Ad: #2 Erik Rufelt   Members   -  Reputation: 1592 Like 0 Likes Like Posted 11 March 2011 - 06:59 AM If you just want a formula, try barycentric coordinates . If you scroll down a bit there are formulas for lambda 1 through 3. Since you have 3 vertices p1=(x1,y1,z1), p2, p3, and want