Quartz2D指定显示范围

CGContextRef context=UIGraphicsGetCurrentContext();
     CGContextAddEllipseInRect(context, CGRectMake(100, 100, 100, 100));
     CGContextClip(context);
     UIImage *img=[UIImage imageNamed:@"img.jpg"];
    [img drawAtPoint:CGPointMake(100, 100)];

 CGContextRef context=UIGraphicsGetCurrentContext();
     UIImage *img=[UIImage imageNamed:@"img.jpg"];
     CGContextMoveToPoint(context, 30, 120);
     CGContextAddLineToPoint(context, 50, 200);
     CGContextAddLineToPoint(context, 200, 60);
     CGContextClosePath(context);
     CGContextClip(context);
    [img drawAtPoint:CGPointMake(30, 120)];