using UnityEngine;

public class Terr : MonoBehaviour
{
      private Terrain terr;
      private byte[,] splatIndex;
      private Vector3 size;
      private Vector3 tPos;
      private int width;
      private int height;

   void Start ()
   {
          terr = GetComponent<Terrain>();
       CalcHiInflPrototypeIndexesPerPoint();
   }

      private void CalcHiInflPrototypeIndexesPerPoint()
      {
          TerrainData terrainData = terr.terrainData;
          size = terrainData.size;
          width = terrainData.alphamapWidth;
          height = terrainData.alphamapHeight;
          int prototypesLength = terrainData.splatPrototypes.Length;
          tPos = terr.GetPosition();

          float[, ,] alphas = terrainData.GetAlphamaps(0, 0, width, height);
          splatIndex = new byte[width, height];

          for (int x = 0; x < width; x++)
          {
              for (int y = 0; y < height; y++)
              {
                  byte ind = 0;
                  float t = 0f;
                  for (byte i = 0; i < prototypesLength; i++)
                      if (alphas[x, y, i] > t)
                      {
                          t = alphas[x, y, i];
                          ind = i;
                      }
                  splatIndex[x, y] = ind;
              }
          }
      }

      public int GetMaterialIndex(Vector3 pos)
      {
          pos = pos - tPos;
          pos.x /= size.x;
          pos.z /= size.z;

          return splatIndex[(int)(pos.z * (width - 1)),
                    (int)(pos.x * (height - 1))];
      }
}

using UnityEngine;

public class TestCaster:MonoBehaviour
{
      private RaycastHit hit;

      private int matIndex;

      void Update()
      {
          if(Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition),out hit))
          {
              Terr t = hit.transform.GetComponent<Terr>();
              if(t!=null)
              {
                  matIndex = t.GetMaterialIndex(hit.point);
              }
          }
      }

      void OnGUI()
      {
          GUI.Box(new Rect(10,10,50,50),matIndex.ToString() );
      }
}