Hi, I have a set of 3D points and a corresponding set of 2D points. I have to find out a projection matrix that defines the mapping from 3D to 2D. (Actually I have 2 images of the same object with different view points and the 3D reconstruction of the part that is common to both the images. I need to find the mapping from 3D to 2D). I tried to derive a (3 x 4) projective camera matrix in matlab but there was a dimensional error. It would be great if anyone could help me with this. Thanks a lot.

You're not going to find a matrix that projects 3D to 2D points; it's a nonlinear transformation. AFAIK, camera estimation problems like this are usually solved using iterative numerical optimization. For example, see Wikipedia on bundle adjustment.