Lesson 16: Channels

Channels is a term that primarily refers to the components that constitute a digital image. Depending on the structural level for which the term is perceived, we can analyze and affect an aspect of the image. We may consider a color channel, to affect color. Or we may consider a layer's transparency as a channel, to affect the layer's transparency. Or we may consider a layer mask in the same fashion. Or we may consider the Quick Mask as a temporary channel, as we have seen in Lesson 8. These conceptual entities may be considered as channels -and basically they are implemented technically as such- been distinguished according to what we want to do with them.

Simply put technically, a channel is a sort of a mask controlling a specific visual property. (I could have said it the other way, "a mask is a sort of a channel" but i am not going to insist on this right now, pardon me. ;-) ) A channel is conceptually implemented as a monochrome (8bit) representation of a specific property, which property is shared by all the pixels of the layer or the image. This property gets values from the well-known range of 0-255 - black means 100% "no", white means 100% "yes", with intermediate values representing the varying degree of the property.

Why channels are useful?

Channels are useful basically for two reasons:

• to affect color, and
• for effective manipulations on selections via selection masks.

Considering color, color channels are the basic channels that refer to the structure of the image. As you already know from the theory, for a color image in the RGB color model there are three color channels: Red, Green, Blue. For a monochrome image in the Grayscale color model, there is one channel: Gray. In case the image has more than one layers, or in case it has one layer with transparency (the layer has an alpha channel), then if you take it logically the image should have an extra basic channel, the so-called Alpha channel which represents transparency, except from the other three (if RGB) or one (if Grayscale).

Considering selection masks, channels offer exquisite flexibility for detailed selective masking, as you have seen if you have followed along to this Lesson. I have been trying to create a learning process so that you conquer new territory little by little, easing for you the learning curve as much as possible. If you have done this School from the beginning as suggested, you know what i am writing about. In this lesson i focus on the technical side of the channels, so that you get the full picture.

The interface for channels in GIMP comes with a special dialog, the Channels Dialog, which has two parts, the upper area with the basic channels, and the area below with the so-called selection masks. The channels in the upper area are derived from the type and the structure of the image as explained above, while the channels in the main dialog area below are extra ones created at will or are produced temporarily when working with your image (i.e. the Quick Mask).

The channels dialog for an RGB image with transparency and with an active selection.

You may ever act on one drawable at a time

Inside the GIMP's application interface, any pixel-affecting action you do on your image acts on one drawable. It is logical, it is simple, it makes sense absolutely, it is the way it should be, of course. So, if you click on a layer, you select this layer to work on - and all your custom channels are deselected. If you click on a channel, you select this channel to work on - and all your layers are deselected.

Working with complex selections

In Lesson 9 you learned how to convert a selection to a channel. As said, this enables you to save selections for later use. So by rightclicking on any channel, with Channel to Selection you can make a selection out of it. And with the next three commands on the context menu, you may add to the selection, subtract, or intersect with the current selection. So you have all possibilities to make complex selections effectively. Do i have to mention that practicing this will help you to be a master on selective masking sooner or later?

Editing and reusing channels

• Clicking on the eye icon, you hide or show channels. Hiding the basic channels is not usually useful, you just do not see them but they do exist.
• Apart from the eye icon, if you click on a basic channel you can deactivate it or activate it back. By default they are all activated and as a beginner you should leave them as such making sure that they are always activated. This is important because anything you do on your image, any action you perform affects the activated channels only. For example if you paint with one channel deactivated, your painting action will exclude that channel and you will get an unexpected result.
• By rightclicking in the dialog you can create a new channel, with some important parameters in the dialog. You also have the option to use a selection for the new channel.
• In the upper part of the dialog you cannot delete channels. They are what your image consists of, they are the basic channels that make the image.
• You can duplicate or delete channels. By dragging a basic channel from the upper part of the dialog to the lower part, you copy it. By dragging a layer inside the Channels Dialog you can create a new channel with its contents.
• You can use any layer or channel of any opened image to create a new custom channel. If you drag-and-drop a layer or channel on your image, a new layer is created, and in case you want it a channel you then drag-and-drop the new layer inside the Channels Dialog (you may disable or delete the layer of course).
• A drag-and-drop of any channel on the Toolbox opens it as a new independent image. (Question: in what color model will this image come out to be? Answer: Grayscale.)
• A channel is a drawable on which you may perform any editing action like drawing, copying or pasting. In case other enabled drawables -layers or channels- get in your way making editing difficult, you have the option to disable the visibility of any of them selectively using their eye icon.
• Shift to Quick Mask View and duplicate the temporary "Quick Mask" channel, to quickly save a selection as a channel.