From Eq. (9), we can know that the derivatives of wrapped phase image after using unwrapping operator $U$ are equal to the derivatives of true phase image. The relationship of Eq. (9) can be extended in the directions of $x$ and $y$, which can be expressed as $\u25a1\phi x(i,j)=\Delta \varphi x(i,j)$, $\u25a1\phi y(i,j)=\Delta \varphi y(i,j)$, where $\u25a1\phi x,\u25a1\phi y$ denotes the derivative of wrapped phase image after using unwrapping operator $U$ and $\Delta \varphi x,\Delta \varphi y$ denotes the derivative of true phase image. Specifically, their derivatives are given by Display Formula
$\u25a1\phi x(i,j)=U[\phi (i+1,j)\u2212\phi (i,j)],$(10)
Display Formula$\u25a1\phi y(i,j)=U[\phi (i,j+1)\u2212\phi (i,j)],$(11)
Display Formula$\Delta \varphi x(i,j)=\varphi (i+1,j)\u2212\varphi (i,j),$(12)
Display Formula$\Delta \varphi y(i,j)=\varphi (i,j+1)\u2212\varphi (i,j).$(13)