Today’s Question: I’ve heard references to an “alpha channel” in Photoshop several times, but I don’t understand what this is. Can you explain?
Tim’s Quick Answer: An alpha channel is essentially any channel other than the channels that define color values for pixels in an image. Generally an alpha channel is used to define areas of a photo, such as to define transparency or to preserve a saved selection.
More Detail: In the context of a digital image, the term “channel” generally refers to the information about individual color values that comprise the overall pixel information. For example, with a typical RGB image there are three channels that individually define the red, green, and blue values for pixels.
The actual definition of individual color values for pixels consists of what are effectively shades of gray, or luminance values. So, for an RGB image you might have a pixel that consists of a red value of 88 (on a scale from 0 to 255), a green value of 142, and a blue value of 222. Those values reflect the luminance value (shade of gray) for the individual pixel on each of the channels that define color.
But you aren’t limited to only using channels that define color. You can also create alpha channels, which can be thought of as defining a selection for the image, where white pixels on the alpha channel represent pixels that are fully selected, black pixels represent pixels that are fully deselected, and pixels with a shade of gray represent areas that are partially selected.
Of course, an alpha channel can be used for more than just a selection, but in general the concept is the same. For example, you could think of the Quick Mask mode in Photoshop, which is used for refining selections, as being a form of alpha channel. Similarly, layer masks used in conjunction with adjustment layers or to create a composite image are also effectively alpha channels.
So, an alpha channel is essentially a “map” that defines specific areas of the image. It is similar in many ways to a channel that defines color, but since it is used for a purpose other than defining overall color for an image, it needed a “special” name. The term “alpha channel” is the name that was given to this feature.
You can learn more about alpha channels, selections, and layer masks in several of the courses included in the “Photoshop for Photographers” bundle available through GreyLearning (https://www.greylearning.com).