Quartz2D打水印

    
    UIImage *bgImage = [UIImage imageNamed:@"bg"];
    
    //上下文 : 基於位圖(bitmap),全部的東西須要繪製到一張新的圖片上去
    
    //1.建立一個基於位圖的上下文(開啓一個基於位圖的上下文)
    //size : 新圖片的此處
    //opaque : 不透明度
    //建立一張新的bitmap,也就是新的UIImage對象
    UIGraphicsBeginImageContextWithOptions(bgImage.size, NO, 0.0);
    
    //2.畫背景
    [bgImage drawInRect:CGRectMake(0, 0, bgImage.size.width, bgImage.size.height)];
    
    //3.畫右下角水印
    UIImage *watermarkImage = [UIImage imageNamed:@"watermark"];
    CGFloat scale = 0.2;
    CGFloat margin = 5;
    CGFloat watermarkW = watermarkImage.size.width * scale;
    CGFloat watermarkH = watermarkImage.size.height * scale;
    CGFloat watermarkX = bgImage.size.width - watermarkW - margin;
    CGFloat watermarkY = bgImage.size.height - watermarkH - margin;
    
    [watermarkImage drawInRect:CGRectMake(watermarkX, watermarkY, watermarkW, watermarkH)];
    NSLog(@"%g",watermarkY);
    
    //4.從上下文中取得製做完畢的UIImange對象
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    
    //5.結束上下文
    UIGraphicsEndImageContext();
    
    //6.將image對象壓縮爲PNG格式的二進制對象.
    NSData *data = UIImagePNGRepresentation(newImage);
    
    //7寫入文件
    NSString *path = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingString:@"new.png"];
    [data writeToFile:path atomically:YES];
    NSLog(@"%@",path);