How to use maketform to get affine transform?

I'm unsure from the documentation how to use maketform.

I'm taking two points each from two images and using them to make a similarity matrix A which I then pass into maketform to get an affine transform:

maketform('affine', inv(A)');

What is A supposed to be and what format is it meant to be in? If I pass in a 2x2 matrix I get an error:

Error using maketform>validate_matrix (line 320)
The final column of A must consist of zeroes, except for a one in the last row.

Answer 1

[Dishant Arora]

A is transformation matrix that is used for scaling, rotating and other spatial transformations. See this blog Spatial transformations: Affine

[t]

Ah thanks. Is it possible to find the transformation matrix if you have the original point and the transformed point?

Answer 2

[Alex Taylor]

1) Why not just use fitgeotrans to construct your transformation from a set of matched control points?

2) What is inv(A)' ?

Unlike many books, MATLAB uses the convention that an affine transformation is of the form:

 T = [A11 A12 0;...
      A21 A22 0;...
      Tx  Ty  1];

Are you possibly using a transposed definition?

3) maketform is not the recommended way of doing this anymore. Try using affine2d/affine3d and imwarp instead of maketform and imtransform.