AlphaChannel

AlphaChannel[color] returns the opacity of color.

AlphaChannel[image] returns the alpha channel of image.

AlphaChannel[video] returns a video containing the alpha channel of the frames in video.

Examples

Get the opacity of a color:

AlphaChannel[RGBColor[1, 0, 0, 0.5]]
(* 0.5 *)

Extract alpha channel from an image:

AlphaChannel[img]

Please visit the official Wolfram Language Reference for more details.