[AGG] perspective troubles

[Vlastimil M. <vla...@gm...>]

Hi all,

I am using the perspective filter from agg and to add or remove
perspective from pictures. It is easy to use with its rectangle to
quadrangle interface, but imagine this situation:

There is a rectangle that is deformed by perspective projection (like
a photo of a painting on a wall taken from an angle). Removing the
perspective projection and getting the original photo is easy with
agg, but is it possible to compute the original aspect ratio of the
rectangle if I only have the deformed quadrangle?

I know, it is possible assuming that the center of perspective is in
the center of the image and knowing the field-of-view angle (or focus
length of the camera). But this seems too much. The readers of this
list are definitely better at math than I am and maybe one of you
could point me in the right direction. How do I compute the aspect
ratio without the FOV angle if it is possible?

I know that my question is only loosely related to agg, and I am sorry
if I have taken your valuable time if it is too loose for you.

Thanks,
Vlasta