[Gimp-user] Yet another question/comment about projective transformations

[David Gowers]

On Sat, Mar 15, 2008 at 9:33 AM, Leonard Evens
<len at math.northwestern.edu> wrote:
> To review, my basic aim is to produce very precise perspective
>  transformations in order to merge panoramas which involve very large
>  images.
Is there some reason you are not using hugin, which is a very capable
panorama stitching software?
http://en.wikipedia.org/wiki/Hugin_%28software%29

>  Unfortunately, there seems to be no way to specify target
>  points except by dragging, but even given that, there is an additional
>  problem.    Typically, I have a facade in which the top and bottom may
>  not be exactly parallel.   In order to get them that way, I need to move
>  corners on top and bottom of the building to desired positions.   I can
>  move the top corner more or less to where I want it.  But when I try to
>  move the bottom corner where I want it, I end up moving the top corner.
>  (The only exception is when the original top was actually at the upper
>  handle on that side.  In that case, it stays at the new position when
>  the bottom is moved.)   If Igo back to correct the top, the bottom
>  moves.  You can iterate this process until you get it about right, which
>  works well for moderate sized images, but for large images and precise
>  positioning, it takes quite a while and is very frustrating.
>
>  I've investigated the mathematics behind this, which involves projective
>  geometry and cross ratios.   Using that, I can determine where I have to
>  move the upper and lower handles to get points in between to go where I
>  want them to go.   That would work, but the formulas are pretty involved
>  and cumbersome to apply.  I experimented with using selections, but that
>  doesn't work because what is outside the selection is not moved and so
>  lines can be bent.
>
>  I don't see any other way to accomplish what I want except successive
>  approximations or using the formulas.  Can anyone suggest an alternate
>  approach?
>
>  --
>  Leonard Evens <len at math.northwestern.edu>
>  Mathematics Department, Northwestern University
>
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