The 6 basic things you need to know are:

1. Channels are simply special layers that store information

A channel is simply a layer on which is stored information about the image. They are called channels to avoid confusion with regular layers. To start with every RGB colour image has three channels: one for the red colour information, one for the green colour information, and one for the blue colour information. If the image is in a different colour format there may be more or less channels. For example, the Lab mode uses four channels.

2. Each channel is a greyscale image

Channels only store greyscale images. This means that a channel is the same as an 8-bit black and white photograph. The channel can store all the values from 0 (black) to 255 (white) but no colour. This is why a colour image has three channels, one each for the three primary colours of the picture.

3. Channels are a place to store selections

When you select a portion of an image using any of the selection tools in Photoshop you can save this selection in a channel (Save Selection… command on the Select menu) so it can be used again in the future. This is especially important when you have created a complex selection which may have taken quite some time to complete. The channel where the selection is stored is often referred to as an Alpha Channel. You can convert an alpha channel back to a normal selection using the Load Selection… command on the Select menu or by using Ctrl+Click (Cmd+Click on Macs) on the channel icon in the Channels palette.

4. You can have as many channels as you wish

You can save as many selections as you like and each one will be stored in its’ own alpha channel in the Channels palette. To see a stored selection overlaid as a red mask on your image click on the channel visibility box next to the channel in the Channels palette You can also view a stored selection as a greyscale image by clicking the visibility box of the required channel to display the red mask and then hide the RGB channel by clicking the RGB channel visibility box to remove the eye icon. The eye icon will appear in the box when the content of a channel is visible.

5. You can treat a channel like a greyscale photograph

Since a stored selection in a channel is a greyscale image you can do almost anything with this image that you can do with a normal black and white photograph. You can change the stored information by painting with a brush on the image using the colours black, white, or any tone of grey. If you paint with a tone of grey you will have created transparency when you load the stored selection to use again.

6. You can apply filters to channels

When you want to create unique or unusual selections, you can apply any of the filters to a channel that work with 8-bit greyscale images. This includes Gaussian blur which is used a lot to soften the edges of the stored selection. This would be similar to using the feather option when using the selection tools.

These 6 basic concepts will give you a head start in understanding the role of channels and what they are used for in your Photoshop image editing.

Photoshop for beginners - The 6 Basics of Channels.

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