Let's say we're transforming a square to an arbitrary 4 points via projective transform.
Is there a way to ensure that the resulting points have homogeneous coordinates that are >0 ? i.e. sending the square (0,0),(1,0),(1,1),(0,1) to (x1,y1,w1),(x2y2,w2),(x3,y3,w3),(x4,y4,w4) -- it is sometimes useful to use w<0 as an indicator that the point is behind a clipping plane attached to a virtual camera.
a matrix may be of the form found here: http://graphics.cs.cmu.edu/courses/15-463/2008_fall/Papers/proj.pdf these are 2d image warps, but may produce homog. points with w<0;