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The final topic for this discussion, and possibly the most important
we will cover, is sharpening. When images are scanned, imported from
digital cameras, or reduced in size, the details begin to blend together
and edges become soft. It is a rare image that does not need some sharpening.
The sharpening tool that is most useful for photographs is the Unsharp
Mask, now available in most raster programs. The Unsharp Mask searches
through your image looking for where colors change, and sharpens those
areas. The Unsharp Mask is superior to any other sharpening because
it makes decisions based on adjacent pixels, not random color changes,
so it usually can find and sharpen just the true edges of color areas.
The Unsharp Mask generally comes with three settings. The first is
the quantity of effect that the filter adds to the image. For Web images,
the amount of sharpening required is always significantly less than
for high quality printing. Monitors display images at 72 dpi, while
printing commonly reached 2500 dpi and above. The larger pixels for
monitor viewing require less sharpening.
The second value for sharpening is Threshold or Clipping. The Threshold
value determines what level of color change the program will accept
for sharpening. A value of 0 sharpens all pixels in the image, and as
the number increases, the amount of contrast between pixels increases
before they are considered for sharpening.
Finally, the Radius value determines how many pixels around the area
to be sharpened will be included. For Web images, this value should
remain quite low considering that 72 dpi features large pixels.
The images at the left illustrate the image quality that sharpening
can deliver. The top photo is typical of a scanned or reduced image.
Although it does not look too bad when seen alone, beside its sharpened
version, the difference screams out.
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