function [T]=persp_pre(zp)
%
% [T]=persp_pre(zp)

% generates "perspective projection into z=0 plane" matrix T
% where input zp is the center of projection
% zp = -1/r or r = -1/zp

% filename must be "persp_pre.m"
% can also write
% function T=persp_pre(zp)
% without square brackets

T= [ 1  0   0  0;
     0  1   0  0;
     0  0   0  0;
     0  0  -1/zp  1];
 