CIImage, UIImage, CGImageRef

這三者都是Image的物件導向表示法
UIImage是用來表示iOS中的image資料，他是內建於UIKit.framework
CIImage是使用CoreImage.framework來對影片做操作的主要datatype
CGImageRef is an opaque type that represents a Quartz 2D drawing environment.

CGImageRef可以視為UIKit與CoreImage之間的介接，如下所示
UIImage *image = [imageView image];
CGImageRef cgImage = image.CGImage;
CIImage *coreImage = [CIImage imageWithCGImage:cgImage];


//從透過CIContext將CIImage繪製成CGImageRef
CGImageRef cgImage = [self.imageContext createCGImage:coreImage fromRect:[coreImage extent]];
//再由CGImageRef轉換成UIImage
[self.imageView setImage:[UIImage imageWithCGImage:cgImage]];