Computer Vision cmput 428/615

Transcription

1 Computer Vision cmput 428/615 Basic 2D and 3D geometry and Camera models Martin Jagersand

2 The equation of projection Intuitively: How do we develop a consistent mathematical framework for projection calculations? Mathematically: Cartesian coordinates: (x, y, z) ( f x z, f y z ) Projectively: x = PX

3 Challenges in Computer Vision: What images don t provide lengths depth

4 Distant objects are smaller

5 Visual ambiguity Will the scissors cut the paper in the middle?

6 Ambiguity Will the scissors cut the paper in the middle? NO!

7 Visual ambiguity Is the probe contacting the wire?

8 Ambiguity Is the probe contacting the wire? NO!

9 Visual ambiguity Is the probe contacting the wire?

10 Ambiguity Is the probe contacting the wire? NO!

11 History of Perspective Prehistoric: Roman

12 Perspective: Da Vinci

13 Visualizing perspective: Dürer Perspectograph 1500 s

14 Parallel lines meet Image plane common to draw image plane in front of the focal point Centre of projection 3D world

15 Perspective Imaging Properties Challenges with measurements in multiple images: Distances/angles change Ratios of dist/angles change Parallel lines intersect 90

16 What is preserved? Invariants: Points map to points Intersections are preserved Lines map to lines Collinearity preserved Ratios of ratios (cross ratio) Horizon horizon What is a good way to represent imaged geometry?

17 Vanishing points each set of parallel lines (=direction) meets at a different point The vanishing point for this direction How would you show this? Sets of parallel lines on the same plane lead to collinear vanishing points. The line is called the horizon for that plane

18 Geometric properties of projection Points go to points Lines go to lines Planes go to whole image Polygons go to polygons Degenerate cases line through focal point to point plane through focal point to line

19 Polyhedra project to polygons (because lines project to lines)

20 Junctions are constrained This leads to a process called line labelling one looks for consistent sets of labels, bounding polyhedra disadv - can t get the lines and junctions to label from real images

21 Back to projection Cartesian coordinates: (x, y, z) ( f x z, f y z ) We will develop a framework to express projection as x=px, where x is 2D image projection, P a projection matrix and X is 3D world point.

22 Basic geometric transformations: Translation A translation is a straight line movement of an object from one postion to another. A point (x,y) is transformed to the point (x,y ) by adding the translation distances T x and T y : x = x + T x y = y + T y z = z + T z

23 Coordinate rotation Example: Around y-axis p = [ x ] y z = cos 0 sin sin 0 cos [ x ] y z = R y p Z P X X

24 Euler angles Note: Successive rotations. Order matters. = [ cosϕ sinϕ ] 0 sinϕ cos ϕ R=R z R y R x cos 0 sin sin 0 cos cosψ sinψ 0 sinψ cos ψ

25 Rotation and translation Translation t in new o coordinates Z P p = cos 0 sin p + t sin 0 cos X X

26 Basic transformations Scaling A scaling transformation alters the scale of an object. Suppose a point (x,y) is transformed to the point (x',y') by a scaling with scaling factors S x and S y, then: x' = x S x y' = y S y z' = z S z A uniform scaling is produced if S x = S y = S z.

27 Basic transformations Scaling The previous scaling transformation leaves the origin unaltered. If the point (x f,y f ) is to be the fixed point, the transformation is: x' = x f + (x - x f ) S x y' = y f + (y - y f ) S y This can be rearranged to give: x' = x S x + (1 - S x ) x f y' = y S y + (1 - S y ) y f

28 Affine Geometric Transforms In general, a point in n-d space transforms by P = rotate(point) + translate(point) In 2-D space, this can be written as a matrix equation: x' y' = Cos( θ ) Sin( θ ) Sin( θ ) Cos( θ ) x y + tx ty In 3-D space (or n-d), this can generalized as a matrix equation: p = R p + T or p = R t (p T)

29 A Simple 2-D Example p = (0,1) p = (1,0) x' y' x' y' Cos( π / 4) = Sin( π / 4) Cos( π / 4) = Sin( π / 4) Sin( π / 4) 1 Cos( π / 4) 0 Suppose we rotate the coordinate system through 45 degrees (note that this is measured relative to the rotated system! x' y' x' y' Cos( π / 4) = Sin( π / 4) = Sin Cos ( π / 4) ( π / 4) Sin( π / 4) 0 Cos( π / 4) 1

30 Matrix representation and Homogeneous coordinates Often need to combine several transformations to build the total transformation. So far using affine transforms need both add and multiply Good if all transformations could be represented as matrix multiplications then the combination of transformations simply involves the multiplication of the respective matrices As translations do not have a 2 x 2 matrix representation, we introduce homogeneous coordinates to allow a 3 x 3 matrix representation.

