crop image from certain portion of screen in iphone programmatically

Question

Hello,

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
CGSize contextSize=CGSizeMake(320,400);
UIGraphicsBeginImageContext(self.view.bounds.size);

UIGraphicsBeginImageContext(contextSize);
[self.view.layer renderInContext:UIGraphicsGetCurrentContext()]; 
UIImage *savedImg = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[self setSaveImage:savedImg];

to extarct some part of image from main screen.

In UIGraphicsBeginImageContext I can only use size, is there any way to use CGRect or some other way to extract image from a specific portion of screen ie (x,y, 320, 400) some thing like this