It's generally understood in the
Photoshop world that almost every image can benefit from
judicious sharpening. Popping into the Unsharp Mask filter
isn't your only option. Understanding sharpening can help
you make the right choices.We sharpen
images to improve the detail, making them seem more "in
focus." This is especially important after resampling a
picture to change its size or resolution. The resampling
process computes the color values of neighboring pixels and
averages them, which reduces the contrast along edges.
Sharpening increases the contrast where different colored
pixels meet. To get a look at how it works, the
now-ubiquitous rubber ducky has been placed on a
highly-contrasting background and a ridiculously-high level
of sharpening has been applied.
As you can see, sharpening has increased the
contrast along the edges. This extreme application of
sharpening has created strong contrast along the edges of
the image. When we zoom in to 800%, we can see that two
separate halos have been created, one on either side of the
edge between the duck and the background. On one side, a
light halo is created, on the other a dark halo has been
added. This "double-haloing" is the core of sharpening in
Photoshop.
Another example of sharpening, this one far
more realistic, shows how the double-haloing can effectively
be used to make even very busy areas of detail "snap" and
jump away from the background. Compare the left
(unsharpened) and right (sharpened) sides of this image.
Zooming in, we can see that Photoshop had
few straight lines to follow in creating the light and dark
halos. The amount of detail and the fineness of that detail
result in halos that are, in some cases, actually a
lightening and darkening of the pixels that make up the
detail. The branches and twigs that are hardly more than a
pixel or two wide are themselves lightened and darkened to
increase the contrast.
We know that sharpening increases contrast
among pixels that already are contrasting. By increasing the
existing contrast, edges and details are made to stand out.
Let's go back to our original sample and see how this is
done, using the color channels. Pictured below are, in
order, the Red, Green, and Blue channels after the large
amount of sharpening was applied.
When we look at the Red channel, we see that
the sharpening was accomplished with a dark halo in the
background and a light halo on the duck. That is the result
of the difference in the red component of the yellow duck
and the lack of red in the background. (In RGB, yellow is
created with red and green.) Because of that lack of red in
the background, the red component was darkened. Because the
red component of the duck was much darker, a light halo was
created on that side of the edge.
The Green channel shows less of a difference
between the duck and its background, although the difference
is still substantial. Both light and dark halos were
created, with the light halo on the background side of the
edge and the dark halo on the yellow side. (The background
has a higher percentage of green.) Zooming in and using the
Eyedropper tool to place color samplers shows the halo.
The color samplers range from the yellow
duck (#1) to the dark halo (#2) to the light halo (#3) to
the background (#4). The actual grayscale values at each
point in the Green channel are shown in the Info palette.
The Blue channel shows a distinct
light-colored halo along the background-side of the edge.
Because the yellow of the duck has no blue component (it
consists entirely of red and green), there can be no dark
halo – the area is already completely black in the Blue
channel.
When we return to the composite channel, we
can see how the halos created on the individual color
channels are combined. Remember that this was an example of
extreme sharpening. The red halo, which we certainly would
not want in a final image, is an unwanted by-product.
Later in this series we'll look at ways to
avoid some sharpening problems (including colored halos) and
how you can sharpen using the color channels to control what
parts of the image are sharpened. The next installment,
however, will look at the Sharpen filters and the Sharpen
tool.