Sobel Filter / Edge Detector in Image Processing

To filter an image it is filtered using both operators the results of which are added together

The Sobel edge detector is a gradient based method. Sobel filters are typically used for edge detection

Sobel Filter / Edge Detector in Image Processing


  • It works with first order derivatives.
  • It calculates the first derivatives of the image separately for the X and Y axes.
  • The derivatives are only approximations (because the images are not continuous).
  • To approximate them, the following kernels are used for convolution:

The kernel on the left approximates the derivative along the X axis.

The one on the right is for the Y axis. Using this information, you can calculate the following:

  • Magnitude or “strength” of the edge:
  • Approximate strength:
  • The orientation of the edge:

Above, I’ve detected horizontal peaks. You can clearly see the horizontal edges highlighted.

You can then threshold this result to get rid of the grey areas and get solid edges.

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