Carrierwave PNG to JPG converter. How to avoid black background?

Question

In paperclip, for example, it is possible to add this to set white background when .png is converted to .jpg:

:convert_options => { :all => '-background white -flatten +matte'}

Once carrierwave uses rmagick too, how to do that?

Obs.: My files are being stored in S3.

My code:

version :square do
    process :resize_to_fill => [200, 200]
    process :convert => 'jpg'
end

Answer 1

I've solved the problem, but I'm not sure if this is the best approach:

def resize_to_fill(width, height, gravity = 'Center', color = "white")
    manipulate! do |img|
      cols, rows = img[:dimensions]
      img.combine_options do |cmd|
        if width != cols || height != rows
          scale = [width/cols.to_f, height/rows.to_f].max
          cols = (scale * (cols + 0.5)).round
          rows = (scale * (rows + 0.5)).round
          cmd.resize "#{cols}x#{rows}"
        end
        cmd.gravity gravity
        cmd.background "rgba(255,255,255,0.0)"
        cmd.extent "#{width}x#{height}" if cols != width || rows != height
      end
      ilist = Magick::ImageList.new
      rows < cols ? dim = rows : dim = cols
      ilist.new_image(dim, dim) { self.background_color = "#{color}" }
      ilist.from_blob(img.to_blob)
      img = ilist.flatten_images
      img = yield(img) if block_given?
      img
    end
  end