Picking point on object

Question

Hi,
maybe I'm to stupid or I'm looking just in the wrong places.
I got a 3D scene containing 1 object... now I want to change the point which is the center of my rotation. A click in the scene shall define around which point the object shall rotate.
I now want to get the point where a ray casted from the point I've clicked hits the object. So that the choosen point is on the surface of this object.
Maybe someone could point me to any source where I can find info about getting the point on a surface by picking.
Even better would be if someone could give me a brief example.
Thanx!

Answer 1

I have some code that may help:
from J3D.org
public static PickRay createPickRay(Canvas3D canvas, int x, int y)
Point3d eye_pos = new Point3d();
Point3d mouse_pos = new Point3d();
canvas.getCenterEyeInImagePlate(eye_pos);
canvas.getPixelLocationInImagePlate(x, y, mouse_pos);
Transform3D motion = new Transform3D();
canvas.getImagePlateToVworld(motion);
motion.transform(eye_pos);
motion.transform(mouse_pos);
Vector3d direction = new Vector3d(mouse_pos);
direction.sub(eye_pos);
return new PickRay(eye_pos, direction);
Given three points in a plane, a camera point Cp, and a camera vector Cv,
calculate the intersection of the line (defined by passing through the camera point
and parallel to the camera vector) and the plane
(defined by the three points p1, p2, p3)
public static Point3d getIntersection(Point3d p1, Point3d p2, Point3d p3,
Tuple3d Cp, Tuple3d Cv )
// normal vector to plane
// n = (p2-p1)X(p3-p1)
Vector3d v1 = new Vector3d(p1);
Vector3d v2 = new Vector3d(p2);
Vector3d v3 = new Vector3d(p3);
v3.sub(v1);
v2.sub(v1);
Vector3d n = new Vector3d();
n.cross(v2,v3);
     // parameter for the parametric equation of a line
// t = ( n.p1 - n.Cp ) / n.Cv
     double t = ( n.dot(v1) - n.dot(new Vector3d(Cp)) ) / n.dot(new Vector3d(Cv));
     // plug parameter t into parametric equation for camera line of sight
Point3d lineIntersect = new Point3d();
     lineIntersect.x = Cp.x + Cv.x * t;
     lineIntersect.y = Cp.y + Cv.y * t;
     lineIntersect.z = Cp.z + Cv.z * t;
     return lineIntersect;
Then the above two methods are called from my mouseClicked method (when user clicks on canvas3D, given MouseEvent m)
Canvas3D canvas = (Canvas3D)m.getSource();
Point3d cameraPosition = new Point3d();
Vector3d cameraVector = new Vector3d();
PickRay ray = createPickRay(canvas, m.getX(), m.getY());
SceneGraphPath pickedPath = locale.pickClosest(ray);
Node pickedNode = pickedPath.getObject();
ray.get(cameraPosition, cameraVector);
// three arbitrary points from the face you want to intersect
// I wanted to intersect the xy plane
Point3d p1 = new Point3d(0,0,0);
Point3d p2 = new Point3d(1,0,0);
Point3d p3 = new Point3d(0,1,0);
Point3d intersect = getIntersection(p1, p2, p3, cameraPosition, cameraVector);
Let me know if this helps!