Quartz2d水印图片做法

• (instancetype)waterImageWithBg:(NSString )bg logo:(NSString )logo
  {
  UIImage *bgImage = [UIImage imageNamed:bg];

  // 1.创建一个基于位图的上下文(开启一个基于位图的上下文)
  UIGraphicsBeginImageContextWithOptions(bgImage.size, NO, 0.0);

  // 2.画背景
  [bgImage drawInRect:CGRectMake(0, 0, bgImage.size.width, bgImage.size.height)];

  // 3.画右下角的水印
  UIImage *waterImage = [UIImage imageNamed:logo];
  CGFloat scale = 0.2;
  CGFloat margin = 5;
  CGFloat waterW = waterImage.size.width * scale;
  CGFloat waterH = waterImage.size.height * scale;
  CGFloat waterX = bgImage.size.width - waterW - margin;
  CGFloat waterY = bgImage.size.height - waterH - margin;
  [waterImage drawInRect:CGRectMake(waterX, waterY, waterW, waterH)];

  // 4.从上下文中取得制作完毕的UIImage对象
  UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

  // 5.结束上下文
  UIGraphicsEndImageContext();

  return newImage;
  }