Image Processing Reference
In-Depth Information
2.8.1 Sharpening Images and Image Elements
No filter or tool can make a major improvement if your photo is strongly
blurred. But if your picture is only slightly out of focus or has a shallow depth
of field, you can sharpen it easily. However, it is not possible to improve the
quality of the picture. Pictures aren't enriched with more detail. Instead, the
existing details and contours are accentuated by enhancing the light/dark
contrasts at the edges of the objects and contours in the pictures (especially
with the Unsharp Mask filter).
GIMP offers various options to sharpen fuzzy photographs. There are two
filters to choose from: Sharpen and Unsharp Mask ( Filters > Enhance > Sharpen/
Unsharp Mask ). These filters affect the entire picture or the active layer. You
can, however, also select a section in the image to restrict the filter's effect to
that area.
The Blur/Sharpen Tool lets you use a brush to sharpen or blur sections of
your photo. This tool becomes interesting if you want to blur sections after
having sharpened them. Pictures that have been sharpened too much get
stark contours and increased noise. These blemishes can be fixed with the
Blur/Sharpen Tool or the Smudge Tool .
Let us have a look at the two available filters.
The Sharpen Filter
Figure 2.70
unsharp.png in its initial state
Before I start with the following examples, I'd like to point out that that these
filters can also greatly improve photographs that are not out of focus. You can
gain more visible detail with most pictures through additional sharpening. I
use one way or another for sharpening almost all of my photographs. The
results make the effort worthwhile. Try it out!
We applied the Sharpen filter ( Filters > Enhance > Sharpen ) in section
2.5.12. This filter has the effect of greatly increasing the sharpness of the
edges while reducing the pixels in pictures (for example, from 300 dpi to 72
dpi while keeping the image size), even after being applied only once.
You can use the Sharpen filter on high-resolution images. However, the
Unsharp Mask filter is more suited for images with larger number of pixels,
as I will explain in the next section. To get clear and detailed photos, you can
apply the Sharpen filter several times. Therefore, you should apply a moderate
value to the filter (in the range 0-100) to avoid suddenly oversharpening the
image (see figure 2.71 ). If this should occur slightly but the picture otherwise
has the desired increase in sharpness, you can apply the Blur/Sharpen and
Smudge tools.
To practice the various sharpening methods and experiment with the
settings, open unsharp.png , which you'll find in the SampleImages folder on
this topic's DVD.
Figure 2.71
Disturbing artifacts on a oversharpened
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