d.values[0][0] = 0.5 * (above.valArr[y][x] - below.valArr[y][x]);
d.values[1][0] = 0.5 * (current.valArr[y + 1][x] - current.valArr[y - 1][x]);
d.values[2][0] = 0.5 * (current.valArr[y][x + 1] - current.valArr[y][x - 1]);
SimpleMatrix b = (SimpleMatrix) d.clone();
b.negate();
// Solve: A x = b
H.solveLinear(b);
ref.val = b.dot(d);
return (b);
}