Replacing a Defective Channel

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 and . 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 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).

(Thanks to Mike Edwards for the question that suggested this column.)