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                /**  vec4 u_projInfo (-2.0f / (width*P[0][0]),
                                  2.0f / (height*P[1][1]),
                                  ( 1.0f - P[0][2]) / P[0][0],
                                  ( -1.0f - P[1][2]) / P[1][1])

                        where P is the projection matrix that maps camera space points
                        to [-1, 1] x [-1, 1].  That is, GCamera::getProjectUnit(). */

                /** Reconstruct camera-space P.xyz from screen-space S = (x, y) in
                        pixels and camera-space z < 0.  Assumes that the upper-left pixel center
                        is at (0.5, 0.5) [but that need not be the location at which the sample tap
                        was placed!]
                */
                float3 reconstructCSPositionUV(float2 S, float z) {
                        return float3((S.xy * u_uvProjInfo.xy + u_uvProjInfo.zw) * z, z);
                }