid Estimate missing components from neighboring values (demosaicing)

Why more green?

Human Luminance Sensitivity Function
Source: Steve Seitz


Problem with demosaicing: color moire

Slide by F. Durand


Color sensing in camera: Prism
· Requires three chips and precise alignment · More expensive
CCD(R)

CCD(G)

CCD(B)


Color sensing in camera: Foveon X3
· CMOS sensor · Takes advantage of the fact that red, blue and green light penetrate silicon to different depths

http://www.foveon.com/article.php?a=67

http://en.wikipedia.org/wiki/Foveon_X3_sensor

better image quality

Source: M. Pollefeys


Digital camera artifacts
Noise
­ low light is where you most notice noise ­ light sensitivity (ISO) / noise tradeoff ­ stuck pixels

In-camera processing
­ oversharpening can produce halos

Compression
­ JPEG artifacts, blocking

Blooming
­ charge overflowing into neighboring pixels

Color artifacts
­ purple fringing from microlenses, ­ white balance

Slide by Steve Seitz



·
· MATLAB . . · (pdf) · c ( .zip) mizotin@cs.msu.ru [ProkudinTask].

·
· · · · FFT (inpainting) (. )



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