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C280, Computer Vision Prof. Trevor Darrell Lecture 2: Image Formation

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Administrivia We’re now in 405 Soda… New office hours: Thurs. 5-6pm, 413 Soda. I’ll decide on waitlist decisions tomorrow. Any Matlab issues yet? Roster…

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Physical parameters of image formation Geometric – Type of projection – Camera pose Optical – Sensor’s lens type – focal length, field of view, aperture Photometric – Type, direction, intensity of light reaching sensor – Surfaces’ reflectance properties Sensor – sampling, etc.

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Physical parameters of image formation Geometric – Type of projection – Camera pose Optical – Sensor’s lens type – focal length, field of view, aperture Photometric – Type, direction, intensity of light reaching sensor – Surfaces’ reflectance properties Sensor – sampling, etc.

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Perspective and art Use of correct perspective projection indicated in 1 st century B.C. frescoes Skill resurfaces in Renaissance: artists develop systematic methods to determine perspective projection (around ) Durer, 1525Raphael K. Grauman

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Perspective projection equations 3d world mapped to 2d projection in image plane Forsyth and Ponce Camera frame Image plane Optical axis Focal length Scene / world points Scene point Image coordinates ‘’‘’ ‘’‘’

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Homogeneous coordinates Is this a linear transformation? Trick: add one more coordinate: homogeneous image coordinates homogeneous scene coordinates Converting from homogeneous coordinates no—division by z is nonlinear Slide by Steve Seitz

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Perspective Projection Matrix divide by the third coordinate to convert back to non- homogeneous coordinates Projection is a matrix multiplication using homogeneous coordinates: Slide by Steve Seitz Complete mapping from world points to image pixel positions?

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Perspective projection & calibration Perspective equations so far in terms of camera’s reference frame…. Camera’s intrinsic and extrinsic parameters needed to calibrate geometry. Camera frame K. Grauman

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Perspective projection & calibration Camera frame Intrinsic: Image coordinates relative to camera Pixel coordinates Extrinsic: Camera frame World frame World frame World to camera coord. trans. matrix (4x4) Perspective projection matrix (3x4) Camera to pixel coord. trans. matrix (3x3) = 2D point (3x1) 3D point (4x1) K. Grauman

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Intrinsic parameters: from idealized world coordinates to pixel values Forsyth&Ponce Perspective projection W. Freeman

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Intrinsic parameters But “pixels” are in some arbitrary spatial units W. Freeman

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Intrinsic parameters Maybe pixels are not square W. Freeman

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Intrinsic parameters We don’t know the origin of our camera pixel coordinates W. Freeman

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Intrinsic parameters May be skew between camera pixel axes W. Freeman

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Intrinsic parameters, homogeneous coordinates Using homogenous coordinates, we can write this as: or: In camera-based coords In pixels W. Freeman

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Extrinsic parameters: translation and rotation of camera frame Non-homogeneous coordinates Homogeneous coordinates W. Freeman

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Combining extrinsic and intrinsic calibration parameters, in homogeneous coordinates Forsyth&Ponce Intrinsic Extrinsic World coordinates Camera coordinates pixels W. Freeman

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Other ways to write the same equation pixel coordinates world coordinates Conversion back from homogeneous coordinates leads to: W. Freeman

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Calibration target Find the position, u i and v i, in pixels, of each calibration object feature point.

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Camera calibration So for each feature point, i, we have: From before, we had these equations relating image positions, u,v, to points at 3-d positions P (in homogeneous coordinates): W. Freeman

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Camera calibration Stack all these measurements of i=1…n points into a big matrix: W. Freeman

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Showing all the elements: In vector form: Camera calibration W. Freeman

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We want to solve for the unit vector m (the stacked one) that minimizes Q m = 0 The minimum eigenvector of the matrix Q T Q gives us that (see Forsyth&Ponce, 3.1), because it is the unit vector x that minimizes x T Q T Q x. Camera calibration W. Freeman

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Once you have the M matrix, can recover the intrinsic and extrinsic parameters as in Forsyth&Ponce, sect Camera calibration W. Freeman

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Recall, perspective effects… Far away objects appear smaller Forsyth and Ponce

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Perspective effects

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Parallel lines in the scene intersect in the image Converge in image on horizon line Image plane (virtual) Scene pinhole

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Projection properties Many-to-one: any points along same ray map to same point in image Points ? – points Lines ? – lines (collinearity preserved) Distances and angles are / are not ? preserved – are not Degenerate cases: – Line through focal point projects to a point. – Plane through focal point projects to line – Plane perpendicular to image plane projects to part of the image.

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Weak perspective Approximation: treat magnification as constant Assumes scene depth << average distance to camera World points: Image plane

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Orthographic projection Given camera at constant distance from scene World points projected along rays parallel to optical access

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2D

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3D

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Other types of projection Lots of intriguing variants… (I’ll just mention a few fun ones) S. Seitz

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360 degree field of view… Basic approach – Take a photo of a parabolic mirror with an orthographic lens (Nayar) – Or buy one a lens from a variety of omnicam manufacturers… See S. Seitz

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Tilt-shift Titlt-shift images from Olivo Barbieri and Photoshop imitationsOlivo Barbieriimitations S. Seitz

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tilt, shift

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Tilt-shift perspective correction

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normal lenstilt-shift lens

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Rollout Photographs © Justin Kerr Rotating sensor (or object) Also known as “cyclographs”, “peripheral images” S. Seitz

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Photofinish S. Seitz

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Physical parameters of image formation Geometric – Type of projection – Camera pose Optical – Sensor’s lens type – focal length, field of view, aperture Photometric – Type, direction, intensity of light reaching sensor – Surfaces’ reflectance properties Sensor – sampling, etc.

