Quartz2D指定显示范围

在qq中，可以看到头像是圆形显示的，通过CGContextClip可以设置

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)];