Working with channels and alpha channels
The Channels panel in Paint by Zaweba: view or paint one channel, load saved selections, split channels into documents and merge them back.
Every colour picture is made of channels. An RGB image has a red, a green and a blue channel, each a greyscale picture of how much of that colour sits in every pixel. Most of the time you never think about them. Now and then, looking at or working on a single channel is the fastest way to solve a problem, and the Channels panel in Paint by Zaweba is where you do it.
What the panel shows
For an RGB document the panel lists the composite RGB channel, then Red, Green and Blue. Shortcuts switch between them: Ctrl+2 for the composite, then Ctrl+3, Ctrl+4 and Ctrl+5 for red, green and blue.
Below those come alpha channels, which are saved selections, and spot colour channels if the document has any. In CMYK or Lab documents the panel lists that document's own channels instead, so you'll see cyan, magenta, yellow and black, or lightness and the two colour channels.
Viewing and painting one channel
You can view a channel on its own, which shows it as a greyscale picture. This is revealing. The blue channel of a portrait is often noisy and dark; the red channel shows skin smooth and bright; the green channel is often the sharpest. For black and white conversions or for choosing a channel to base a mask on, a quick look through all three tells you a lot.
You can also paint on one channel alone. That is specialist work, but it's useful for fixing a colour cast in one area or for removing a flaw that only shows in one channel.
Alpha channels and selections
An alpha channel is a selection stored as a greyscale image in the document. White is selected, black isn't. Save Selection in the Select menu creates one, and Ctrl+click on any channel loads it as a selection.
That Ctrl+click works on the colour channels too, which gives you a quick brightness-based selection. Ctrl+click the blue channel, for instance, and you get a selection that is strongest where the picture has the most blue, which can be a good start for a sky mask.
The buttons along the panel load a channel as a selection, save the current selection as a channel, duplicate a channel, delete one, and invert one. Duplicating a colour channel and then adjusting the copy's contrast is a classic way to build a mask for hair or tree branches against a bright sky. The copy is an alpha channel, so it doesn't change the picture.
Split and merge
Split channels turns each channel into its own greyscale document. Merge channels does the reverse, building one document from open greyscale documents. These are handy for scientific images, for special effects such as shifting one channel relative to another, and for treating each channel as a separate black and white picture.
Where channels fit in
Most people use channels in two ways. The first is storing selections. Any selection that took effort should be saved as an alpha channel, and the Channels panel is where you find it again. The second is mask making: duplicate the channel with the best contrast between subject and background, push its contrast hard with Levels or Curves, clean it up with a brush, then load it as a selection.
Channel slips to watch for
Editing a colour channel when you meant to edit a copy is the classic mistake, because it changes the picture's colours. Duplicate first, then adjust the copy.
Painting while a single channel is visible and forgetting to switch back can also confuse, since the canvas looks grey. Press Ctrl+2 to return to the full colour view.
The panel takes a little getting used to, but it opens up mask techniques that no single selection tool can match. Paint by Zaweba keeps it close to the way photo editors have long worked, so guides you may have read elsewhere about channel masks will make sense here too.
