Question

I’m reading some paper about computer vision. It looks like a simple fact but I can’t understand. It is about homogenous [3×3] matrix that is used for planar projective transformation. And it is said to have eight independent ratios of matrix elements. I don’t know what the ratio is, and what the eight independent ratios are? Please help me this problem.

Thank you.

Answer 1

It means that two projective transformations P and kP are equivalent.

Consider a point in 2D: it can be expressed in non-homogeneous coordinates by a vector [x,y] . The same point expressed in homogeneous coordinates would be [x',y',w] where

x = x' / w
y = y' / w

As you can see, w behave as a scaling factor.
Dividing the homogeneous coordinates by w you get [x'/w, y'/w, 1] = [x,y,1]. Thus a 2D point has only two degrees of freedom.

You can apply the same reasoning to a 3×3 matrix. Of the 9 elements only 8 are independent, while the last one can be seen as a scaling factor. It doesn’t matter actually which one of the nine you choose.

For additional informations: Homogeneous coordinates

EDIT:
The number of DOF is the number of independent parameters. In the example of the 2D point, even though we have three parameters (x',y',w), there are only two independent ratios: as I shown before, if you divide by w your firsts two parameters become fractions (“ratio” means division), while the third one is simply 1.

For a 3D point it’s the same reasoning, but you have to consider the z axes: a generic 3D point is [x',y',z',w] (4 parameters), but, if we divide by w it becomes [x'/w, y'/w, z'/w, 1] so three independent ratios.

I’m always dividing by w because the ratios x'/w, y'/w, z'/w have a particular meaning (non-homogeneous coordinates of the point), but to count the dof you can use any other parameter.

Let’s consider the example of a 2×2 matrix (for a 3×3 it’s the same, it’s just longer to type):

m11  m12
m21  m22

4 parameters. Dividing by one of those at your choice (well, actually at my choice…), say m12 it becomes

m11      1
---    
m12

m21     m22
---     ---
m12     m12

3 ratios so three degrees of freedom (for a generic 2×2 matrix). If, by instance, we have m21 = m12 we would get

m11      
---      1
m12

        m22
 1      ---
        m12

thus in this case we would have only 2 dof! Don’t get confused by the fact that you see m11,m22 and m12 (three parameters), because actually you can consider a = m11/m12 and b= m22/m12, thus it becomes

a   1
1   b

that means two independent parameters, thus two dof.

Hope it’s clearer now