designed.rb

require 'mini_magick'

class Design
  TEMP_TEXT_PATH = "./test_text.png"

  def initialize (design)
    @design = design
  end

  def opened_template
    @opened_template ||= MiniMagick::Image.open(@design[:template])
  end

  def apply_text
    opened_template.combine_options do |c|
      c.font @design[:typeface]
      c.fill @design[:color]
      c.pointsize @design[:pointsize]

      c.draw "translate #{@design[:x] + half_text_width},#{@design[:y]} "\
             "rotate #{@design[:rotation]} "\
             "text #{-half_text_width},0 '#{@design[:content]}'"
    end

    opened_template
  end

  def text_output
    @text_output ||= begin
      `convert -pointsize #{@design[:pointsize]} -font '#{@design[:typeface]}' label:'#{@design[:content]}' #{TEMP_TEXT_PATH}`
      MiniMagick::Image.open(TEMP_TEXT_PATH)
    end
  end

  def half_text_width
    text_output[:width] * 0.5
  end

end

example.rb

# Produce a fake template for us to print upon
template = "./result.png"
`convert -size 400x400 xc:white #{template}`

# Generate a new design object
design = Design.new({
  template: template,
  color: '#1496bb',
  content: 'Viget Labs',
  pointsize: 50,
  rotation: 45,
  typeface: 'Helvetica',
  x: 80,
  y: 220
})

design.apply_text.write(template);