在网上发现很多人使用

    CGContextDrawImage(context,CGRectMake(0,0,self.width,self.height),[image CGImage]); //原图
    CGContextDrawImage(context,rect,[mask CGImage]); //水印图

添加水印，图像会颠倒，还需要改变坐标

    CGAffineTransform myTr = CGAffineTransformMake(1, 0, 0, -1, 0, self.height);
    CGContextConcatCTM(context, myTr);

其实用drawInRect很方便，图片、文字都可以加

原理可参看前面转的图片相关操作（截取，缩略，翻转。。。。）

做了Category，下载

// 画水印
- (UIImage *) imageWithWaterMask:(UIImage*)mask inRect:(CGRect)rect
{
#if __IPHONE_OS_VERSION_MAX_ALLOWED >= 40000
if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 4.0)
{
UIGraphicsBeginImageContextWithOptions([self size], NO, 0.0); // 0.0 for scale means "scale for device‘s main screen".
}
#else
if ([[[UIDevice currentDevice] systemVersion] floatValue] < 4.0)
{
UIGraphicsBeginImageContext([self size]);
}
#endif

//原图
[self drawInRect:CGRectMake(0, 0, self.size.width, self.size.height)];
//水印图
[mask drawInRect:rect];

UIImage *newPic = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

return newPic;
}

IOS给图片增加水印(图片、文字)