### Author Topic: a point inside polygon  (Read 3955 times)

#### rolz

• float
• Posts: 280
• Technocrat
##### a point inside polygon
« on: February 28, 2006, 09:28:05 pm »
I've got stuck with a math question.

How to determine Z coordinate for a point inside a polygon with given x and y coordinates ?

Assume we have SimpleVector p1, p2, p3 - vertices of the polygon.
then we have float x, float y - coordinates of point inside this polygon. How do we find Z coordinate that belongs to this polygon ?

Spent 30 minutes trying to figure this out and finally gave up. .. any suggestions ?
Regards,
Andrei

#### EgonOlsen

• Posts: 12066
##### a point inside polygon
« Reply #1 on: February 28, 2006, 10:17:19 pm »
Maybe like so...(but i'm not sure as i don't have a valid test case for this. I've just done this on paper and i can hardly read my own writings...:wink: ):

Code: [Select]
`import com.threed.jpct.*;public class TestPoint {  public static void main(String[] args) {    // The vertices    SimpleVector p0=new SimpleVector(10,10,40);    SimpleVector p1=new SimpleVector(0,10,20);    SimpleVector p2=new SimpleVector(10,0,10);    // The "target" vertex    SimpleVector px=new SimpleVector(10,0,0);    // The vectors from p0 to the other vertices    SimpleVector a=p1.calcSub(p0);    SimpleVector b=p2.calcSub(p0);    float d=0.00000000001f;    float div=(b.y*a.x-b.x*a.y);    float div2=a.y;    if (div==0) {      div=d;    }    if (div2==0) {      div2=d;    }    // Some linear algebra...could be just plain wrong...:-)    float y=((-p0.y+px.y)*a.x+(p0.x-px.x)*a.y)/div;    float x=(-p0.y+px.y-y*b.y)/div2;    px.z=p0.z+x*a.z+y*b.z;    System.out.println(px);  }}`

#### EgonOlsen

• Posts: 12066
##### a point inside polygon
« Reply #2 on: March 03, 2006, 05:56:47 pm »
Have you tried it? Did it work?

#### rolz

• float
• Posts: 280
• Technocrat
##### a point inside polygon
« Reply #3 on: March 08, 2006, 03:43:32 pm »
Sorry for late response. Nope, it didnt worked. Sometimes player flies over the ground, sometimes falls through.
Regards,
Andrei

#### rolz

• float
• Posts: 280
• Technocrat
##### a point inside polygon
« Reply #4 on: March 08, 2006, 04:27:08 pm »
My bad, i was putting vertices in wrong order. It works now Thanks !
Regards,
Andrei