The Sharpen tool in digiKam offers three sharpening methods: simple sharp, unsharp mask, and refocus. Each method has its advantages and drawbacks. The simple sharp technique uses a standard convolution matrix algorithm to improve image details. If you are curious about the nitty-gritty of the convolution matrix, the GIMP documentation provides a brief description of the algorithm.
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This is a companion discussion topic for the original entry at https://digikam.org/news/2013-05-08_sharpening_in_digikam/