[Radiance-general] Transform from 3D to 2D Coordinates

[radiance-general@radiance-online.org]

Dear Radiance list,

I am currently using radiance to render some pictures which I would like
to apply some post processing to. This will involve pasting an image
onto a specific location in the .pic file i have created. 

In order to do this i need to know the location of certain points. I
know where they are in my 3D scene, but cannot work out a 3D to 2D
coordinate transform. I know that with a traditional pinhole perspective
projection i can work out new 2D coordinates if i know the old
coordinates, the focal point and the focal length. But i cant find any
way of determining what the focal length used by rpict is!

If anyone has, ideally, an equation for transforming 3D radiance
coordinates into 2D .pic coordinates based on an (x,y,z), view point and
view direction i would be incredibly grateful if you could let me know.
And if anyone could even help me determine the focal length i need that
would be a big help

Thanks a lot for your time

Jon Bright