[Gimp-user] How can I specify the tranformation matrix for a perpsective directly

[Elwin Estle]

Have you tried using guides to snap the handles?

--- Leonard Evens <len at math.northwestern.edu> wrote:

> I don't see a way to specify the matrix used to perform a desired
> perspective transformation manually.  Of course, I can adjust it by
> dragging handles, but it would be much easier for me to calculate what I
> need and then enter the elements of the matrix by hand.
> 
> Needless to say, a typical gimp user would not be able to do this, but I
> am a mathematician who has taught geometry many times during my
> professional life, so I know what to do.  I don't need an explanation of
> how the matrix is formed and what it does since I can work that out by
> experimentation.
> 
> The gimp manual is not helpful.  It refers the reader to the section on
> convolution filters, but that appears to have to do with modifying
> pixels in place rather than moving the pixels---a rather misleading
> reference.  Perhaps something entirely different happens if you use the
> full matrix, but the documentation is silent on that matter.
> 
> Of course, I can try to produce what I want by moving the handles, but
> for slight adjustments of large images, that is difficult if not
> impossible.  It helps if I increase the magnification enormously, but it
> is still awkward.  It would be a lot simpler to be able to enter the
> matrix elements directly. 
> 
> 
> -- 
> Leonard Evens <len at math.northwestern.edu>
> Mathematics Department, Northwestern University
> 
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