(PECL gmagick >= Unknown)
Gmagick::separateimagechannel — Separates a channel from the image
Separates a channel from the image and returns a grayscale image. A channel is a particular color component of each pixel in the image.
channelIdentify which channel to extract: RedChannel, GreenChannel, BlueChannel, OpacityChannel, CyanChannel, MagentaChannel, YellowChannel, BlackChannel.
The Gmagick object on success
错误时抛出 GmagickException。