alpha function

The alpha function extracts the alpha (opacity) component of a color and returns it. Colors contain four components: red, green, blue, and alpha. To construct a color from the individual components, use the rgba function.

Examples:

alpha(rgb(1, 0.2, 0))          ; 1.0
alpha(rgba(0.5, 1, 0.3, 0.8))  ; 0.8
alpha(hsla(4, 1, 0.5, 0.2))    ; 0.2

See Also
red function
hue function
Writing direct coloring algorithms