Alpha blending is accomplished by combining the color components of the source image with the color components of the destination image using the linear interpolation formula, where “source” is one color component of one pixel of the new paint and “destination” is one color component of the background image.
Core Graphics supports premultiplied alpha only for images. You should not premultiply any other color values specified in Core Graphics.
This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software.