void ComputeHeightfieldNormals(int xsize, int ysize, float *Heightmap, fvector *Normalmap)
{
        int x, y;
        float l;
        fvector *N;
        int xsm = xsize - 1;
        int ysm = ysize - 1;        

        for( y = 0; y < ysize; y++ )
	   for( x = 0; x < xsize; x++ ) {

	      // Access current normalmap grid point
              N = &NormalMap[y*xsize+x];

	      // Compute normal by using the height differential
              N->x = HeightMap[y*xsize + ( !x ? 0 : x-1)] - HeightMap[y*xsize + ( x == xsm ? xsm : x+1 )];
              N->y = HeightMap[( !y ? 0 : y-1)*xsize + x] - HeightMap[( y == ysm ? ysm : y+1 )*xsize + x];
              N->z = (2.0f / (float)xsize) + (2.0f / (float)ysize);

	      // Normalize it
              l = sqrt(N->x*N->x + N->y*N->y + N->z*N->z); 
	      if( l != 0 ) { 
	         N->x /= l; 
	         N->y /= l;
	         N->z /= l; 
	      } else N->x = N->y = N->z = 0.0f;

           }  
}