18. High-pass Filters

Edge Handling

Kernel convolution relies on centering a pixel and looking at it's surrounding neighbors. So, what do you do if there are no surrounding pixels like on an image corner or edge? Well, there are a number of ways to process the edges, which are listed below. It’s most common to use padding, cropping, or extension. In extension, the border pixels of an image are copied and extended far enough to result in a filtered image of the same size as the original image.

Extend
The nearest border pixels are conceptually extended as far as necessary to provide values for the convolution. Corner pixels are extended in 90° wedges. Other edge pixels are extended in lines.

Padding
The image is padded with a border of 0's, black pixels.

Crop
Any pixel in the output image which would require values from beyond the edge is skipped. This method can result in the output image being slightly smaller, with the edges having been cropped.