Check out example codes for "let the plaer draw in unity". It will help you in understanding the concepts better.

Code Example 1

// 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,;

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