I attempted a calculation of normal vectors for this mesh2 object.
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camera {location <0,0,-5> look_at 0}
light_source {<-10,20,-50>, rgb 1}

#declare GridSize = 7;
#declare PointCount = pow(GridSize,2);
#declare NumberOfFaces = 2*pow(GridSize-1,2);
#declare use_normal_vectors = 1;

#declare XCoord =
   array[PointCount] {
     -1.5, -1.0, -0.5, 0,  0.5,   1.0,  1.5,
     -1.5, -1.0, -0.5, 0,  0.5,   1.0,  1.5,
     -1.5, -1.0, -0.5, 0,  0.5,   1.0,  1.5,
     -1.5, -1.0, -0.5, 0,  0.5,   1.0,  1.5,
     -1.5, -1.0, -0.5, 0,  0.5,   1.0,  1.5,
     -1.5, -1.0, -0.5, 0,  0.5,   1.0,  1.5,
     -1.5, -1.0, -0.5, 0,  0.5,   1.0,  1.5
   }

#declare YCoord =
   array[PointCount] {
     -1.5, -1.5, -1.5, -1.5, -1.5,  -1.5, -1.5,
     -1.0, -1.0, -1.0, -1.0, -1.0,  -1.0, -1.0,
     -0.5, -0.5, -0.5, -0.5, -0.5,  -0.5, -0.5,
     0,   0,   0,   0,    0,    0 ,   0,
     0.5,  0.5,  0.5,  0.5,  0.5,   0.5,  0.5,
     1.0,  1.0,  1.0,  1.0,  1.0,   1.0,  1.0,
     1.5,  1.5,  1.5,  1.5,  1.5,   1.5,  1.5
   }

#declare ZCoord =
   array[PointCount] {
     0.7781,  0.9975,  0.6816, 0, -0.6816,  -0.9975, -0.7781,
     0.9975,  0.8415,  0.4794, 0, -0.4794,  -0.8415, -0.9975,
     0.6816,  0.4794,  0.2474, 0, -0.2474,  -0.4794, -0.6816,
     0,       0,       0,      0,       0,  0,       0,
     -0.6816, -0.4794, -0.2474, 0,  0.2474,   0.4794,  0.6816,
     -0.9975, -0.8415, -0.4794, 0,  0.4794,   0.8415,  0.9975,
     -0.7781, -0.9975, -0.6816, 0,  0.6816,   0.9975,   0.7781
   }

mesh2 {
   vertex_vectors {
     PointCount,
     #local I = 0;
     #while (I < PointCount)
       #if (I != PointCount-1)
	<XCoord[I],YCoord[I],ZCoord[I]>,
       #else
	<XCoord[I],YCoord[I],ZCoord[I]>
       #end
       #local I = I + 1;
     #end
   }
   #if (use_normal_vectors)
     normal_vectors {
       PointCount,
       #local I = 0;
       #while (I < GridSize)
	#local I1 = ((I = 0) ? 0 : I - 1);
	#local I2 = ((I = GridSize - 1) ? I : I + 1);

	#local J = 0;
	#while (J < GridSize)
	  #local J1  = ((J = 0) ? 0 : J - 1);
	  #local J2  = ((J = GridSize - 1) ? J : J + 1);

	  #local dx  = XCoord[I2 + J * GridSize] - XCoord[I1 + J * GridSize];
	  #local dy  = YCoord[I + J2 * GridSize] - YCoord[I + J1 * GridSize];
	  #local dzx = ZCoord[I2 + J * GridSize] - ZCoord[I1 + J * GridSize];
	  #local dzy = ZCoord[I + J2 * GridSize] - ZCoord[I + J1 * GridSize];
	  #local vnorm = vnormalize( vcross(<dx, 0, dzx>, <0, dy, dzy>) );
	  #if ((J < GridSize - 1) | (I < GridSize - 1))
	    vnorm,
	  #else
	    vnorm
	  #end
	  #local J = J + 1;
	#end
	#local I = I + 1;
       #end
     }
   #end
   face_indices {
     NumberOfFaces
     #local I = 0;
     #while (I<GridSize-1)
       #local J = 0;
       #while (J<GridSize-1)
	#local K = I*(GridSize)+J;
	<K,K+1,K+GridSize>,
	#if (K!=NumberOfFaces-1)
	  <K+1,K+GridSize+1,K+GridSize>,
	#else
	  <K+1,K+GridSize+1,K+GridSize>
	#end
	#local J = J + 1;
       #end
       #local I = I + 1;
     #end
   }
   pigment {rgb 1}
}

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