Figure 16.28. Applying example for the Unsharp Mask filter
Out-of-focus photographs and most digitized images often need a
sharpness correction. This is due to the digitizing process that must
chop a color continuum up in points with slightly different colors:
elements thinner than sampling frequency will be averaged into an
uniform color. So sharp borders are rendered a little blurred. The same
phenomenon appears when printing color dots on paper.
The Unsharp Mask filter (what an odd name!) sharpens edges of the
elements without increasing noise or blemish. It is the king of the
Some scanners apply a sharpen filter while scanning. It's worth
disabling it so that you keep control on your image.
Some imaging devices like digital cameras or scanners offer to sharpen
the created images for you. We strongly recommend you disable the
sharpening in this devices and use the GIMP filters instead. This way
you regain the full control over the sharpening of your images.
To prevent color distortion while sharpening, Decompose your image to
HSV and work only on Value. Then Compose the image to HSV. Go to
Image/Mode and click on Decompose. Make sure the
Decompose to Layers
box is checked. Choose HSV and click OK. You will get a new grey-level
image with three layers, one for Hue, one for Saturation, and one for
Value. (Close the original image so you won't get confused). Select
the Value layer and apply your sharpening to it. When you are done,
with that same layer selected, reverse the process. Go to Image/Mode
and click on Compose. Again choose HSV and click OK. You will get back
your original image except that it will have been sharpened in the
3.7.4. How does an unsharp mask work?
Using an unsharp mask to sharpen an image can seem rather weird. Here is
Think of an image with a contrast in some place. The intensity curve of
the pixels on a line going through this contrast will show an abrupt
increase of intensity: like a stair if contrast is perfectly sharp, like
an S if there is some blur.
Now, we have an image with some blur we want to sharpen (black curve).
We apply some more blur: the intensity variation will be more gradual
Let us subtract the blurredness intensity from the intensity of the
image. We get the red curve, which is more abrupt : contrast and
sharpness are increased. QED.
Unsharp mask has first been used in silver photography. The photograph
first creates a copy of the original negative by contact, on a film,
placing a thin glass plate between both; that will produce a blurred
copy because of light diffusion. Then he places both films, exactly
corresponding, in a photo enlarger, to reproduce them on paper. The dark
areas of the positive blurred film, opposed to the clear areas of the
original negative will prevent light to go through and so will be
subtracted from the light going through the original film.
In digital photography, with GIMP, you will go through the following
Open your image and duplicate it
In the copy, duplicate the layer
→ , then drop the Filters menu down and apply
to the duplicated layer with the default IIR option and radius 5.
In the layer dialog of the duplicated image, change Mode to
“Subtract”, and in the right-clic menu, select
Click and drag the only layer you got into the original image, where
it appears as a new layer.
Change the Mode in this layer dialog to “Addition”.
Voilà. TheUnsharp Mask
plug-in does the same for you.
At the beginning of the curve, you can see a dip. If blurring is
important, this dip is very deep; the result of the subtraction can be
negative, and a complementary color stripe will appear along the
contrast, or a black halo around a star on the light background of a
nebula (black eye effect).
Figure 16.30. Black eye effect