The panel where selections go to live

Three colour channels, and as many alpha channels as you like

The Channels panel starts with the composite and the three colour channels. Below them you can store alpha channels: extra greyscale images, the same dimensions as the document, that are not displayed and carry no colour. They are storage.

Any selection can be written into one. Select > Save Selection in Photoshop and Photopea; GIMP calls the same thing Select > Save to Channel. Ctrl/Cmd-click the channel’s thumbnail to load it back as a selection. That is the mechanism behind the most useful habit in this lesson: a selection that took you twenty minutes should never be allowed to die when you press Ctrl+D.

Alpha channels travel inside PSD and TIFF. They add about a third of a colour channel’s worth of size each, which is negligible, and they are the reason a professional cut-out file can be re-cut six months later.

Building a mask out of a channel

The shortcut you will use most: Ctrl/Cmd-click the RGB composite thumbnail and you get a selection whose strength at every pixel equals that pixel’s brightness: a luminosity selection with no edge anywhere. Ctrl/Cmd-click an individual channel and you get a selection based on that channel alone.

That second one is how difficult masks get made. Look at the three channels of a photograph of dark hair against a bright sky: in the blue channel the sky is near-white and the hair near-black, a far better separation than any selection tool will find. Duplicate the blue channel to a new alpha channel, apply a hard Levels move to push the sky to pure white and the hair to pure black, paint the middle of the head solid black with a brush, and you have a mask that took ninety seconds and holds every strand.

GIMP does the same through Colours > Components > Decompose, or by adding a layer mask with the Grayscale copy of layer option and then working on it. Krita has Image > Separate Image. None of this is a Photoshop-only technique.

Channel arithmetic: Apply Image and Calculations

Two dialogues that look forbidding and do something simple: they combine two channels with a blend mode and write the result somewhere.

Apply Image blends a chosen channel or layer into the currently active one. Point it at a layer mask and you can, for example, blend the blue channel into an existing mask with Multiply to knock back an area, in one step, with no brushwork.

Calculations takes two sources and writes the result to a new alpha channel, a selection or a new document. This is how multi-level luminosity masks are built, and it is the subject of a lesson of its own in the next block.

The practical pattern worth remembering: Multiply two masks to get their intersection; Screen them to get their union. Everything else in channel arithmetic is a variation on that.

intersection: result = maskA × maskB        (both must be white)
union:        result = 1 − (1−maskA)(1−maskB)  (either being white is enough)

Spot channels, for print

A spot channel is a different animal: it represents a specific named ink: a Pantone, a varnish, a white underbase for printing on dark garments, a die-cut line. It sits outside the CMYK separation and the printer produces a separate plate for it.

You will meet this if you do packaging, screen printing or anything with a metallic. The rules are strict: the channel must be named exactly as the printer expects, the ink must be specified, and overprint behaviour has to be agreed. Get it wrong and the job either prints as an extra unwanted plate or does not print at all. Ask the printer for their specification before building it; this is one area where guessing has a four-figure cost.

GIMP does not support spot channels. Scribus, free, handles spot colours properly for layout, and for a raster file destined for a spot plate the usual route is to supply a separate greyscale file per plate with clear naming.

Where channels mislead

Two honest cautions.

A channel-derived mask carries the channel’s noise. In a high-ISO photograph the blue channel is the noisiest, so a mask built from it will have speckled edges. Apply a small blur to the alpha channel, or build from green instead and accept less separation.

Pushing Levels on a channel to make a mask destroys the partial values that make edges believable. Driving the sky to pure white and the subject to pure black gives you a crisp mask and no transition zone, which produces exactly the cut-with-scissors look the selections lesson warned about. The craft is to push hard in the flat regions and leave the transition zone alone, which usually means doing the Levels move, then painting the interior solid by hand rather than turning the contrast up further.

And a structural limit: alpha channels are per-document and per-file. They do not follow a layer when you copy it into another document, and they are silently discarded when you export to JPEG, PNG or WebP. The mask is a working asset. Save the PSD or TIFF, or it was never saved at all.