You can find this filter through
→ → .Sobel's filter detects horizontal and vertical edges separately on a scaled image. Color images are turned into RGB scaled images. As with the Laplace filter, the result is a transparent image with black lines and some rest of colors.
Sobel Horizontally: Renders near horizontal edges.
Sobel Vertically: Renders near vertical edges.
Keep sign of result: This option works when only one direction is selected: it gives a flat relief with bumps and hollows to the image.