# 776 Computer Vision Jan-Michael Frahm, Enrique Dunn Spring 2013.

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776 Computer Vision Jan-Michael Frahm, Enrique Dunn Spring 2013

Last class

Last Class World to camera coord. trans. matrix (4x4) Perspective projection matrix (3x4) Camera to pixel coord. trans. matrix (3x3) = 2D point (3x1) 3D point (4x1)

Facing Real Cameras There are undesired effects in real situations o perspective distortion Camera artifacts o aperture is not infinitely small o lens o vignetting

No distortionPin cushionBarrel Radial Distortion o Caused by imperfect lenses o Deviations are most noticeable near the edge of the lens slide: S. Lazebnik

Radial Distortion Brown’s distortion model o accounts for radial distortion o accounts for tangential distortion (distortion caused by lens placement errors) typically K 1 is used or K 1, K 2, P 1, P 2 (x u, y u ) undistorted image point as in ideal pinhole camera (x d,y d ) distorted image point of camera with radial distortion (x c,y c ) distortion center K n n-th radial distortion coefficient P n n-th tangential distortion coefficient

Facing Real Cameras There are undesired effects in real situations o perspective distortion Camera artifacts o aperture is not infinitely small o lens o vignetting, radial distortion

Depth of Field http://www.cambridgeincolour.com/tutorials/depth-of-field.htm Slide by A. Efros

How can we control the depth of field? Changing the aperture size affects depth of field o A smaller aperture increases the range in which the object is approximately in focus o But small aperture reduces amount of light – need to increase exposure Slide by A. Efros

F Number of the Camera f number (f-stop) ratio of focal length to aperture

Varying the aperture Large aperture = small DOFSmall aperture = large DOF Slide by A. Efros

Facing Real Cameras There are undesired effects in real situations o perspective distortion Camera artifacts o aperture is not infinitely small o lens o vignetting, radial distortion o depth of field

Field of View Slide by A. Efros What does FOV depend on?

f Field of View Smaller FOV = larger Focal Length Slide by A. Efros f FOV depends on focal length and size of the aperture

Field of View / Focal Length Large FOV, small f Camera close to car Small FOV, large f Camera far from the car Sources: A. Efros, F. Durand

Same effect for faces standard wide-angletelephoto Source: F. Durand

The dolly zoom Continuously adjusting the focal length while the camera moves away from (or towards) the subject http://en.wikipedia.org/wiki/Dolly_zoom slide: S. Lazebnik

The Dolly Zoom

Facing Real Cameras There are undesired effects in real situations o perspective distortion Camera artifacts o aperture is not infinitely small o lens o vignetting, radial distortion o depth of field o field of view

Digital camera A digital camera replaces film with a sensor array o Each cell in the array is light-sensitive diode that converts photons to electrons o Two common types Charge Coupled Device (CCD) Complementary metal oxide semiconductor (CMOS) o http://electronics.howstuffworks.com/digital-camera.htm http://electronics.howstuffworks.com/digital-camera.htm Slide by Steve Seitz

Color sensing in camera: Color filter array Source: Steve Seitz Estimate missing components from neighboring values (demosaicing) Why more green? Bayer grid Human Luminance Sensitivity Function

Problem with demosaicing: color moire Slide by F. Durand

The cause of color moire detector Fine black and white detail in image misinterpreted as color information Slide by F. Durand

Color sensing in camera: Prism Requires three chips and precise alignment More expensive CCD(B) CCD(G) CCD(R) slide: S. Lazebnik

Color sensing in camera: Foveon X3 Source: M. Pollefeys http://en.wikipedia.org/wiki/Foveon_X3_sensorhttp://www.foveon.com/article.php?a=67 CMOS sensor Takes advantage of the fact that red, blue and green light penetrate silicon to different depths better image quality

Facing Real Cameras There are undesired effects in real situations o perspective distortion Camera artifacts o Aperture is not infinitely small o Lens o Vignetting, radial distortion o Depth of field o Field of view o Color sensing

Rolling Shutter Cameras Many cameras use CMOS sensors (mobile, DLSR, …) To save cost these are often rolling shutter cameras o lines are progressively exposed o line by line image reading Rolling shutter artifacts image source: Wikipedia

Rolling Shutter regular camera (global shutter) rolling shutter camera

Facing Real Cameras There are undesired effects in real situations o perspective distortion Camera artifacts o Aperture is not infinitely small o Lens o Vignetting, radial distortion o Depth of field o Field of view o Color sensing o Rolling shutter cameras

Digital camera artifacts Noise low light is where you most notice noisenoise light sensitivity (ISO) / noise tradeoff stuck pixels In-camera processing oversharpening can produce haloshalos Compression JPEG artifacts, blocking Blooming charge overflowing into neighboring pixelsoverflowing Smearing o columnwise overexposue Color artifacts purple fringing from microlenses, purple fringing white balance modified from Steve Seitz

Conventional versus light field camera slide: Marc Levoy

Conventional versus light field camera slide: Marc Levoy

Conventional versus light field camera slide: Marc Levoy

Prototype camera 4000 × 4000 pixels ÷ 292 × 292 lenses = 14 × 14 pixels per lens Contax medium format cameraKodak 16-megapixel sensor Adaptive Optics microlens array125μ square-sided microlenses slide: Marc Levoy

Digitally stopping-down stopping down = summing only the central portion of each microlens Σ Σ f / N light field camera, with P × P pixels under each microlens, can produce views as sharp as an f / (N × P) conventional camera slide: Marc Levoy

Digital refocusing refocusing = summing windows extracted from several microlenses Σ Σ f/N light field camera can produce views with a shallow depth of field ( f / N ) focused anywhere within the depth of field of an f / (N × P) camera images: Marc Levoy

Example of digital refocusing images: Marc Levoy

Extending the depth of field conventional photograph, main lens at f / 22 conventional photograph, main lens at f / 4 light field, main lens at f / 4, after all-focus algorithm [Agarwala 2004] images: Marc Levoy

Digitally moving the observer moving the observer = moving the window we extract from the microlenses Σ Σ images: Marc Levoy

Example of moving the observer slide: Marc Levoy

Moving backward and forward slide: Marc Levoy

Historic milestones Pinhole model: Mozi (470-390 BCE), Aristotle (384-322 BCE) Principles of optics (including lenses): Alhacen (965-1039 CE) 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

Early color photography Sergey Prokudin-Gorskii (1863-1944) Photographs of the Russian empire (1909- 1916) http://www.loc.gov/exhibits/empire/ http://en.wikipedia.org/wiki/Sergei_Mikhailovich_Prokudin-Gorskii Lantern projector

First digitally scanned photograph 1957, 176x176 pixels http://listverse.com/history/top-10-incredible-early-firsts-in-photography/