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Alpha (alpha channel) - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

Colors are represented in digital form as a collection of numbers, each representing the strength or intensity level of a given component of the color. Each of these components is called a channel. In a typical image file, the color channels describe how much red, green, and blue are used to make up the final color. To represent a color through which the background can be seen to some extent, a fourth channel is added to the color: the alpha channel. The alpha channel specifies how opaque the color is.

For example, the color #8921F2 (also described as rgb(137, 33, 242) or hsl(270, 89%, 54)) is a nice shade of purple. Below you see a small box of that color in the top-left corner and a box of the same color but with an alpha channel set at 0.5 (50% opacity). The two boxes are drawn on top of a paragraph of text.

Image showing the effect of an alpha channel on a color.

As you can see, the color without an alpha channel completely blocks the background text, while the box with the alpha channel leaves it visible through the purple background color.