I'm am trying to reflect a ray of a triangle and not sure I got the code right. I'm using two bits of pseudo code from two locations:

1. Intersection of a Line with a Plane from Paul Bourke's site.
2. Reflecting a vector from 3DKingdoms site

Here's my code so far:

2. PVector[] face = new PVector[3];
3. float ai = TWO_PI/3;//angle increment
4. float r  = 300;//overall radius
5. float ro = 150;//random offset
6. 
   
7. PVector n;//normal
8. Ray r1;
9. 
   
10. void setup(){
11.   size(500,500,P3D);
12.   for(int i = 0 ; i < 3; i++) face[i] = new PVector(cos(ai * i) * r + random(ro),random(-50,50),sin(ai * i) * r + random(ro));
13.   r1 = new Ray(new PVector(-100,-200,-300),new PVector(100,200,300));
14. }
15. void draw(){
16.   background(255);
17.   lights();
18.   translate(width/2, height/2,-500);
19.   rotateX(map(mouseY,0,height,-PI,PI));
20.   rotateY(map(mouseX,0,width,-PI,PI));
21.   //draw plane
22.   beginShape(TRIANGLES);
23.   for(PVector p : face) vertex(p.x,p.y,p.z);
24.   endShape();
25.   //normals
26.   PVector c = new PVector();//centroid
27.   for(PVector p : face) c.add(p);
28.   c.div(3.0);
29.   PVector cb = PVector.sub(face[2],face[1]);
30.   PVector ab = PVector.sub(face[0],face[1]);
31.   n = cb.cross(ab);//compute normal
32.   line(c.x,c.y,c.z,n.x,n.y,n.z);//draw normal
33. 
    
34.   pushStyle();
35.   //http://paulbourke.net/geometry/planeline/
36.   //line to plane intersection u = N dot ( P3 - P1 ) / N dot (P2 - P1), P = P1 + u (P2-P1), where P1,P2 are on the line and P3 is a point on the plane
37.   PVector P2SubP1 = PVector.sub(r1.end,r1.start);
38.   PVector P3SubP1 = PVector.sub(face[0],r1.start);
39.   float u = n.dot(P3SubP1) / n.dot(P2SubP1);
40.   PVector P = PVector.add(r1.start,PVector.mult(P2SubP1,u));
41.   strokeWeight(5);
42.   point(P.x,P.y,P.z);//point of ray-plane intersection
43. 
    
44.   //vector reflecting http://www.3dkingdoms.com/weekly/weekly.php?a=2
45.   //R = 2*(V dot N)*N - V
46.   //Vnew = -2*(V dot N)*N + V
47.   //PVector V = PVector.sub(r1.start,r1.end);
48.   PVector V = PVector.sub(r1.start,P);
49.   PVector R = PVector.sub(PVector.mult(n,3 * (V.dot(n))),V);
50.   strokeWeight(1);
51.   stroke(0,192,0);
52.   line(P.x,P.y,P.z,R.x,R.y,R.z);
53.   stroke(192,0,0);
54.   line(r1.start.x,r1.start.y,r1.start.z,P.x,P.y,P.z);
55.   stroke(0,0,192);
56.   line(P.x,P.y,P.z,r1.end.x,r1.end.y,r1.end.z);
57.   popStyle();
58. 
    
59. }
60. void keyPressed(){   setup();  }//reset
61. class Ray{
62.   PVector start = new PVector(),end = new PVector();
63.   Ray(PVector s,PVector e){   start = s ; end = e;  }
64. }