Where the leaves are backlit and very nearly identical in colour to the background, however, the decision as to whether any given pixel belongs to a leaf or to the background becomes anything but clear-cut. That’s what USM tries to do the crux is where to put the masking tape? The edges where the leaves are dark are fairly clear-cut. One wants to take a piece of masking tape, lay it down along one’s best guess where an edge should be, then colour everything on one side scarlet or dark green and everything on the other side pea soup green. Looking at these edges at high magnification what we see is like looking at the original scene through a screen door - crucial transitions are missing. By way of example, consider the boundaries between the leaves and the pale green background in Fig 3. The principle behind unsharp masking is exaggerating the light-dark contrast between the two sides of an edge. We throw away even more information in the hopes that what is left gives the illusion of the original acutance Lacking that, we resort to the old law that two wrongs make a right. ( See also Understanding Sharpnessfor more information.)īut if the edges have been blurred during digitization, how can the lost detail that defined the edges be recovered? One option would be to re-assemble the team of PhDs that corrected the Hubble Space Telescopes optics by means of a computer program. Fuzzy edges cause us to rub our eyes, wondering if we need new glasses, so high acutance tends to be one of the holy grails of photography. acutanceis Photography 101 speak for how precisely the edges of things are defined. Sharpening is about restoring acutance that has been lost previously along the image capture chain of events. You can read Brian’s succinct statement here the verbose version follows. Buck to shatter the Graphics Programmers’ unwritten vow of silence by revealing the Arcane Secret of the Digital Unsharp Mask. Finally, a discussion about this on the Luminous Landscapeforum caused Brian D. The mystery became even more acute when Michael Reichmannposted a tutorial on using USM to achieve Local Contrast Enhancement. 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 USM dialogue has three sliders - Amount, Radius, Threshold. Buck (penetrating insight) Fuzzy Concepts of Sharp PicturesĪpparently, I have not been alone in feeling that the digital implementation of Unsharp Mask (USM)was one of the great mysteries of the universe.
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