RMagick convert image with alpha

I'm trying to convert an image to 25% opacity via RMagick. The following works from the command line but I can't figure out how to do it in Ruby.

convert input.png -alpha on -channel A -evaluate set 25% +channel output.png

I've tried messing with Magick::AlphaChannelType::ActivateAlphaChannel but can't figure out how to chain the commands together. Any help would be greatly appreciated!

Solution

plz try this function to solve your problem

 def watermark(opacity = 0.25, size = 'input')
    manipulate! do |img|
      logo = Magick::Image.read("#{Rails.root}/app/assets/images/{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 = logo.composite(logo_opacity, Magick::NorthWestGravity, 0, 0, Magick::OverCompositeOp)
    end
  end