|
Sometimes you'll find that one color channel in
an image is just not salvageable, but the other channels are
fine. It's possible to, with luck and patience, recreate the
damaged channel.
Especially with digital cameras, you may find
that one color channel in an image has a large amount of noise,
usually the blue channel. Often you can select that channel in
the Channels palette and apply a Gaussian Blur to remedy the
problem. By blurring only one channel, the image's detail is
retained in the other channels and the picture itself doesn't
suffer too much from the blurring.
Sometimes, however, the channel is simply beyond
hope. In our exaggerated sample image, the amount of noise in
the blue channel has created both blue and yellow spots in the
image.
The Channels palette allows us to view each
channel individually. Clicking the eyeball icons next to the
individual channels toggles the visibility. (Clicking the
eyeball icon next to the composite channel shows all channels.)
Looking at the channels, we can confirm that the problem is in
the blue channel.
Remember the difference between a visible
channel and an active channel. When the channel's name is
highlighted in the Channels palette it is active. Clicking on a
single channel activates it. Shift-clicking allows you to active
more than one channel. Clicking the composite channel (named RGB
or CMYK, depending upon color mode) activates all channels.
It's possible to have one (or two) channel(s)
active and all channels visible. To do so, click on a channel to
activate it, then click in the eyeball column next to the
composite channel to make all channels visible. That allows you
to work on one channel (or a couple of channels) and monitor the
effect on the overall image.
As mentioned above, one way to handle noise is a
single channel is to apply a blur filter to that problem
channel. As you can see in this image, by the time the blur
radius is large enough to eliminate the noise, it's creating
other problems. A yellow halo is appearing along edges and a
general discoloration of the picture is apparent. (Note that the
blue channel is active and all channels are visible.)
In an attempt to rescue this image, we'll
replace the blue channel. Normally, we could start by comparing
the contrast in the other two channels to see which is a closer
match to the problem channel. The extreme noise in the blue
channel makes it very difficult to get an accurate comparison
for this image.
Instead, we'll make some copies of the image and
dive right into the problem. The left-most button at the bottom
of the History palette allows you to make a copy of an open
image with a single click. In this case, we'll make two copies.
By default, the new files are named for the current state in the
History palette. However, we'll save the files with the names
<CopyRed> and <CopyGreen>. This makes it easy to track which
technique is used with which file.
With CopyRed active, we can go to the Channels
palette and make the red channel active by clicking on it.
Clicking the eyeball next to the RGB channel allows us to track
our work's effect on the entire image. The commands Select All
and Copy place the grayscale information from the red channel on
the Clipboard. Clicking on the blue channel makes it active.
Since we haven't deselected, everything in the channel remains
selected. Deleting the content of the blue channel removes the
noise from the image… and a whole lot more.
Using the Paste command puts the red channel
info into the blue channel.
After making <GreenCopy> active, the same
technique can be used to copy the content of the green channel
to the blue channel. The effect on the overall image is somewhat
different. Comparing the two copies of the image to the original
shows that duplicating the green channel, in this case, produces
a much more pleasing image.
However, there are still some touch-ups
required. Notice that while the rug under the tree looks fine in
GreenCopy, the tree itself, the subject of the image, is a
little off. In addition, several books and objects on the
shelves need some color correction. (On the plus side, it looks
like someone finally polished the silver….)
A variety of techniques can be used to do the
final correction, including using the Curves or Levels command
on the new blue channel. The tree could also be copied from the
red channel into the new blue channel. However, greater control
often can be achieved with the command Image> Adjust> Selective
Color.
The Selective Color dialog box allows you to
adjust the appearance of each color individually, as well as
work with Neutrals, Blacks, and Whites.
In addition to Selective Color, a little general
"tweaking" of the image is in order, using Curves or Levels.
Zooming in on an area to the lower right of the
tree shows the distinct improvement from the original (top) to
the image with the reconstructed blue channel (bottom).
 |