Image gradient is a value that describes changes in images. It defines as follows: $\nabla I =(\frac{\partial I}{\partial x},\frac{\partial I}{\partial y})$ It can be approximated by discrete derivatives: $d_0 = [1, -1], f \circ d_0 = f[n] - f[n-1]$ $d_1 = [1, 0, -1]/2, f \circ d_1 =\frac{f[n+1] - f[n-1]}{2}$ In practice, we can use a filter such as $[1, -1]$, $[1, 0 ,-1]$, $[1, -1]^T$ and convolute it with our image.