On Mon, 2008-03-10 at 18:22 -0500, Leonard Evens 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.

I've gotten one response so far, and while interesting, it didn't work

Is there some way to do what I want using a script?

Is there software except gimp which runs under Linux which I could use

If someone could direct me to the section of the gimp code which
performs perspective transformations, I might be able to extract it and
write my own program to do what I want, but from past experience I don't
place much hope in that.  Gimp code forms a complex network and it is
usually impossible to isolate one particular part and make much sense of
it.  Of course it would be nice if the gimp programmers added the
capability I want to a future version of gimp, but I won't hold my
breath waiting for that.

> 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. 

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