RMagick autocropper

autocrop.rb

require 'RMagick'

# Auto crop an image by detecting solid edges around them.
class AutoCropper

  # How small we accept a cropped picture to be. E.G. if it was 100x100 and
  # ratio 0.1, min output should be 10x10
  MIN_CROP_RATIO = 0.1

  attr_reader :img, :x0, :y0, :x1, :y1, :min_width, :min_height, :rows, :columns, :is_border

  def initialize(img, is_border_proc = nil, min_ratio = MIN_CROP_RATIO)
    @img, @min_ratio = img, min_ratio

    # Coordinates
    @x0, @y0 = 0, 0; @x1, @y1 = img.columns, img.rows
    @min_width, @min_height = img.columns * @min_ratio, img.rows * @min_ratio

    # We need a border finder proc. Provide one if none was given.
    @is_border = is_border_proc || self.class.default_border_finder(img)

    find_edges
  end

  def width
    x1 - x0
  end

  def height
    y1 - y0
  end

  def output
    img.crop x0, y0, width, height, true
  end

  # Returns a Proc that, given a set of pixels (an edge of the image) decides
  # whether that edge is a border or not.
  def self.default_border_finder(img, samples = 5, threshold = 0.95, fuzz = 0.05)
    fuzz = (2**16 * fuzz).to_i

    # Returns true if the edge is a border.
    lambda do |edge|
      border, non_border = 0.0, 0.0

      pixels = (0...samples).map { |n| edge[n * edge.length / samples] }
      pixels.combination(2).each { |a, b| a.fcmp(b, fuzz) ? border += 1 : non_border += 1 }

      border.to_f / (border + non_border) > threshold
    end
  end

private

  def find_edges
    return unless is_border

    u = x1 - 1
    x0.upto(u)     { |x| width_croppable?  && is_border[vline x] ? @x0 = x + 1 : break }
    (u).downto(x0) { |x| width_croppable?  && is_border[vline x] ? @x1 = x - 1 : break }

    u = y1 - 1
    0.upto(u)      { |y| height_croppable? && is_border[hline y] ? @y0 = y + 1 : break }
    (u).downto(y0) { |y| height_croppable? && is_border[hline y] ? @y1 = y - 1 : break }
  end

  def vline(x)
    img.get_pixels x, y0, 1, height
  end

  def hline(y)
    img.get_pixels x0, y, width, 1
  end

  def width_croppable?
    width > min_width
  end

  def height_croppable?
    height > min_height
  end
end