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Pinhole size / aperture Smaller Larger How does the size of the aperture affect the image we’d get? K. Grauman

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Adding a lens A lens focuses light onto the film –Rays passing through the center are not deviated –All parallel rays converge to one point on a plane located at the focal length f Slide by Steve Seitz focal point f

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Pinhole vs. lens K. Grauman

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Cameras with lenses focal point F optical center (Center Of Projection) A lens focuses parallel rays onto a single focal point Gather more light, while keeping focus; make pinhole perspective projection practical K. Grauman

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Human eye Fig from Shapiro and Stockman Pupil/Iris – control amount of light passing through lens Retina - contains sensor cells, where image is formed Fovea – highest concentration of cones Rough analogy with human visual system:

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Thin lens Rays entering parallel on one side go through focus on other, and vice versa. In ideal case – all rays from P imaged at P’. Left focus Right focus Focal length fLens diameter d K. Grauman

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Thin lens equation Any object point satisfying this equation is in focus K. Grauman

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Focus and depth of field Image credit: cambridgeincolour.com

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Focus and depth of field Depth of field: distance between image planes where blur is tolerable Thin lens: scene points at distinct depths come in focus at different image planes. (Real camera lens systems have greater depth of field.) Shapiro and Stockman “circles of confusion”

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Focus and depth of field How does the aperture affect the depth of field? A smaller aperture increases the range in which the object is approximately in focus Flower images from Wikipedia Slide from S. Seitz

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Depth from focus [figs from H. Jin and P. Favaro, 2002] Images from same point of view, different camera parameters 3d shape / depth estimates

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Field of view Angular measure of portion of 3d space seen by the camera Images from K. Grauman

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As f gets smaller, image becomes more wide angle – more world points project onto the finite image plane As f gets larger, image becomes more telescopic – smaller part of the world projects onto the finite image plane Field of view depends on focal length from R. Duraiswami

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Field of view depends on focal length Smaller FOV = larger Focal Length Slide by A. Efros

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Vignetting

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Vignetting “natural”: “mechanical”: intrusion on optical path

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Chromatic aberration

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Physical parameters of image formation Geometric – Type of projection – Camera pose Optical – Sensor’s lens type – focal length, field of view, aperture Photometric – Type, direction, intensity of light reaching sensor – Surfaces’ reflectance properties Sensor – sampling, etc.

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Environment map

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BDRF

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Diffuse / Lambertian

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Foreshortening

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Specular reflection

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Phong Diffuse+specular+ambient:

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Physical parameters of image formation Geometric – Type of projection – Camera pose Optical – Sensor’s lens type – focal length, field of view, aperture Photometric – Type, direction, intensity of light reaching sensor – Surfaces’ reflectance properties Sensor – sampling, etc.

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Digital cameras Film sensor array Often an array of charge coupled devices Each CCD is light sensitive diode that converts photons (light energy) to electrons camera CCD array optics frame grabber computer K. Grauman

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Historical context Pinhole model: Mozi ( BCE), Aristotle ( BCE) Principles of optics (including lenses): Alhacen ( CE) Camera obscura: Leonardo da Vinci ( ), Johann Zahn ( ) 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 Slide credit: L. Lazebnik K. Grauman

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Digital Sensors

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Resolution sensor: size of real world scene element a that images to a single pixel image: number of pixels Influences what analysis is feasible, affects best representation choice. [fig from Mori et al]

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Digital images Think of images as matrices taken from CCD array. K. Grauman

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im[176][201] has value 164 im[194][203] has value 37 width 520 j=1 500 height i=1 Intensity : [0,255] Digital images K. Grauman

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Color sensing in digital cameras Source: Steve Seitz Estimate missing components from neighboring values (demosaicing) Bayer grid

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RG B Color images, RGB color space K. Grauman Much more on color in next lecture…

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Physical parameters of image formation Geometric – Type of projection – Camera pose Optical – Sensor’s lens type – focal length, field of view, aperture Photometric – Type, direction, intensity of light reaching sensor – Surfaces’ reflectance properties Sensor – sampling, etc.

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Summary Image formation affected by geometry, photometry, and optics. Projection equations express how world points mapped to 2d image. Homogenous coordinates allow linear system for projection equations. Lenses make pinhole model practical Photometry models: Lambertian, BRDF Digital imagers, Bayer demosaicing Parameters (focal length, aperture, lens diameter, sensor sampling…) strongly affect image obtained. K. Grauman

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Slide Credits Bill Freeman Steve Seitz Kristen Grauman Forsyth and Ponce Rick Szeliski and others, as marked…

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` Next time Color Readings: – Forsyth and Ponce, Chapter 6 – Szeliski, 2.3.2

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