# Thread: How to identify if a point is inside a mesh

1. ## How to identify if a point is inside a mesh

Hello, this is my second post here in the message boards. I have another problem that I'm really stuck on. I'm using c++.

My question is, if given a point in 3d space, how am I going to identify if it is INSIDE, ON THE SURFACE, or OUTSIDE of my mesh. I represented my mesh by identifying it's vertices and its polygons with its faces. So thats where I'm stuck. My mesh is assumed to be enclosed and no hollow areas are available.

Thanks.

2. ## Re: How to identify if a point is inside a mesh

For inside of a surface, you can look up barycentric coordinates.

For inside or outside of a mesh, you can shoot a ray from the point to some random direction. If it crosses the mesh an odd number of times, it is inside, else outside. I have a lib that can tell you if an intersection occured. http://ee.1asphost.com/vmelkon/glhlibrary.html
Search for the word "intersection" in the header file.

3. ## Re: How to identify if a point is inside a mesh

pseudocodes/codes are very much appreciated.
Thanks.

4. ## Re: How to identify if a point is inside a mesh

I assume P is the point you're interested in.

R is a ray originating in P in some arbitrary direction (let's say (1,0,0)).

Code :
```Ray R = Ray(P, Vector(1,0,0));
int intersections = 0;
foreach(Triangle T in your Mesh) {
if(R intersects T)
intersections++;
}
if(intersections % 2 == 1) {
// INSIDE
} else {
// OUTSIDE
}```
As for "exactly on the surface", you should code a special case in the ray-triangle intersection. If the intersection is only a short way from the beginning of the ray, you can count this as "on the surface", and abort the whole algorithm.

It shouldn't be a problem to find some code for ray-triangle intersection with google, it's pretty common.

5. ## Re: How to identify if a point is inside a mesh

My question is, if given a point in 3d space, how am I going to identify if it is INSIDE, ON THE SURFACE, or OUTSIDE of my mesh.
Just out of curiosity, what kind of mesh is this (e.g., is it closed, is it convex), and what do you need this test for?

6. ## Re: How to identify if a point is inside a mesh

It's closed, the mesh is imported from 3ds max. This test will be utilized for a wind flow application implemented in VC++ with openGL and glut.

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