Enumeration

CGImageAlphaInfo

Storage options for alpha component data.

Declaration

typedef enum CGImageAlphaInfo : uint32_t {
    ...
} CGImageAlphaInfo;

Overview

A CGImageAlphaInfo constant specifies (1) whether a bitmap contains an alpha channel, (2) where the alpha bits are located in the image data, and (3) whether the alpha value is premultiplied. You can obtain a CGImageAlphaInfo constant for an image by calling the CGImageGetAlphaInfo function. (You provide a CGBitmapInfo constant to the function CGImageCreate, part of which is a CGImageAlphaInfo constant.)

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.

Topics

Constants

kCGImageAlphaFirst

The alpha component is stored in the most significant bits of each pixel. For example, non-premultiplied ARGB.

kCGImageAlphaLast

The alpha component is stored in the least significant bits of each pixel. For example, non-premultiplied RGBA.

kCGImageAlphaNone

There is no alpha channel.

kCGImageAlphaNoneSkipFirst

There is no alpha channel. If the total size of the pixel is greater than the space required for the number of color components in the color space, the most significant bits are ignored.

kCGImageAlphaOnly

There is no color data, only an alpha channel.

kCGImageAlphaNoneSkipLast

There is no alpha channel.

kCGImageAlphaPremultipliedFirst

The alpha component is stored in the most significant bits of each pixel and the color components have already been multiplied by this alpha value. For example, premultiplied ARGB.

kCGImageAlphaPremultipliedLast

The alpha component is stored in the least significant bits of each pixel and the color components have already been multiplied by this alpha value. For example, premultiplied RGBA.