Computer Vision : CISC4/689 Onto 3D Coordinate systems 3-D homogeneous transformations...

Computer Vision : CISC4/689

Onto 3D

• Coordinate systems• 3-D homogeneous transformations

– Translation, scaling, rotation

• Changes of coordinates– Rigid transformations

Vector Projection

• The projection of vector a onto u is that component

of a in the direction of u

Vector Cross Product

• Definition: If a = (xa, ya, za)T and

• b = (xb, yb, zb)T, then:

c = a X b

c is orthogonal to both a and b

from Hill

• Let x = (x, y, z)T be a point in 3-D space (R3). What do these values mean?

• A coordinate system in Rn is defined by an origin o and n orthogonal basis vectors – In R3, positive direction of each axis X, Y, Z is indicated by unit vector i, j, k,

respectively, where k = i X j (in a right-handed system)– Coordinate is length of projection of vector from origin to point onto axis basis

vector—e.g., x = x ¢ i

Coordinate System: Definitions

3-D Camera Coordinates• Right-handed system• From point of view of camera looking out into scene:

+X right, {X left

+Y down, {Y up

+Z in front of camera, {Z behind

Going from 2-D to 3-D

• Points: Add z coordinate

• Transformations: Become 4 x 4 matrices with extra

row/column for z component—e.g., translation:

3-D Scaling

3-D Rotations

• In 2-D, we are always rotating in the plane of the image, but in 3-D the axis of rotation itself is a variable

• Three canonical rotation axes are the

coordinate axes X, Y, Z• These are sometimes referred to • in aviation terms: pitch, yaw or heading, and roll, respectively

from Hill

Pitch is the angle that its longitudinal axis (running from tail to nose and along n) makes with horizontal plane.

3-D Euler Rotation Matrices

• Similar to 2-D rotation matrices, but with coordinate corresponding to rotation axis held constant

• E.g., a rotation about the X axis of µ radians:

3-D Rotation Matrices

• General form is:

• Properties

– RT = R-1

– Preserves vector lengths, angles between vectors

– Upper-left block R3£3 is orthogonal matrix• Rows form orthonormal basis (as do columns): Length = 1, mutually

orthogonal

• So R3£3 x projects point x onto unit vectors represented by rows of

• Camera coordinates C: Origin at center of camera, Z axis pointed in viewing direction

• World coordinates W: Arbitrary origin, axes– Way to specify camera location, orientation (aka pose) in same frame as

scene objects (we like to move camera to world, so as to convert world coordinates into camera coordinates)

• Cx, Wx,: Same point in different coordinates

Coordinate System Conversion

scene objects

Change of Coordinates: Special Case of Same Axes

• Distinct origins, parallel basis vectors: If B is world, Ax (camera) can be obtained by Bx (world) minusits CG.

Change of Coordinates: Special Case of Same Origin

• Just need to rotate basis vectors so that they are aligned• Rotation matrix is projection of basis vectors in new frame ia ib ja 0 ka 0 ia 0 ja jb ka 0 ia 0 ja 0 ka kb

Check by multing(ib 0 0), etc.i.e, take A coordinate systemAs (1 0 0), (0 1 0), (0 0 1)

3-D Rigid Transformations

• Combination of rotation followed by translation without scaling

• “Moves” an object from one 3-D position and orientation (pose) to another

3-D Transformations: Arbitrary Change of Coordinates

• A rigid transformation can be used to represent a general change in the coordinate system that “expresses” a point’s location

• Points in one coordinate system are transformed to the other as follows:

• takes the camera to the world origin, transforming world coordinates to camera coordinates

• If A is camera and B is world, inverse translationand inverse rotation

Rigid Transformations: Homogeneous Coordinates

Camera Projection Matrix

• Using homogeneous coordinates, we can describe perspective projection as the result of multiplying by a 3 x 4 matrix P:

(by the rule for converting between homo-geneous and regular coordinates—this is perspective division)

Camera Projection Matrix: Image Offsets

Center of CCD matrixusually does not coincide with the principalpoint C0. This addsu0 and v0 to definein pixel units of C0 inretinal coordinate system.

