How do you capture only a sub-set of the entire screen using Objective-C on an iOS device?

I found the following code:

UIWindow *screenWindow = [[UIApplication sharedApplication] keyWindow];
UIGraphicsBeginImageContext(screenWindow.frame.size);
[screenWindow.layer renderInContext:UIGraphicsGetCurrentContext()]
UIImage *screenshot = UIGraphicsGetImageFromCurrentImageContext();

Which works great but I only want to create an image from a piece of the entire window. How would I go about doing that?

• iphone
• objective-c
• ios
• screenshot

replace

UIGraphicsBeginImageContext(screenWindow.frame.size);

with

UIGraphicsBeginImageContext(CGSizeMake(width, height));