[Octave-bug-tracker] [bug #44830] image package: imtransform gives different numbers on simple rotation

[H. G.]

Follow-up Comment #3, bug #44830 (project octave):

I found some more information on the Matlab behavior here:

http://www.mathworks.com/matlabcentral/answers/67114-what-is-the-difference-between-imwarp-and-imtransform

Two short quotes from this link:

"...imwarp determines and computes the world limits of the input and output
images in terms of the outer extents of the first and last pixel in each
dimension where as imtransform computes input and output boundaries in terms
of the center of the first and last pixel in each dimension."

"I understand that new functions (IMWARP etc..) and the old functions
(IMTRANSFORM) differ in the point of application- for example rotation. In the
new ones it is image center while in the old ones it is top left corner."

I haven't come to any useful conclusions, though...

There is also a link to this:

http://blogs.mathworks.com/steve/2006/08/01/spatial-transformations-controlling-the-input-and-output-grids-with-imtransform/

Here I found the suspected value of 0.5 for example:

"imtransform assumes that an M-by-N input image lives in input space inside a
rectangle spanning from 0.5 to M+0.5 horizontally, and from 0.5 to N+0.5
vertically."

@Pantxo: Maybe those explanations and the code examples help you understand
the Matlab behavior of imtransform a bit more...



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