Programming iOS 6, 3rd Edition

CGImage Drawing

The Core Graphics version of UIImage is CGImage (actually a CGImageRef). They are easily converted to one another: a UIImage has a CGImage property that accesses its Quartz image data, and you can make a UIImage from a CGImage using imageWithCGImage: or initWithCGImage: (in real life, you are likely to use their more configurable siblings, imageWithCGImage:scale:orientation: and initWithCGImage:scale:​orientation:).

A CGImage lets you create a new image directly from a rectangular region of the original image, which you can’t do with UIImage. (A CGImage has other powers a UIImage doesn’t have; for example, you can apply an image mask to a CGImage.) I’ll demonstrate by splitting the image of Mars in half and drawing the two halves separately (Figure 15-9). Observe that we are now operating in the CFTypeRef world and must take care to manage memory manually; ARC won’t help us here (Chapter 12):

UIImage* mars = [UIImage imageNamed:@"Mars.png"]; // extract each half as a CGImage CGSize sz = [mars size]; CGImageRef marsLeft = CGImageCreateWithImageInRect([mars CGImage], CGRectMake(0,0,sz.width/2.0,sz.height)); CGImageRef marsRight = CGImageCreateWithImageInRect([mars CGImage], CGRectMake(sz.width/2.0,0,sz.width/2.0,sz.height)); // draw each CGImage into an image context UIGraphicsBeginImageContextWithOptions( CGSizeMake(sz.width*1.5, sz.height), NO, 0); CGContextRef con = UIGraphicsGetCurrentContext(); CGContextDrawImage(con, CGRectMake(0,0,sz.width/2.0,sz.height), marsLeft); ...