autocrop.rb

#!/usr/bin/env ruby

require 'RMagick'
include Magick


infile, outfile = ARGV
unless infile && outfile
  puts "Usage: #{$0} <infile> <outfile>"
  exit 1
end

class ImageData
  BLOCK_WIDTH = 10
  BLOCK_HEIGHT = 10

  def self.from_file(filename)
    new Image.read(filename).first
  end

  def write_file(filename)
    @img.write(filename)
  end

  def initialize(img)
    @img = img
    clamp!
  end

  def clamp!
    crop!(0, 0, width, height)
  end

  def crop!(bx, by, bw, bh)
    puts "crop #{bx}x#{by}+#{bw}x#{bh} (#{@img.columns}x#{@img.rows})"
    x, y = bx * BLOCK_WIDTH, by * BLOCK_HEIGHT
    w, h = bw * BLOCK_WIDTH, bh * BLOCK_HEIGHT
    @img = @img.crop(x, y, w, h, true)
    puts "=> #{@img.columns}x#{@img.rows}"
    self
  end

  def entropy_of(bx, by)
    # Raw pixels:
    pixels = @img.get_pixels(bx * BLOCK_WIDTH, by * BLOCK_HEIGHT,
                             BLOCK_WIDTH, BLOCK_HEIGHT)
    # Intensities as [ 0.0, 0.1, 0.2 .. 1.0 ]
    intensities = pixels.map { |pixel| (pixel.intensity / 6553.5).to_i / 10.0 }
    amounts = Hash.new(0)
    intensities.each do |i|
      amounts[i] += 1
    end
    probabilities = {}
    amounts.each do |intensity,amounts|
      probabilities[intensity] = amounts / (BLOCK_WIDTH * BLOCK_HEIGHT).to_f
    end
    - intensities.inject(0) do |sum,intensity|
      sum + probabilities[intensity] * Math.log(probabilities[intensity])
    end
  end

  def width
    @img.columns / BLOCK_WIDTH
  end
  def height
    @img.rows / BLOCK_HEIGHT
  end

  def block_void?(x, y)
    entropy_of(x, y) < 1.0
  end
  AREA_VOID_THRESHOLD = 0.98
  def area_void?(x, y, w, h)
    voids = 0

    (y..(y+h-1)).each do |by|
      (x..(x+w-1)).each do |bx|
        voids += 1 if block_void?(bx, by)
      end
    end

    puts "#{x}x#{y}+#{w}x#{h} => #{voids / (w * h).to_f}"
    voids / (w * h).to_f > AREA_VOID_THRESHOLD
  end

  def print_out
    height.times { |y|
      width.times { |x|
        if block_void?(x, y)
          print " "
        else
          print "x"
        end
      }
      puts
    }
  end
end


d = ImageData.from_file(infile)

puts "Before:"
#d.print_out

begin
  done = true

  # From top:
  unless d.area_void?(0, 0, d.width, 1)
    d.crop!(0, 1, d.width, d.height - 1)
    done = false
  end
  # From left:
  unless d.area_void?(0, 0, 1, d.height)
    d.crop!(1, 0, d.width - 1, d.height)
    done = false
  end
  # From bottom:
  unless d.area_void?(0, d.height - 1, d.width, 1)
    d.crop!(0, 0, d.width, d.height - 1)
    done = false
  end
  # From right:
  unless d.area_void?(d.width - 1, 0, 1, d.height)
    d.crop!(0, 0, d.width - 1, d.height)
    done = false
  end

end while not done

d.write_file(outfile)

puts "After:"
d.print_out