help with perspective transforms.

I''m looking for a formula for my photo-paint type app. Anyone know of any websites with a formula like this on it? What I want is the perspective transformation from a rectangle to a quadrilateral given that you know the four points in on the quadrilateral. I have tried working this out, by first working out where the 2 vannishing points would be and the rotating it so the vannishing points lie on the horizon, then undoing the perspective by applying: X->X/Y Y->1/Y to get a rotated rectangle, from then it is an easy step to relate this to the orginal rectangle. But my formula doesn''t work very well and I want to see if there is a website with this transformation on it somewhere with the direct transformation. In Paint Shop Pro it is called the "Perspective Transformation" so you know what I''m talking about. Thanks.