RaycastHit.textureCoord

Description

The uv texture coordinate at the collision location.

A ray is fired into the Scene. The textureCoord is the location where the ray has hit a collider. RaycastHit._textureCoord is a texture coordinate when a hit occurs. A Vector2 zero is returned if no mesh collider is present in the GameObject.

Note: A textureCoord requires the collider to be a MeshCollider.

// Write black pixels onto the GameObject that is located
// by the script. The script is attached to the camera.
// Determine where the collider hits and modify the texture at that point.
//
// Note that the MeshCollider on the GameObject must have Convex turned off. This allows
// concave GameObjects to be included in collision in this example.
//
// Also to allow the texture to be updated by mouse button clicks it must have the Read/Write
// Enabled option set to true in its Advanced import settings.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Camera cam;

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

    void Update()
    {
        if (!Input.GetMouseButton(0))
            return;

        RaycastHit hit;
        if (!Physics.Raycast(cam.ScreenPointToRay(Input.mousePosition), out hit))
            return;

        Renderer rend = hit.transform.GetComponent<Renderer>();
        MeshCollider meshCollider = hit.collider as MeshCollider;

        if (rend == null || rend.sharedMaterial == null || rend.sharedMaterial.mainTexture == null || meshCollider == null)
            return;

        Texture2D tex = rend.material.mainTexture as Texture2D;
        Vector2 pixelUV = hit.textureCoord;
        pixelUV.x *= tex.width;
        pixelUV.y *= tex.height;

        tex.SetPixel((int)pixelUV.x, (int)pixelUV.y, Color.black);
        tex.Apply();
    }
}

Additional resources: Physics.Raycast, Physics.Linecast, Physics.RaycastAll.