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Posted on 2008-10-20

Hi,

I have a set of 3 point pairs. A point is defined like:

pt = { x, y, scale, rotation }

I want to find the affine transformation that maps the 3 points to their point matches. Is there any way to do this in matlab? I know there is the tformfwd() function which looked promising but I don't know if it can handle a 3rd and 4th parameter attached to each point (scale and rotation). Any thoughts?

Thanks

I have a set of 3 point pairs. A point is defined like:

pt = { x, y, scale, rotation }

I want to find the affine transformation that maps the 3 points to their point matches. Is there any way to do this in matlab? I know there is the tformfwd() function which looked promising but I don't know if it can handle a 3rd and 4th parameter attached to each point (scale and rotation). Any thoughts?

Thanks

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3 Comments

Yes, you are quite right. maketform and tformfwd are the suitable functions to find and validate the affine transformation. However, you can't use them when your points are represented in this form. You can use the function below to convert your points to the proper form.

```
function [x y] = applytrans(p)
% applytrans converts points from pt = { x, y, scale, rotation }
% to equivalent pt = { x, y}
% By applying the scale and rotation transformation to original x, y.
% For example:
% A =[
% 1.0000 2.0000 3.0000 3.1416;
% 5.0000 6.0000 7.0000 1.5708;
% 4.0000 3.0000 1.0000 -1.5708;
% ];
%
% [x y]=applytrans(A)
%
% Gives
% x =
% -3.0000
% -42.0001
% 3.0000
% y =
% -6.0000
% 34.9998
% -4.0000
x=p(:,1).*cos(p(:,4))-p(:,2).*sin(p(:,4));
y=p(:,1).*sin(p(:,4))+p(:,2).*cos(p(:,4));
x=p(:,3).*x;
y=p(:,3).*y;
```

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