{
        curtain.cal

        A surface normal perterbation texture for wavy curtains.

        by Charles Ehrlich

        Curtain are hanging in z-direction,
        and has a period of one unit in the x-direction.
}
                        { jerk surface back and forth }
cur_dx = cur_dfx-cur_dot*Nx;
cur_dy = cur_dfy-cur_dot*Ny;
cur_dz = 0;

cur_dfx = cos(Px);
cur_dfy = sin(Px);
cur_dot = cur_dfx*Nx+cur_dfy*Ny;