These filters increase the differences between adjacent pixels and give
an apparent sharpness to the image;
when it is digitized or compressed or sent to an output device, any image
always lose some sharpness; as a result, most images will appear soft: in
this case we can use the sharpness filters;
the sharpen category of the Effects menu, contains five filters that provide
a wide range of sharpening effects that can be used to both improve image
quality and create special effects; |
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1.Adaptive Unsharp;
2.Directional Sharpen;
3.High Pass;
4.Sharpen;
5.Unsharp Mask;
the Image menu contains the Tune Sharpen, that allows you to apply all but
High Pass filters from the same dialogue box;
1/2/5 act in roughly the same manner, introducing small amount of distortion
to the image to reduce noise enhancement;
4 is a true sharpen filter that sharpen both the image and its noise equally;
3 removes low frequency detail and shading, while emphasizing higlights
and lighter areas of an image.
The human eye is influenced by the presence of edges in an image and without
edges it appears dull;
by increasing the difference (contrast) between neighboring pixels edges
can be enhanced and thus the image appear to be sharper to the viewer, whether
it is or not;
while sharpening filters help to compensate for images that are out of focus,
don't expect sharpening to bring a blurred photograph into sharp focus;
all computer images include noise: noise consist of pixels that may produce
a grainy pattern;
all images from photograph will have noise of some sort, the only exception
is the Uniform color fill which has no noise... or details;
when we sharpen an image, we sharpen the noise as well: in fact, the noise
generally sharpens up much better and faster then the rest of image, because
noise pixels contain the one component that sharpening filters look for
the difference between adjoining pixels: since the act of sharpening seeks
out the differences (edges) and increases the contrast, the edges of the
noise are enhanced more than the rest of the pixels in the image; |
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The Unsharp Mask filter: Percentage = 150
Treshold = 15
Radius = 10 |
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Unsharp mask filters can minimize the effect of the noise by
distorting the image; that is, the effect of toning down the sharp borders
of noise while providing general sharpening of the other pixels in the image:
the result is an overall sharpening of the image without enhancing the noise;
Unsharping is named the tecnique that highlights the edges in an image by
combining a blurred film negative with the original film positive.
The Unsharp Mask filter compares each pixel in the image to
its neighboring pixels: it then looks for amounts of contrast between adjacent
pixels: after it has found pixels that appear to be an edge, it creates
a light corona around those pixels, so, can also produce an undesired effect
by creating halos around detected edges when applied in excessive amounts. |
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The Adaptive Unsharp filter: Percentage = 100 |
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The Adaptive Unsharp filter is a local control over the sharpening
process around each pixel, rather than a global sharpening amount applied
to the image in general.
It uses a process that evaluates statistical differences (adaptive) between
adjacent pixels to determine the sharpening amount for each pixel; it's
very similar in its effects to the other two Unsharp filters (Directional
Sharpen and Unsharp Mask): mainly Adaptive Unsharp appears to produce slightly
less contrast than the other two. |
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The Directional Sharpen filter: Percentage = 100 |
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Also The Directional Sharpen filter has a local sharpening control:
sharpening amounts for each pixel are computed for several compass dicections
and the greatest amount of these will be used as the final sharpening amount
for that pixel.
It analyzes values of pixels of similar color shades to determine the direction
in which to apply the greatest amount of sharpening: it usually increases
the contrast of the image more than the Unsharp Mask filter does and also
produces good sharpening, but with higher contrast than either the Unsharp
Mask or Adaptive Unsharp filter. |
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The Sharpen filter:
Level = 50, Treshold = 50 Preserve Colors |
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The Sharpen filter is rarely the best filter to use because doesn't
care about noise: it's just sharpens everything in the photograph.
It is a powerful that sharpens the appearence of the image or a masked area
by intensifying the contrast of neighboring pixels.
Its dialog box contains three controls:
the Edge level slider controls the amount of the sharpening applied
to the image (note:use this filter higher settings with some degreed
of caution: higher values usually produce blowouts); the Threshold slider determines the level of contrast between adjoining pixels that is necessary
for the filter action to occur: if value is set high, more pixels meet the
minimum requirement and the sharpening effect will be applied to more of
the image; if value is set low, only the high contrast elements of the image
will be affected;
the Preserve color check box, when enabled, prevents dramatic shifts
in hue when applying a sharpening effect. |
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The High Pass filter: Percentage = 50
Radius = 10 |
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| The High Pass filter, official, removes low-frequency detail
and shading, while emphasizing highlights and luminous areas of an image:
this filter makes a high contrast image into a murky gray one (high contrast
image areas are isolated from their low contrast counterparts); this filter
is best used as preparatory to other filter actions. |
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The Tune Sharpen filter is a dialog box where it is possible
to see all sharpening filters at once (except High Pass filter).
While having the filters grouped together facilitates multiple applications
of different filters, there is a loss of control.
The Edges level, Preserve Colors and Thrashold controls found in the Sharpen
Filter dialog box are missing in the Tune Sharpen Filter as are the Radius
and Treshold controls of the Unsharp Mask Filter.
Remember that excessive sharpening brings out noise, lots of noise! |
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