A script to get a random point on a mesh, for Unity3D

gistfile1.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class RandomPointOnMesh : MonoBehaviour
{
    public MeshCollider lookupCollider;

    public bool bangGetPoint;
    private Vector3 randomPoint;

    public List<Vector3> debugPoints; 
  
	void Update () {

        //click the checkbox to generate a point, and have it shown in a debug gizmo.
        //here's a blogpost on it http://nottheinternet.com/blog/banging-things-in-Unity/
	    if (bangGetPoint)
	    {
	        Vector3 randomPoint = GetRandomPointOnMesh(lookupCollider.sharedMesh);
	        randomPoint += lookupCollider.transform.position;
	        debugPoints.Add(randomPoint);
            Debug.Log(randomPoint);
	        bangGetPoint = false;
	    }
	}


    public void OnDrawGizmos()
    {
        foreach (Vector3 debugPoint in debugPoints)
        {
            Gizmos.DrawSphere(debugPoint, 1f);
        }
    }


    Vector3 GetRandomPointOnMesh(Mesh mesh)
    {
        //if you're repeatedly doing this on a single mesh, you'll likely want to cache cumulativeSizes and total
        float[] sizes = GetTriSizes(mesh.triangles, mesh.vertices);
        float[] cumulativeSizes = new float[sizes.Length];
        float total = 0;

        for (int i = 0; i < sizes.Length; i++)
        {
            total += sizes[i];
            cumulativeSizes[i] = total;
        }

        //so everything above this point wants to be factored out

        float randomsample = Random.value* total;

        int triIndex = -1;
        
        for (int i = 0; i < sizes.Length; i++)
        {
            if (randomsample <= cumulativeSizes[i])
            {
                triIndex = i;
                break;
            }
        }

        if (triIndex == -1) Debug.LogError("triIndex should never be -1");

        Vector3 a = mesh.vertices[mesh.triangles[triIndex * 3]];
        Vector3 b = mesh.vertices[mesh.triangles[triIndex * 3 + 1]];
        Vector3 c = mesh.vertices[mesh.triangles[triIndex * 3 + 2]];

        //generate random barycentric coordinates

        float r = Random.value;
        float s = Random.value;

        if(r + s >=1)
        {
            r = 1 - r;
            s = 1 - s;
        }
        //and then turn them back to a Vector3
        Vector3 pointOnMesh = a + r*(b - a) + s*(c - a);
        return pointOnMesh;

    }

    float[] GetTriSizes(int[] tris, Vector3[] verts)
    {
        int triCount = tris.Length / 3;
        float[] sizes = new float[triCount];
        for (int i = 0; i < triCount; i++)
        {
            sizes[i] = .5f*Vector3.Cross(verts[tris[i*3 + 1]] - verts[tris[i*3]], verts[tris[i*3 + 2]] - verts[tris[i*3]]).magnitude;
        }
        return sizes;

        /*
         * 
         * more readably:
         * 
for(int ii = 0 ; ii < indices.Length; ii+=3)
{
    Vector3 A = Points[indices[ii]];
    Vector3 B = Points[indices[ii+1]];
    Vector3 C = Points[indices[ii+2]];
    Vector3 V = Vector3.Cross(A-B, A-C);
    Area += V.magnitude * 0.5f;
}
         * 
         * 
         * */
    }
}

Works only on mesh whit scale 1, 1, 1.. Any update for custom scale?

multiply the Vector3 output by your scale buddy

Just gonna leave a warning that accessing vertices and triangles on a mesh creates copies of these arrays every time, so you might want to store these in some local variable.

Works well! I'm curious if there is a solution to having already rotated objects or applying rotation to the point to match the game object

Edit: Nevermind, realized it wasn't too much more to do that. Here's my code:

Vector3 RotatePointAroundPivot(Vector3 point, Vector3 pivot, Vector3 angles)
{
    Vector3 dir = point - pivot;
    dir = Quaternion.Euler(angles) * dir;
    point = dir + pivot;
    return point;
}