31 How to translate a 2D point: Old way: New way:

32 Relationship between 3D homogeneous and inhomogeneous The Homogeneous coordinate corresponding to the point (x,y,z) is the triple (x h, y h, z h, w) where: x h = wx y h = wy z h = wz We can (initially) set w = 1. Suppose a point P = (x,y,z,1) in the homogeneous coordinate system is mapped to a point P' = (x',y',z,1) by a transformations, then the transformation can be expressed in matrix form.

33 Matrix representation and Homogeneous coordinates For the basic transformations we have: Translation Scaling P = P = x y z w x y z w = = T x T y T z s x s y s z x y z w x y z w

34 Geometric Transforms Using the idea of homogeneous transforms, we can write: p' = 0 R 0 0 T R and T both require 3 parameters. 1 p R = [ cosϕ sinϕ ] 0 sinϕ cosϕ cos 0 sin sin 0 cos cosψ sinψ 0 sinψ cosψ

35 Geometric Transforms If we compute the matrix inverse, we find that ' ' ' p R T R p = R and T both require 3 parameters. These correspond to the 6 extrinsic parameters needed for camera calibration

36 Rotation about a Specified Axis It is useful to be able to rotate about any axis in 3D space This is achieved by composing 7 elementary transformations (next slide)

37 Rotation through θ about Specified Axis y P2 y y z y P1 x initial position z y x translate P1 to origin z x rotate so that P2 lies on z-axis (2 rotations) y P2 z x rotate through requ d angle, θ z x rotate axis z to orig orientation P1 x translate back

38 Comparison: Homogeneous coordinates Rotations and translations are represented in a uniform way Successive transforms are composed using matrix products: y = Pn*..*P2*P1*x Affine coordinates Non-uniform representations: y = Ax + b Difficult to keep track of separate elements

39 Camera models and projections Geometry part 2. x Using geometry and homogeneous transforms to describe: Perspective projection Weak perspective projection Orthographic projection y z y x

40 The equation of projection Cartesian coordinates: We have, by similar triangles, that (x, y, z) -> (f x/z, f y/z, -f) Ignore the third coordinate, and get (x, y, z) ( f x z, f y z )

41 The camera matrix Homogenous coordinates for 3D four coordinates for 3D point equivalence relation (X,Y,Z,T) is the same as (k X, k Y, k Z,k T) Turn previous expression into HC s HC s for 3D point are (X,Y,Z,T) HC s for point in image are (U,V,W) U V W = f 0 X Y Z T ), ( ), ( ),, ( v u W V W U W V U =

42 Camera parameters Issue camera may not be at the origin, looking down the z-axis extrinsic parameters one unit in camera coordinates may not be the same as one unit in world coordinates intrinsic parameters - focal length, principal point, aspect ratio, angle between axes, etc. U V W = Transforma tion representi ng intrinsic parameters Transforma tion Transforma tion representi ng representi ng projection model extrinsic parameters Note: f moved from proj to intrinsics! X Y Z T

43 Intrinsic Parameters Intrinsic Parameters describe the conversion from metric to pixel coordinates (and the reverse) x mm = - (x pix o x ) s x y mm = - (y pix o y ) s y p M w y x o s f o s f w y x mm y y x x pix int / 0 0 / = = or Note: Focal length is a property of the camera and can be incorporated as above

44 Example: A real camera Laser range finder Camera

45 Relative location Camera-Laser Camera Laser R=10deg T=(16,6,-9)

46 In homogeneous coordinates Rotation: Translation R = cos 10 0 sin sin 10 0 cos 10 T =

47 Full projection model Camera internal parameters Camera projection p camera = ( )( ) Extrinsic rot and translation = ( )

48 Camera parameters Issue camera may not be at the origin, looking down the z-axis extrinsic parameters one unit in camera coordinates may not be the same as one unit in world coordinates intrinsic parameters - focal length, principal point, aspect ratio, angle between axes, etc. U V W = Transforma tion representi ng intrinsic parameters Transforma tion Transforma tion representi ng representi ng projection model extrinsic parameters Note: f moved from proj to intrinsics! X Y Z T

49 Result Camera image Laser measured 3D structure

50 Hierarchy of different camera models x orth x wp x parap x persp Image plane Object plane X 0 (origin) Camera center Perspective: non-linear Weak perspective: linear approx Orthographic: lin, no scaling Para-perspective: lin

51 Orthographic projection u v = = x y

52 The fundamental model for orthographic projection U V W = X Y Z T

53 Perspective and Orthographic Projection perspective Orthographic (parallel)

54 Weak perspective Issue perspective effects, but not over the scale of individual objects u v T = = Tx Ty = f / Z collect points into a group at about the same depth, then divide each point by the depth of its group Adv: easy Disadv: wrong

55 The fundamental model for weak perspective projection = T Z Y X Z f W V U * / Note Z* is a fixed value, usually mean distance to scene

56 Weak perspective projection = k t t y x 1/ 0 r r 1 α α P 2 2T 1 1T (7dof) Weak perspective projection for an arbitrary camera pose R,t

