Projective geometry- 2D

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Projective geometry- 2D. Acknowledgements Marc Pollefeys: for allowing the use of his excellent slides on this topic http://www.cs.unc.edu/~marc/mvg/ Richard Hartley and Andrew Zisserman, " Multiple View Geometry in Computer Vision ". Homogeneous representation of points. on. - PowerPoint PPT Presentation

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  • Projective geometry- 2D

    AcknowledgementsMarc Pollefeys: for allowing the use of his excellent slides on this topic http://www.cs.unc.edu/~marc/mvg/Richard Hartley and Andrew Zisserman, "Multiple View Geometry in Computer Vision"

    Projective Geometry 2D

  • Homogeneous coordinatesHomogeneous representation of linesequivalence class of vectors, any vector is representativeSet of all equivalence classes in R3(0,0,0)T forms P2The point x lies on the line l if and only if xTl=lTx=0

    Projective Geometry 2D

  • Points from lines and vice-versaIntersections of lines The intersection of two lines and is Example

    Projective Geometry 2D

  • Ideal points and the line at infinityIntersections of parallel lines Note that in P2 there is no distinction between ideal points and othersNote that this set lies on a single line,

    Projective Geometry 2D

  • SummaryThe set of ideal points lies on the line at infinity, intersects the line at infinity in the ideal point

    A line parallel to l also intersects in the same ideal point, irrespective of the value of c.

    In inhomogeneous notation, is a vector tangent to the line.It is orthogonal to (a, b) -- the line normal. Thus it represents the line direction.As the lines direction varies, the ideal point varies over .--> line at infinity can be thought of as the set of directions of lines in the plane.

    Projective Geometry 2D

  • A model for the projective planeexactly one line through two pointsexaclty one point at intersection of two linesPoints represented by rays through originLines represented by planes through origin

    x1x2 plane represents line at infinity

    Projective Geometry 2D

  • Duality

    Projective Geometry 2D

  • ConicsCurve described by 2nd-degree equation in the plane5DOF:

    Projective Geometry 2D

  • Conics http://ccins.camosun.bc.ca/~jbritton/jbconics.htm

    Projective Geometry 2D

  • Five points define a conicFor each point the conic passes through

    Projective Geometry 2D

  • Tangent lines to conicsThe line l tangent to C at point x on C is given by l=CxlxC

    Projective Geometry 2D

  • Dual conicsA line tangent to the conic C satisfies Dual conics = line conics = conic envelopesC* : Adjoint matrix of C.

    Projective Geometry 2D

  • Degenerate conicsA conic is degenerate if matrix C is not of full ranke.g. two lines (rank 2)e.g. repeated line (rank 1)Degenerate line conics: 2 points (rank 2), double point (rank1)

    Projective Geometry 2D

  • Projective transformationsA projectivity is an invertible mapping h from P2 to itself such that three points x1,x2,x3 lie on the same line if and only if h(x1),h(x2),h(x3) do.Definition:projectivity=collineation=projective transformation=homography

    Projective Geometry 2D

  • Mapping between planescentral projection may be expressed by x=Hx(application of theorem)

    Projective Geometry 2D

  • Removing projective distortionselect four points in a plane with know coordinates(linear in hij)(2 constraints/point, 8DOF 4 points needed)Remark: no calibration at all necessary, better ways to compute (see later)

    Projective Geometry 2D

  • Transformation of lines and conicsTransformation for linesFor a point transformation

    Projective Geometry 2D

  • Distortions under center projectionSimilarity: squares imaged as squares.Affine: parallel lines remain parallel; circles become ellipses.Projective: Parallel lines converge.

    Projective Geometry 2D

  • Class I: Isometries(iso=same, metric=measure)special cases: pure rotation, pure translation3DOF (1 rotation, 2 translation) Invariants: length, angle, area

    Projective Geometry 2D

  • Class II: Similarities(isometry + scale)also know as equi-form (shape preserving)metric structure = structure up to similarity (in literature)4DOF (1 scale, 1 rotation, 2 translation) Invariants: ratios of length, angle, ratios of areas, parallel lines

    Projective Geometry 2D

  • Class III: Affine transformationsnon-isotropic scaling! (2DOF: scale ratio and orientation)6DOF (2 scale, 2 rotation, 2 translation) Invariants: parallel lines, ratios of parallel lengths, ratios of areas

    Projective Geometry 2D

  • Class VI: Projective transformationsAction non-homogeneous over the plane8DOF (2 scale, 2 rotation, 2 translation, 2 line at infinity) Invariants: cross-ratio of four points on a line (ratio of ratio)

    Projective Geometry 2D

  • Action of affinities and projectivities on line at infinityLine at infinity becomes finite, allows to observe vanishing points, horizon.Line at infinity stays at infinity, but points move along line

    Projective Geometry 2D

  • Decomposition of projective transformationsupper-triangular,decomposition unique (if chosen s>0)Example:

    Projective Geometry 2D

  • Overview transformationsProjective8dofAffine6dofSimilarity4dofEuclidean3dofConcurrency, collinearity, order of contact (intersection, tangency, inflection, etc.), cross ratioParallellism, ratio of areas, ratio of lengths on parallel lines (e.g midpoints), linear combinations of vectors (centroids). The line at infinity lRatios of lengths, angles.The circular points I,J

    lengths, areas.

    Projective Geometry 2D