Factoring the Camera Matrix

• Another way to write it:

P = K ( Id 0 )

Camera calibrationmatrix

Identity form of rigid transformation

(with 4th row dropped)

Camera Calibration Matrix• More general matrix allows:

– Image coordinates with an offset origin (e.g., convention of upper left corner)– Non-square pixels = Different effective horizontal vs. vertical focal length

• These four variables are known as the camera’s intrinsic parameters

fu=f*sufv=f*sv

Dealing with World Coordinates

• Thus far we have assumed that points are in camera coordinates

• Recall the definition of the world-to-camera coordinate rigid transformation:

• In simpler form:

Combining Intrinsic & Extrinsic Parameters

• The transformation performed by a pinhole camera on an arbitrary point in world coordinates can be written as:

3 x 4 projective camera matrix P has 10 degrees of freedom (DOF): 4 intrinsic, 3 rotation, 3 translation

Skew ignored

• The textbook has skew parameter included (pp. 29).

• Since the camera coordinate system may also be skewed due to some manufacturing error, the angle between the two image axes is not equal (maybe close to 90 degrees). This adds up another unknown parameter

• Easy to incorporate, just makes it 11 unknowns

Applications

• Estimates of the camera matrix parameters are critical in order to:– Know where the camera is and how it is moving

– Deduce structural characteristics of the scene (i.e., 3-D information)

– Place known objects (e.g., computer graphics) into a camera image correctly

Camera Matrix

• Linear systems of equations• Least-squares estimation• Application: Estimating the camera matrix

Linear System

• A general set of m simultaneous linear equations

in n variables can be written as:

Matrix Form of Linear System

• This can be represented as a matrix-vector product:

• Compactly, we write this as A x = b

Solving Linear Systems

• If m = n (A is a square matrix), then we can obtain the solution by simple inversion:

• If m > n, then the system is over-constrained and A is not invertible – Use the pseudoinverse A+ = (ATA)-1AT to obtain least-

squares solution x = A+b(Ax=B, multiply both sides by A^t,

Fitting Lines

• A 2-D point x = (x, y) is on a line with slope m and

intercept b if and only if y = mx + b • Equivalently,

• So the line defined by two points x1, x2 is the solution to

the following system of equations:

Fitting Lines

• With more than two points, there is no guarantee that they will all be on the same line

• Least-squares solution obtained from pseudoinverse is line that is “closest” to all of the points

courtesy ofVanderbilt U.

Example: Fitting a Line

• Suppose we have points (2, 1), (5, 2), (7, 3), and (8, 3)

• Then

and x = A+b = (0.3571, 0.2857)T

Example: Fitting a Line

Homogeneous Systems of Equations

• Suppose we want to solve A x = 0• There is a trivial solution x = 0, but we don’t want this. For

what other values of x is A x close to 0?• This is satisfied by computing the singular value

decomposition (SVD) A = UDVT (a non-negative diagonal matrix between two orthogonal matrices) and taking x as the last column of V (unit singular vector

corresponding to the least eigenvalue.– Note that Matlab returns [U, D, V] = svd(A)

This is usually subject to constraints such as norm of x=1

Line-Fitting as a Homogeneous System

• A 2-D homogeneous point x = (x, y, 1)T is on the

line l = (a, b, c)T only when ax + by + c = 0

• We can write this equation with a dot product: x ¢ l = 0, and hence the following system is implied for multiple

points x1, x2, ..., xn:

Example: Homogeneous Line-Fitting

• Again we have 4 points, but now in homogeneous form: (2, 1, 1), (5, 2, 1), (7, 3, 1), and (8, 3, 1)

• Our system is:

• Taking the SVD of A, we get: b=-1, a=.3571, c=0.2857..scaled differently

C ompare to x = (0.3571, 0.2857)T

Camera Calibration

• Camera calibration is the name given to the process of discovering the projection matrix (and its decomposition into camera matrix and the position and orientation of the camera) from an image of a controlled scene. For ex., we might set up the camera to view a calibrated grid of some sort.

