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Structure from motion

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Given a set of corresponding points in two or more images, compute the camera parameters and the 3D point coordinates Camera 1 Camera 2 Camera 3 R 1,t 1 R 2,t 2 R 3,t 3 ? ? ? Slide credit: Noah Snavely ?

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Structure from motion Given: m images of n fixed 3D points λ ij x ij = P i X j, i = 1, …, m, j = 1, …, n Problem: estimate m projection matrices P i and n 3D points X j from the mn correspondences x ij x1jx1j x2jx2j x3jx3j XjXj P1P1 P2P2 P3P3

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Is SfM always uniquely solvable? Source: N. Snavely Necker cube

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Structure from motion ambiguity If we scale the entire scene by some factor k and, at the same time, scale the camera matrices by the factor of 1/k, the projections of the scene points in the image remain exactly the same: It is impossible to recover the absolute scale of the scene!

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Structure from motion ambiguity If we scale the entire scene by some factor k and, at the same time, scale the camera matrices by the factor of 1/k, the projections of the scene points in the image remain exactly the same More generally, if we transform the scene using a transformation Q and apply the inverse transformation to the camera matrices, then the images do not change:

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Types of ambiguity Projective 15dof Affine 12dof Similarity 7dof Euclidean 6dof Preserves intersection and tangency Preserves parallellism, volume ratios Preserves angles, ratios of length Preserves angles, lengths With no constraints on the camera calibration matrix or on the scene, we get a projective reconstruction Need additional information to upgrade the reconstruction to affine, similarity, or Euclidean

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Projective ambiguity

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Affine ambiguity Affine

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Affine ambiguity

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Similarity ambiguity

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Structure from motion Lets start with affine or weak perspective cameras (the math is easier) center at infinity

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Recall: Orthographic Projection Image World Projection along the z direction

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Orthographic Projection Parallel Projection Affine cameras

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A general affine camera combines the effects of an affine transformation of the 3D space, orthographic projection, and an affine transformation of the image: Affine projection is a linear mapping + translation in non-homogeneous coordinates x X a1a1 a2a2 Projection of world origin

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Affine structure from motion Given: m images of n fixed 3D points: x ij = A i X j + b i, i = 1,…, m, j = 1, …, n Problem: use the mn correspondences x ij to estimate m projection matrices A i and translation vectors b i, and n points X j The reconstruction is defined up to an arbitrary affine transformation Q (12 degrees of freedom): We have 2mn knowns and 8m + 3n unknowns (minus 12 dof for affine ambiguity) Thus, we must have 2mn >= 8m + 3n – 12 For two views, we need four point correspondences

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Affine structure from motion Centering: subtract the centroid of the image points in each view For simplicity, set the origin of the world coordinate system to the centroid of the 3D points After centering, each normalized 2D point is related to the 3D point X j by

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Affine structure from motion Lets create a 2m × n data (measurement) matrix: cameras (2 m) points (n) C. Tomasi and T. Kanade. Shape and motion from image streams under orthography: A factorization method. IJCV, 9(2):137-154, November 1992.Shape and motion from image streams under orthography: A factorization method.

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Affine structure from motion Lets create a 2m × n data (measurement) matrix: cameras (2 m × 3) points (3 × n) The measurement matrix D = MS must have rank 3! C. Tomasi and T. Kanade. Shape and motion from image streams under orthography: A factorization method. IJCV, 9(2):137-154, November 1992.Shape and motion from image streams under orthography: A factorization method.

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Factorizing the measurement matrix Source: M. Hebert

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Factorizing the measurement matrix Singular value decomposition of D: Source: M. Hebert

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Factorizing the measurement matrix Singular value decomposition of D: Source: M. Hebert

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Factorizing the measurement matrix Obtaining a factorization from SVD: Source: M. Hebert

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Factorizing the measurement matrix Obtaining a factorization from SVD: Source: M. Hebert This decomposition minimizes |D-MS| 2

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Affine ambiguity The decomposition is not unique. We get the same D by using any 3×3 matrix C and applying the transformations M MC, S C -1 S That is because we have only an affine transformation and we have not enforced any Euclidean constraints (like forcing the image axes to be perpendicular, for example) Source: M. Hebert

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Transform each projection matrix A to another matrix AC to get orthographic projection Image axes are perpendicular and scale is 1 This translates into 3m equations: (A i C)(A i C) T = A i (CC T )A i = Id, i = 1, …, m Solve for L = CC T Recover C from L by Cholesky decomposition: L = CC T Update M and S: M = MC, S = C -1 S Eliminating the affine ambiguity Source: M. Hebert x X a1a1 a2a2 a 1 · a 2 = 0 |a 1 | 2 = |a 2 | 2 = 1 AA T = Id

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Reconstruction results C. Tomasi and T. Kanade. Shape and motion from image streams under orthography: A factorization method. IJCV, 9(2):137-154, November 1992.Shape and motion from image streams under orthography: A factorization method.

