RaycastHit.triangleIndex

Description

The index of the triangle that was hit.

Triangle index is only valid if the collider that was hit is a MeshCollider.

#pragma strict
// This script draws a debug line around mesh triangles 
// as you move the mouse over them.
var camera: Camera;
function Start() {
	camera = GetComponent.<Camera>();
}
function Update() {
	var hit: RaycastHit;
	if (!Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), hit))return ;
	var meshCollider: MeshCollider = hit.collider as MeshCollider;
	if (meshCollider == null || meshCollider.sharedMesh == null)return ;
	var mesh: Mesh = meshCollider.sharedMesh;
	var vertices: Vector3[] = mesh.vertices;
	var triangles: int[] = mesh.triangles;
	var p0: Vector3 = vertices[triangles[hit.triangleIndex * 3 + 0]];
	var p1: Vector3 = vertices[triangles[hit.triangleIndex * 3 + 1]];
	var p2: Vector3 = vertices[triangles[hit.triangleIndex * 3 + 2]];
	var hitTransform: Transform = hit.collider.transform;
	p0 = hitTransform.TransformPoint(p0);
	p1 = hitTransform.TransformPoint(p1);
	p2 = hitTransform.TransformPoint(p2);
	Debug.DrawLine(p0, p1);
	Debug.DrawLine(p1, p2);
	Debug.DrawLine(p2, p0);
}

// This script draws a debug line around mesh triangles 
// as you move the mouse over them.
using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
	Camera camera;
	
	void Start() {
		camera = GetComponent<Camera>();
	}
	
	
    void Update() {
        RaycastHit hit;
        if (!Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), out hit))
            return;
        
        MeshCollider meshCollider = hit.collider as MeshCollider;
        if (meshCollider == null || meshCollider.sharedMesh == null)
            return;
        
        Mesh mesh = meshCollider.sharedMesh;
        Vector3[] vertices = mesh.vertices;
        int[] triangles = mesh.triangles;
        Vector3 p0 = vertices[triangles[hit.triangleIndex * 3 + 0]];
        Vector3 p1 = vertices[triangles[hit.triangleIndex * 3 + 1]];
        Vector3 p2 = vertices[triangles[hit.triangleIndex * 3 + 2]];
        Transform hitTransform = hit.collider.transform;
        p0 = hitTransform.TransformPoint(p0);
        p1 = hitTransform.TransformPoint(p1);
        p2 = hitTransform.TransformPoint(p2);
        Debug.DrawLine(p0, p1);
        Debug.DrawLine(p1, p2);
        Debug.DrawLine(p2, p0);
    }
}

See Also: Physics.Raycast, Physics.Linecast, Physics.RaycastAll.