Understanding USM

By: Dale Cotton (glib tongue)and Brian D. Buck (penetrating insight)
Fuzzy Concepts of Sharp Pictures
Apparently, I have not been alone in feeling that the digital implementation ofUnsharp Mask (USM)was one of the great mysteries of the universe. The USM dialogue has three sliders —Amount,Radius,Threshold. They all have an effect, but finding an optimal setting seemed a hit and miss process because one never knew quite what the sliders were actually doing. The mystery became even more acute whenMichael Reichmannposted a tutorial on using USM to achieveLocal Contrast Enhancement. Finally, a discussion about this on theLuminous Landscapeforum caused Brian D. Buck to shatter the Graphics Programmers' unwritten vow of silence by revealing theArcane Secret of the Digital Unsharp Mask. You can read Brian's succinct statementhere; the verbose version follows.
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Please Clarify
Fig 1. A picture before applying USM
Sharpening is a...