rasterize-v1.rb

#!/usr/bin/env ruby
#
# Convert .PNG to rasterized SVG for laster etching. Save PNG at 50.8
# DPI (2 lines per mm) or adjust 'dots_per_mm' below for different
# DPI. Horizontal resolution is also locked to this DPI in this
# version.

require 'chunky_png'
require 'nokogiri'

image = ChunkyPNG::Image.from_file(ARGV[0])

lines = []
image.height.times { |y|
  line_start = nil
  image.width.times { |x|
    value = image[x, y]
    if line_start && value > 128
      # continue
    elsif line_start
      line_end = [x, y]
      lines << [line_start, line_end]
      line_start = nil
    elsif value > 128
      # begin line
      line_start = [x, y]
    end
  }
  if line_start
    line_end = [image.width, y]
    lines << [line_start, line_end]
  end
}

dots_per_mm = 2.0
target_dpi = 90
mm_per_inch = 25.4
px_per_mm = target_dpi / mm_per_inch
scale = px_per_mm / dots_per_mm

builder = Nokogiri::XML::Builder.new do |xml|
  xml.svg(xmlns: 'http://www.w3.org/2000/svg', version: '1.1') do
    lines.each do |p1, p2|
      xml.line(x1: p1[0] * scale,
               y1: p1[1] * scale,
               x2: p2[0] * scale,
               y2: p2[1] * scale,
               stroke: 'red', 'stroke-width' => 4)
    end
  end
end
puts builder.to_xml