Convolution by Unsharp Mask

Unsharp masking is a convolution technique that, at first sight, appears to be something else. It works because the linearity property of convolution allows you to break a kernel into additive component kernels. First, the image is convolved with a kernel that averages pixel values over a region:

1 4 6 4 1 4 16 24 16 4 6 24 36 24 6 • 4 16 24 16 4 1 4 6 4 1

The soft image is called the unsharp mask, after the masking process used during the last century by photographers and photolithographers. Next, the image is convolved with an expanded version of the unity kernel:

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