Sobel filter

Reference resources sobel How operators work

If there is a color change as shown in the figure

The change of pixel depth can be shown in the figure below

We can see that the change of its curve in the middle position is the steepest . Calculate its first derivative , Observe further from the orange line in the figure below .

therefore , By calculating the derivative of the pixel value , Its boundary can be judged . But the pixel value of the image is discrete , There is no way to calculate the derivative . So calculate by convolution .

Use a convolution kernel as shown in the figure , To cover a piece 3x3 Region , You can capture the level / The characteristics of the vertical edge .

See the original text for other details .