struct VECTOR{	float x, y, z;};VECTOR VecSub(VECTOR a, VECTOR b){	VECTOR v;	v.x = a.x - b.x;	v.y = a.y - b.y;	v.z = a.z - b.z;	return v;}VECTOR VecCross(VECTOR a, VECTOR b){	VECTOR v;		v.x=a.y*b.z-a.z*b.y;	v.y=a.z*b.x-a.x*b.z;	v.z=a.x*b.y-a.y*b.x;	return v;}VECTOR VecNormalize(VECTOR a){	float l = sqrtf(a.x * a.x + a.y * a.y + a.z * a.z);	a.x /= l;	a.y /= l;	a.z /= l;	return a;}int main(){	VECTOR p1 = { -0.1f, +0.1f, -1.0f };	VECTOR p2 = { +0.1f, -0.1f, -1.0f };	VECTOR n = { 0, 0, 1 };	VECTOR u, v;		VECTOR p1d, p2d;	u = VecSub(p2, p1);		v = VecCross(u, n);	u = VecNormalize(u);	v = VecNormalize(v);	printf("%0.5f %0.5f %0.5f\n", u.x, u.y, u.z);	printf("%0.5f %0.5f %0.5f\n", v.x, v.y, v.z);	printf("%0.5f %0.5f %0.5f\n", n.x, n.y, n.z);	p1d.x = p1.x * u.x + p1.y * u.y + p1.z * u.z;	p1d.y = p1.x * v.x + p1.y * v.y + p1.z * v.y;	p1d.z = p1.x * n.x + p1.y * n.y + p1.z * n.z;	p2d.x = p2.x * u.x + p2.y * u.y + p2.z * u.z;	p2d.y = p2.x * v.x + p2.y * v.y + p2.z * v.y;	p2d.z = p2.x * n.x + p2.y * n.y + p2.z * n.z;	printf("\n");	printf("%0.5f, %0.5f %0.5f\n", p1d.x, p1d.y, p1d.z);	printf("%0.5f, %0.5f %0.5f\n", p2d.x, p2d.y, p2d.z);	return 0;}

Quote:Original post by Zipster


You should also have a dot product function to make this operation easier in the future:

VECTOR VecDot(VECTOR a, VECTOR b){	VECTOR v;		v.x=a.x*b.x;	v.y=a.y*b.y;	v.z=a.z*b.z;	return v;}