First of all, the most fundamental difference between the channel layer and the image layer is that the attribute of each pixel in the layer is expressed by the numerical value of the three primary colors of red, green and blue, while the pixel color in the channel layer is composed of the brightness values of a group of primary colors. Let's talk about the popular point first: there is only one color with different brightness in the channel, which is a gray image. A channel can actually be understood as a mapping of a selection area.
Channels were originally used to store selections and other information in image files. The well-known transparent GIF diagram actually contains a channel to tell the application (browser) which parts need to be transparent and which parts need to be displayed.
For example, you have painstakingly sketched out some extremely irregular constituencies from the image. After saving, these "choices" will soon disappear. At this point, we can use the channel to store the "selection" in an independent channel layer; When you need any options, you can easily call them from the channel. This function has been fully applied in the photo coloring example of special effects. Of course, you can save the "selection" as a different layer, but this is far less convenient than the channel, because the image is 24 bits and the channel layer is 8 bits, so saving as a channel will greatly save space; Most importantly, a multi-layer image can only be saved in a special Photoshop format, and many standard image formats, such as TIF and TGA, can contain channel information, which greatly facilitates information sharing between different applications.
In addition, another main function of the channel is to calculate and synthesize with the image layer, thus generating many incredible special effects. This function is mainly used for the production of special effects characters. In addition, there are many functions of the channel, so I won't list them one by one here, and then explain them in detail in the example operation.
In short, the concept of channel is broad and profound, and it can't be said in one or two sentences. Need to be carefully understood in actual operation.