Module: Polylabel

Defined in:
lib/polylabel.rb,
lib/polylabel/cell.rb,
lib/polylabel/version.rb

Defined Under Namespace

Classes: Cell

Constant Summary collapse

VERSION = 
"0.1.0"

Class Method Summary collapse

Class Method Details

.compute(polygon, precision = 1.0) ⇒ Object 



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# File 'lib/polylabel.rb', line 8

def self.compute(polygon, precision = 1.0)
  # find the bounding box of the outer ring
  first_item = polygon[0][0]

  min_x = first_item[0]
  min_y = first_item[1]
  max_x = first_item[0]
  max_y = first_item[1]

  polygon[0].each do |p|
    min_x = p[0] if p[0] < min_x
    min_y = p[1] if p[1] < min_y
    max_x = p[0] if p[0] > max_x
    max_y = p[1] if p[1] > max_y
  end

  width = max_x - min_x
  height = max_y - min_y
  cell_size = [width, height].min
  h = cell_size / 2.0

  return { x: min_x, y: min_y, distance: 0 } if cell_size.zero?

  # a priority queue of cells in order of their "potential" (max distance to polygon)
  cell_queue = PQueue.new { |a, b| a.max > b.max }

  # cover polygon with initial cells
  x = min_x
  while x < max_x
    y = min_y
    while y < max_y
      cell_queue.push Cell.new(x + h, y + h, h, polygon)
      y += cell_size
    end
    x += cell_size
  end

  # take centroid as the first best guess
  best_cell = get_centroid_cell(polygon)

  #  second guess: bounding box centroid
  bbox_cell = Cell.new(min_x + width / 2, min_y + height / 2, 0, polygon)

  best_cell = bbox_cell if bbox_cell.d > best_cell.d

  num_probes = cell_queue.length

  until cell_queue.empty?
    #  pick the most promising cell from the queue
    cell = cell_queue.pop

    #  update the best cell if we found a better one
    best_cell = cell if cell.d > best_cell.d

    # do not drill down further if there's no chance of a better solution
    next if cell.max - best_cell.d <= precision

    # split the cell into four cells
    h = cell.h / 2
    cell_queue.push(Cell.new(cell.x - h, cell.y - h, h, polygon))
    cell_queue.push(Cell.new(cell.x + h, cell.y - h, h, polygon))
    cell_queue.push(Cell.new(cell.x - h, cell.y + h, h, polygon))
    cell_queue.push(Cell.new(cell.x + h, cell.y + h, h, polygon))
    num_probes += 4
  end

  {
    x: best_cell.x,
    y: best_cell.y,
    distance: best_cell.d
  }
end

.get_centroid_cell(polygon) ⇒ Object 



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# File 'lib/polylabel.rb', line 81

def self.get_centroid_cell(polygon)
  area = 0
  x = 0
  y = 0
  points = polygon[0]

  b = points[-1]
  points.each do |a|
    f = a[0] * b[1] - b[0] * a[1]
    x += (a[0] + b[0]) * f
    y += (a[1] + b[1]) * f
    area += f * 3
    b = a
  end

  if area.zero?
    Cell.new(points[0][0], points[0][1], 0, polygon)
  else
    Cell.new(x / area, y / area, 0, polygon)
  end
end