Image Processing Reference
In-Depth Information
4.3.3 Using Channels to Extract an Object from
the Background
What Are Channels?
As you know from section 1.3.3, the colors you see on your monitor are
created from three primary colors—red, green, and blue. The same holds
true for images in RGB mode: All colors in an image are mixed from these
three primary colors. Accordingly, each image in RGB mode consists of three
channels: red, green, and blue. Each channel is a chromatic component
representing the share of the corresponding color in the image.
You have been introduced to channels in the previous exercises, even though
it may have been in the user interface of a tool or function dialog, such as
correcting the color-cast image in the Levels dialog (tonality correction),
where only the red channel was edited.
Decomposing and Composing the Channels of an Image
You can use the Colors > Components > Decompose menu item to decompose
an image into its color channels. These channels can be accessed for editing
purposes in the Channels dialog. For example, you can target a single color and
apply a filter or option to it. The Colors > Components > Compose menu item
will combine the channels again, so you can go back to working with the full
RGB channel in the image. However, when the image is decomposing, it will
be converted into a grayscale image ( Image > Grayscale ). When it's composed,
the image will be set to true color again. This is true for decomposing and
composing, using the Color model: RGB in the settings of the Decompose dialog.
For decomposing to CMYK, I've encountered some problems. The channels
were decomposed, but the resulting layers were too dark. On composing
again as RGB, the colors were shifted and the image was too bright.
Consider the following information about color channels:
• The red channel ofers the best contrasts.
• The green channel has the highest sharpness.
• The blue channel shows the image quality the most.
The Channels Dialog
You can find the Channels dialog in the Layers, Channels and Paths dock, or
you can find it via the Windows > Dockable Dialogs menu item in the image
The Channels dialog works similarly to the Layers dialog. However, the
Channels dialog is separated into two parts. The upper part shows the red,
green, and blue color channels. In addition, there is an alpha channel to control
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