Color Channel

What is a Color Channel in a Digital Image?

A color channel in an image would be something like the red, green, and blue channels of a RGB image (plus the alpha channel of a RGBA image). More specifically, a color channel is essentially a grayscale image of same size that only captures one aspect of the image as a whole.

For example, an image in the RGB color space has the colors of its pixels encoded as a tuple of red, green, and blue intensities. A yellow pixel would be 100% red, 100% green, and 0% blue. Each and every pixel of an image has its own tuple of 3 separate values. When we separate only the red values of all pixels in an image, we call that the red channel of the image. Since these values range from 0% to 100% (or from 0 to 255 in 8 bits per pixel), we can display the values of a channel as a grayscale image, the 0% intensity being represented by black, and the 100% intensity being represented by white (50% would be gray in this case).

Some images have an alpha channel that encodes the opacity (transparency) of each pixel. Putting it in another way, since every pixel needs to have a separate value for its opacity, that means we have an entire extra channel: the alpha channel.

In some image editing applications, it's also possible to add a "mask" to a layer. This mask is a grayscale image that can hide pixels, just like alpha, but that is applied on top of the alpha channel. Since every pixel has a mask value, the mask can be considered a channel as well.

There are other color spaces besides RGB, such as L*a*b*, in which L is luminance, and a and b are color. So in this color space, instead of a red channel, we have a luminance channel. You may have heard of HSL, which as a lightness value. HSL is not a color space, it's just an alternative way to interpret RGB values. Basically some math is applied and RGB becomes HSL, then the reverse is applied to convert back. In LAB's case, the L data is part of the image, which means that operations on luminance can be done directly on that channel alone. For example, it's possible to copy the luminance channel and directly paste it in the alpha to overwrite the alpha values in some applications, and doing so would make black pixels transparent and white pixels opaque.

Observations

In some image editing applications, there's a panel to change which channels of the image are visible or locked. This panel is typically found close to the layers panel by default. In some cases, such as in GIMP, it's possible to directly edit the alpha channel by selecting it in such panel.