What does the find edges filter do ?

Uses a Sobel edge detector to highlight sharp changes in intensity in the active image or selection. Two 3×3 convolution kernels (shown below) are used to generate vertical and horizontal derivatives. The final image is produced by combining the two derivatives using the square root of the sum of the squares.

   1  2  1     1  0 -1
   0  0  0     2  0 -2
  -1 -2 -1     1  0 -1