High Pass Sharpening
Imaging - Imaging
Some imaging application such as Photoshop offer a so-called "high pass" filter. The filter is capable to detect edge details within a specified radius. Other information are suppressed. In other words - the filter provides you with the raw data that is important for sharpening.

Now what can be done with this information. The raw data alone is pretty much useless but there is a workflow which takes advantage of the high-pass filtering:

  1. make a copy of your image and change the color model to Lab color (PS: image menu -> mode -> Lab color)
  2. apply the high pass filter (PS: filter menu -> other -> high pass). A low filter setting (1.0-2.0) extract extremely fine details (incl. noise if present), a higher value extracts coarse information (excl. noise). You've to experiment a little here to find a value that suits your specific needs.
  3. now omit the color information (e.g. by converting to grayscale and back to RGB) - this reduces color noise
  4. copy the filtered image to the original as a new layer
  5. now combine the two layers by choosing "hard" or "soft" light in the menu (the high-pass filtered image must be the active layer) of the layer box and play around with the opacity in order to define the degree of sharpening
Variation: you may repeat the workflow using further high pass filters with different radius settings. Unlike other approaches high-pass filtering does not produce sharpening halos. Here's how it looks ...


the original image (raw)


the resulting B&W high-passfilter (radius 1.5)


the final result (hard light, opacity 40%)


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