function persp (dp, p) { // compute perspective projection of data point
into viewplane
	// (vp+t(dp-vp),vd)=0 so t=-(vd,vp)/(vd,dp-vp) // doesn't require
basis for that plane
	// so it can be used to set i2 in initialization!
	var t; dir = new Array(3); centerv = new Array(3);
	for (i=0; i<3; i++) dir[i] = dp[i]-vp[i];
	t = -dot3(vd, vp)/dot3(vd, dir);
	for (i=0; i<3; i++) p[i] = vp[i]+t*dir[i]; }

function cross (v0, v1, v2) {
	v2[0] = v1[2]*v0[1]-v1[1]*v0[2];
	v2[1] = v1[0]*v0[2]-v1[2]*v0[0];
	v2[2] = v1[1]*v0[0]-v1[0]*v0[1];}

function icoord (ip, i1, i2, ic) {
	ic[0] = dot3(ip, i1);
	ic[1] = dot3(ip, i2); }

function co2pix(ic, pix){
		pix[0]=xcenter+xconv*ic[0];
		pix[1]=ycenter-yconv*ic[1]; }

function point2pixels(point,pixels){
	proj = new Array(3); implco = new Array(2);
	persp(point,proj); // ip vector of intersection of vp v and image
	icoord(proj, i1, i2, implco);// ic image coords
	co2pix(implco,pixels); }