(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.
channel
One of the Channel constant
(Gmagick::CHANNEL_*).
The Gmagick object.
错误时抛出 GmagickException。