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updated : wed 21 aug 2002 4:29pm bst
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Real Unsharp Masking

The section on basic image sharpening showed how to use Photodesk's built-in unsharp mask facility to enhance a picture of the Moon. Here we'll look in more detail at unsharp masking, and show how to do it 'by hand'. This offers more control and much better quality, but does require extra work and is harder to get right.

The technique can appear almost magical at first. You take a copy of a blurred image, make it much worse, then subtract it from the original. It seems counter-intuitive, but the results can be astonishing.



Malcolm Ripley did some experimenting after reading this page, and has some useful advice:
The relation between the equalise adjustment and the gain factor in the calculate step should be :

gain = 255 / (255 - equalise factor)

The higher the equalise factor the greater the sharpening although as it gets "too sharp" it tends towards high contrast edges. Values between 150 - 190 seem to be about right depending on how sharp the original is.

The blur radius can be assessed by magnifying the image and examining a supposed sharp edge. The radius should be set to the approximate width of this edge. The overall sharpening varies very little if the blurring is out by +/- a pixel or so.

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