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Algorithm summary Given: m images and n features x ij For each image i, center the feature coordinates Construct a 2m × n measurement matrix D: Column j contains the projection of point j in all views Row i contains one coordinate of the projections of all the n points in image i Factorize D: Compute SVD: D = U W V T Create U 3 by taking the first 3 columns of U Create V 3 by taking the first 3 columns of V Create W 3 by taking the upper left 3 × 3 block of W Create the motion and shape matrices: M = U 3 W 3 ½ and S = W 3 ½ V 3 T (or M = U 3 and S = W 3 V 3 T ) Eliminate affine ambiguity Source: M. Hebert

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Dealing with missing data So far, we have assumed that all points are visible in all views In reality, the measurement matrix typically looks something like this: cameras points

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Dealing with missing data Possible solution: decompose matrix into dense sub- blocks, factorize each sub-block, and fuse the results Finding dense maximal sub-blocks of the matrix is NP- complete (equivalent to finding maximal cliques in a graph) Incremental bilinear refinement (1)Perform factorization on a dense sub-block (2) Solve for a new 3D point visible by at least two known cameras (linear least squares) (3) Solve for a new camera that sees at least three known 3D points (linear least squares) F. Rothganger, S. Lazebnik, C. Schmid, and J. Ponce. Segmenting, Modeling, and Matching Video Clips Containing Multiple Moving Objects. PAMI 2007.Segmenting, Modeling, and Matching Video Clips Containing Multiple Moving Objects.

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Projective structure from motion Given: m images of n fixed 3D points λ ij x ij = P i X j, i = 1,…, m, j = 1, …, n Problem: estimate m projection matrices P i and n 3D points X j from the mn correspondences x ij x1jx1j x2jx2j x3jx3j XjXj P1P1 P2P2 P3P3

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Projective structure from motion Given: m images of n fixed 3D points λ ij x ij = P i X j, i = 1,…, m, j = 1, …, n Problem: estimate m projection matrices P i and n 3D points X j from the mn correspondences x ij With no calibration info, cameras and points can only be recovered up to a 4x4 projective transformation Q: X QX, P PQ -1 We can solve for structure and motion when 2mn >= 11m +3n – 15 For two cameras, at least 7 points are needed

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Projective SFM: Two-camera case Compute fundamental matrix F between the two views First camera matrix: [I | 0] Second camera matrix: [A | b] Then b is the epipole ( F T b = 0 ), A = –[b × ]F F&P sec. 8.3.2

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Sequential structure from motion Initialize motion from two images using fundamental matrix Initialize structure by triangulation For each additional view: Determine projection matrix of new camera using all the known 3D points that are visible in its image – calibration cameras points

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Sequential structure from motion Initialize motion from two images using fundamental matrix Initialize structure by triangulation For each additional view: Determine projection matrix of new camera using all the known 3D points that are visible in its image – calibration Refine and extend structure: compute new 3D points, re-optimize existing points that are also seen by this camera – triangulation cameras points

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Sequential structure from motion Initialize motion from two images using fundamental matrix Initialize structure by triangulation For each additional view: Determine projection matrix of new camera using all the known 3D points that are visible in its image – calibration Refine and extend structure: compute new 3D points, re-optimize existing points that are also seen by this camera – triangulation Refine structure and motion: bundle adjustment cameras points

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Bundle adjustment Non-linear method for refining structure and motion Minimizing reprojection error x1jx1j x2jx2j x3jx3j XjXj P1P1 P2P2 P3P3 P1XjP1Xj P2XjP2Xj P3XjP3Xj

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Self-calibration Self-calibration (auto-calibration) is the process of determining intrinsic camera parameters directly from uncalibrated images For example, when the images are acquired by a single moving camera, we can use the constraint that the intrinsic parameter matrix remains fixed for all the images Compute initial projective reconstruction and find 3D projective transformation matrix Q such that all camera matrices are in the form P i = K [R i | t i ] Can use constraints on the form of the calibration matrix: zero skew Can use vanishing points

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Modern SFM pipeline N. Snavely, S. Seitz, and R. Szeliski, "Photo tourism: Exploring photo collections in 3D," SIGGRAPH 2006."Photo tourism: Exploring photo collections in 3D,"

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Feature detection Detect features using SIFT [Lowe, IJCV 2004] Source: N. Snavely

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Feature detection Detect features using SIFT [Lowe, IJCV 2004] Source: N. Snavely

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Feature matching Match features between each pair of images Source: N. Snavely

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Feature matching Use RANSAC to estimate fundamental matrix between each pair Source: N. Snavely

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Image connectivity graph (graph layout produced using the Graphviz toolkit: http://www.graphviz.org/)http://www.graphviz.org/ Source: N. Snavely

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Incremental SFM Pick a pair of images with lots of inliers (and preferably, good EXIF data) Initialize intrinsic parameters (focal length, principal point) from EXIF Estimate extrinsic parameters (R and t) Five-point algorithm Use triangulation to initialize model points While remaining images exist Find an image with many feature matches with images in the model Run RANSAC on feature matches to register new image to model Triangulate new points Perform bundle adjustment to re-optimize everything

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The devil is in the details Handling degenerate configurations (e.g., homographies) Eliminating outliers Dealing with repetitions and symmetries Handling multiple connected components Closing loops ….

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Review: Structure from motion Ambiguity Affine structure from motion Factorization Dealing with missing data Incremental structure from motion Projective structure from motion Bundle adjustment Modern structure from motion pipeline

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Summary: 3D geometric vision Single-view geometry The pinhole camera model –Variation: orthographic projection The perspective projection matrix Intrinsic and extrinsic parameters Calibration Single-view metrology, calibration using vanishing points Multiple-view geometry Triangulation The epipolar constraint –Essential matrix and fundamental matrix Stereo –Binocular, multi-view Structure from motion –Reconstruction ambiguity –Affine SFM –Projective SFM

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