... if anyone is still interested - i found a way for homography transformation in pd:
Badgeek wrote an (great and fantastic) object called fux_anysurface: https://github.com/badgeek/fux_homo
... it was a bit confusing, because he renamed it from fux_homo to fux_anysurface...

i also found a glsl shader for keystone transformation (see attachment) in a quartz composer patch, that's also working in pd (after i changed the "uniform sampler2DRect" to "uniform sampler2D") ... sadly i can't find the website where i downloaded it anymore.

maybe that's going to help somebody, cheers, q

http://www.pdpatchrepo.info/hurleur/qkeystone.zip

actually, i am wondering if there is a homography object like in vvvv or the _1024perspective transform plugin for quartz composer, in pure data as well?
in my expereince the curved3d is not an homographic transformation so it's not the best choice for videomapping for example....
you can also see the difference for example using 4 points and than drag one point across the center. in pd it is just going to be curved, in vvvv or quartzcomposer the object flips in the other direction when crossing the center of the object with one point.

it would be great to have homographic transformation in pure data..... i like pd very much!

...you can change more points of the curved3d by sending messages to it looking like that:
|set 0 0 x y z< (0 0 = top left)
|set 1 0 x y z< (1 0 = top middle)
|set 2 0 x y z< (2 0 = top rigth)
...

i think the differences (and for me a little problem) between the curved3d object in pd and the homography object in vvvv is, that the curved3d isn't a perspective transformation. is that right?

(by the way, an alternative to the curved3d object in pd is using the GL shader from the Extended-view-toolkit but it isn't a perspective transformation as well, i think)

in my experience there is no perspectiv correct distortion in pd (except of course rotating a rectangle or so for example)
it would be great if i'm wrong!
....so it would be nice if anybody knows something about that topic.