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.

// This script draws a debug line around mesh triangles 
// This script draws a debug line around mesh triangles 
// as you move the mouse over them.

#pragma strict

var cam: Camera;

function Start() {
	cam = GetComponent.<Camera>();
}

function Update() {
	var hit: RaycastHit;
	
	if (!Physics.Raycast(cam.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.