57 Full Affine linear camera Affine camera (8dof) 1T α x s r t1 m11 2T PA = α y r t 2 A = m / k P A = 3 3 affine affine [ ] [ ] 1. Affine camera=camera with principal plane coinciding with Π 2. Affine camera maps parallel lines to parallel lines 3. No center of projection, but direction of projection P A D=0 (point on Π ) m m 0 m m 0 t1 t P

58 Hierarchy of camera models Camera center Image plane Object plane X 0 (origin) x persp Perspective: x parap Para-perspective: First order approximation of perspective = z y x persp t t t f f P k j i 1 x orth Orthographic: = 1 T y x orth t t P 0 j i Weak perspective: x wp = 1 1 T y x wp t t k k P 0 j i

59 Camera Models Internal calibration: Weak calibration: Affine calibration: Stratification of stereo vision: - characterizes the reconstructive certainty of weakly, affinely, and internally calibrated stereo rigs C sim up to a similarity (scaled Euclidean transformation) C aff up to an affine transformation of task space C proj up to a projective transformation of task space C inj reconstruction up to a bijection of task space

60 Visual Invariance sim aff aff proj proj proj inj inj inj inj

61 Perspective Camera Model Structure Assume R and T express camera in world coordinates, then p R T R p w c = ' ' Combining with a perspective model (and neglecting internal parameters) yields p f R T T R T R f R R R p M u w z y x z y x w c = = ' ' ' ' Note the M is defined only up to a scale factor at this point! If M is viewed as a 3x4 matrix defined up to scale, it is called the projection matrix.

62 Perspective Camera Model Structure Assume R and T express camera in world coordinates, then p R T R p w c = ' ' Combining with a weak perspective model (and neglecting internal parameters) yields p f T P R T R T R R R p M u w z y x y x w c = = ) ( ' ' 0 ' ' Where is the nominal distance to the viewed object P

63 Other Models The affine camera is a generalization of weak perspective. The projective camera is a generalization of the perspective camera. Both have the advantage of being linear models on real and projective spaces, respectively. But in general will recover structure up to an affine or projective transform only. (ie distorted structure)

64 Camera Internal Calibration Recall: Intrinsic Parameters Intrinsic Parameters describe the conversion from metric to pixel coordinates (and the reverse) x mm = - (x pix o x ) s x y mm = - (y pix o y ) s y p M w y x o s o s w y x mm y y x x pix int / 0 0 1/ = = or

65 CAMERA INTERNAL CALIBRATION Compute Sx Known distance d Focal length = 1/ Sx rki = ( x d r ( x d = i i+ 1 o x x i ) s ) s x x known regular offset r A simple way to get scale parameters; we can compute the optical center as the numerical center and therefore have the intrinsic parameters

66 Camera calibration Issues: what are intrinsic parameters of the camera? what is the camera matrix? (intrinsic+extrinsic) General strategy: view calibration object identify image points obtain camera matrix by minimizing error obtain intrinsic parameters from camera matrix Error minimization: Linear least squares easy problem numerically solution can be rather bad Minimize image distance more difficult numerical problem solution usually rather good, but can be hard to find start with linear least squares Numerical scaling is an issue

67 GOAL: Passive 2- camera system for triangulating 3D position of points in space to generate a depth map of a world scene. Humans use stereo vision to obtain depth Stereo Vision

68 Stereo depth calculation: Simple case, aligned cameras DISPARITY= (XL - XR) Similar triangles: Z = (f/xl) X Z= (f/xr) (X-d) Z Solve for X: (f/xl) X = (f/xr) (X-d) X = (XL d) / (XL - XR) Solve for Z: f XL XR Z = d*f (XL - XR) (0,0) (d,0) X

69 Epipolar constraint Special case: parallel cameras epipolar lines are parallel and aligned with rows

70 Stereo measurement example: Left image Resolution = 1280 x 1024 pixels f = 1360 pixels Right image Baseline d = 1.2m Q: How wide is the hallway

71 How wide is the hallway? General strategy Similar triangles: W = Z Need depth Z v f Then solve for W W Z v f

72 How wide is the hallway? Steps in solution: 1. Compute focal length f in meters from pixels 2. Compute depth Z using stereo formula (aligned camera planes) Z = d*f (XL - XR) 3. Compute width: W = Z v f

73 Focal length: Here screen projection is metric image plane. f = 1360 pixels f = *0.224 = 0.238m 0.224m is 1280 pixels

74 How wide Depth calculation XL = 0.144m XR = 0.074m Disparity: XL XR = 0.07m (Note in the disparity calculation the choice of reference (here the edge) doesn t matter. But in the case of say X-coordinate calculation it should be w.r.t. the center of the image as in the stereo formula derivation Depth 1.2*0.238 Z = = 4. 1m 0.07

75 Similar triangles: How wide? Answer: W = Z The width of the hallway is: v f W = 4.1* = 2. 3m V = 0.135m W Z v f