A Vision Problem: Estimating P

• Given a number of correspondences between 3-D

points and their 2-D image projections Xi $ xi, we

would like to determine the camera projection

matrix P such that xi = PXi for all i

A Calibration Target

courtesy of B. Wilburn

Estimating P: The Direct Linear Transformation (DLT) Algorithm

• xi = PXi is an equation involving homogeneous vectors

(powers are equal), so PXi and xi need only be in the same

direction, not strictly equal

• We can specify “same directionality” by using a cross product formulation:

DLT Camera Matrix Estimation: Preliminaries

• Let the image point xi = (xi, yi, wi)T

(remember that Xi has 4 elements)

• Denoting the jth row of P by pjT (a 4-element row vector), we have:

DLT Camera Matrix Estimation: Step 1

• Then by the definition of the cross product, xi £

PXi is:

Definition of cross product:U x V = uy vz – uz vy, uz vx – ux vz, ux vy – uy vx

• The dot product commutes, so pjT Xi = XT

and we can rewrite the preceding as:

• Collecting terms, this can be rewritten as a matrix product:

where 0T = (0, 0, 0, 0). This is a 3 x 12

matrix times a 12-element column vector p = (p1T, p2T, p3T)T

What We Just Did

• There are only two linearly independent rows here

– The third row is obtained by adding xi times the first row to yi times the

second and scaling the sum by -1/wi

• So we can eliminate one row to obtain the following linear matrix equation for the ith pair of corresponding points:

• Write this as Ai p = 0

• Remember that there are 11 unknowns which generate the 3 x 4 homogeneous matrix P (represented in vector

form by p)• Each point correspondence yields 2 equations (the two

rows of Ai) We need at least 5 ½ point correspondences to solve for

p• Stack Ai to get homogeneous linear system A p = 0

Direct Linear Transform (DLT)(summary)

ii PXx ii PXx

rank-2 matrix

Direct Linear Transform (DLT)

Minimal solution

Over-determined solution

5½ correspondences needed (say 6)

P has 11 dof, 2 independent eq./points

n 6 points (usually, around 30 points needed?)

Apminimize subject to constraint

use SVD

Degenerate configurations

(i) Points are collinear or single line passing through projection center

(ii) Camera and points on a twisted cubic

Scale data to values of order 1

1. move center of mass to origin2. scale to yield order 1 values

Data normalization

Geometric error

Gold Standard algorithmObjective

Given n≥6 2D to 3D point correspondences {Xi↔xi’}, determine the Maximum Likelyhood Estimation of P

Algorithm

(i) Linear solution:

(a) Normalization:

(b) DLT

(ii) Minimization of geometric error: using the linear estimate as a starting point minimize the geometric error:

(iii) Denormalization:

ii UXX~ ii Txx~

Calibration example

(i) Canny edge detection(ii) Straight line fitting to the detected edges(iii) Intersecting the lines to obtain the images corners

typically precision <1/10

(H&Z rule of thumb: 5n constraints for n unknowns)

Errors in the world

Errors in the image and in the world

ii XPx

Errors in the image

iPXx̂

(standard case)

Radial distortion

• Due to spherical lenses (cheap)

• Model:

yxyxKyxKyx ...))()(1(),( 44

http://foto.hut.fi/opetus/260/luennot/11/atkinson_6-11_radial_distortion_zoom_lenses.jpgstraight lines are not straight anymore

pincushion dist.

barrel dist.

Radial distortion example

Some typical calibration algorithmsTsai calibration

Zhangs calibration

http://research.microsoft.com/~zhang/calib/

Z. Zhang. A flexible new technique for camera calibration. IEEE Transactions on Pattern Analysis and Machine Intelligence, 22(11):1330-1334, 2000.

Z. Zhang. Flexible Camera Calibration By Viewing a Plane From Unknown Orientations. International Conference on Computer Vision (ICCV'99), Corfu, Greece, pages 666-673, September 1999.

Jean-Yves Bouguet’s matlab implementation:http://www.vision.caltech.edu/bouguetj/calib_doc/

Reg Willson’s implementation: http://www-2.cs.cmu.edu/~rgw/TsaiCode.html

Recovery of world position

• Given u,v we cannot uniquely determine the position of the point in the world.

• Each observed image point (u,v) gives us two equations in three unknowns (X,Y,Z). These equations define a line (i.e, ray) in space, on which the world point must lie.

• For general 3D scene interpretation, we need to use more than one view. Later in this course we will take a detailed look at stereo vision and structure from motion.

Computer Vision : CISC4/689 Onto 3D Coordinate systems 3-D homogeneous transformations...

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