Rmagick在水印中设置透明度的不透明度

Question

我正在尝试创建具有不同不透明度值的水印(从0不透明值到1完全透明).

我在ruby中有以下RMagick方法:

  # 0 = opaque (Magick::OpaqueOpacity) 1= transparent (Magick::TransparentOpacity)
  def watermark(opacity = 0.99, size = 'm')
    manipulate! do |img|
      logo = Magick::Image.read("#{Rails.root}/app/assets/images/watermark#{size}.png").first
      logo.alpha(Magick::ActivateAlphaChannel) 
      logo.opacity = (1 - opacity.to_f) * Magick::QuantumRange
      img.alpha(Magick::ActivateAlphaChannel)
      img = img.composite(logo, Magick::NorthWestGravity, 0, 0, Magick::OverCompositeOp)
    end
  end

我的问题是它似乎工作,但复合模式或alpha复合或设置不透明度或alpha失败,因为我在图像中得到黑色透明度.例如,如果我的水印是带有文字的完全透明的图像,我将其放在汽车图像上,那么我会得到一个带有水印的更暗或夜间图像,因此水印的背景不能正确混合.

有没有正确设置水印图像中不透明度的建议？也许有一些解决水印的方法？

编辑:添加图像示例:

http://uppix.com/f-watermarkg53925b100016ab8e.png(水印) http://oi62.tinypic.com/2us8rxl.jpg(基本图像) http://oi60.tinypic.com/2pt6mg3.jpg(组合物)

Answer 1

感谢Neil Slater,我终于找到了正确的解决方案.我需要在最终结果中结合使用 DstIn + Over 的复合操作:

  def watermark(opacity = 0.99, size = 'm')
    manipulate! do |img|
      logo = Magick::Image.read("#{Rails.root}/app/assets/images/watermark#{size}.png").first
      logo.alpha(Magick::ActivateAlphaChannel) 

      white_canvas = Magick::Image.new(logo.columns, logo.rows) { self.background_color = "none" }
      white_canvas.alpha(Magick::ActivateAlphaChannel)
      white_canvas.opacity = Magick::QuantumRange - (Magick::QuantumRange * opacity)

      # Important: DstIn composite operation (white canvas + watermark)
      logo_opacity = logo.composite(white_canvas, Magick::NorthWestGravity, 0, 0, Magick::DstInCompositeOp)
      logo_opacity.alpha(Magick::ActivateAlphaChannel)

      # Important: Over composite operation (original image + white canvas watermarked)
      img = img.composite(logo_opacity, Magick::NorthWestGravity, 0, 0, Magick::OverCompositeOp)
    end
  end