I'm reading this article about shape context algorithm so for example we have pairs of points http://dl.dropbox.com/u/8841028/shape%20context/point_corespondence.png http://dl.dropbox.com/u/8841028/shape%20context/transform_estimation.png what is mean by matrix of homogeneous coordinates?
Linear transformation can be represented as matrix. Assume that you have an object that is set of points, each in the form of p=(x,y). If you want to apply rotation, scaling or shear to it, you just need to find 2x2 matrix A, and for each point p calculate A*p. As a result objected is rotated, scaled and sheared. But some of extreamely frequent transformations are not linear. Shift for example. Or projective transformation. As a result you can't describe them as matrix operation. Homogeneous coordinates is a workaround for this problem. Instead of represnting point as (x,y) you represent it as (x,y,1). Now you can store those transformations as 3x3 matrix B (that part of it is 1 and 0).
Nice explanation but do you know how to explain in a bit more detail why isn't it possible to describe non-linear transformations as a matrix and how does adding an extra coordinate with homogeneous coordinates helps solving that problem?
Rui, it is too long to explain here. Check those: http://en.wikipedia.org/wiki/Transformation_matrix, http://en.wikipedia.org/wiki/Translation_matrix, http://en.wikipedia.org/wiki/3D_projection#Perspective_projection. They explain this issue pretty well.
Opencv has affineTransform and perspectiveTransform functions that can be applied to sets of points, buddy :)
I never tried to solve least square equations for perspective transformation, so I don't know. Sorry.
@Rui Marques the problem is that perspectiveTransform can't handle with small errors in coordinates,with outliners when we use overdetermined system of linear equations. I tried to solve it with pseudoinverse matrix http://opencv.willowgarage.com/documentation/cpp/algebraic_operations.html but it give me all 0.
Asked: 2012-08-24 03:59:09 -0600
Seen: 1,610 times
Last updated: Aug 26 '12
