IOS UIGraphicsBeginImageContext 截图不清晰

我们经常使用 UIGraphicsBeginImageContext  截图，发现放在手机上不清晰

  UIGraphicsBeginImageContext(v.frame.size);

    [v.layerrenderInContext:UIGraphicsGetCurrentContext()];

    UIImage*  viewImage=UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

需要将 UIGraphicsBeginImageContext 换成 UIGraphicsBeginImageContextWithOptions