cp2tform / tformfwd - are points outside controlpoints correctly transformed ?

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Emmanuel on 2 May 2012

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https://in.mathworks.com/matlabcentral/answers/37197-cp2tform-tformfwd-are-points-outside-controlpoints-correctly-transformed

Hi,

I am getting erratic results when trying to transform a set of 2D points using cp2tform and tformfwd.

When the points to be transformed are situated outside the perimeter of controlpoints, I sometime get negative coordinates (ok knowing the position of my CP), but sometimes the transformed coordinates seem to default to (0,y) or (0,0).

I thought this was caused by the piecewise linear transformation I'm using (as matlab probably cannot draw triangles beyond CP limits), bu it seems I get the same behavior when using projective or polynomial transformations, and tformfwd or tforminv.

What surprises me the most is that I do not get any error or warning message when these outlier points are being (wrongly) transformed ...

Any ideas ?

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Answer 1

Emmanuel on 22 May 2012

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https://in.mathworks.com/matlabcentral/answers/37197-cp2tform-tformfwd-are-points-outside-controlpoints-correctly-transformed#answer_48642

An update about the original question, after more torough tests:

- the problem I got initially was caused by my own code, sorry about that.

- however, If this can help anybody, I discovered that piecewise linear TFORM behaves strangely if control points describe a non-convex polygon, creating triangles with zero area in the output space. These triangles are transformed to a segment that seem to "attract" even points outside the cp polygon. I don't have the skills to test it properly, but this may suggest that piecewise linear transform might be unreliable in some cases, e.g. for processing images with barrel distorsion.