Chequerboard Corner Detection

grid.rb

require 'rubygems'
require 'hornetseye_ffmpeg'
require 'hornetseye_xorg'
include Hornetseye
class Node
  def nms(threshold)
    self >= dilate.major(threshold)
  end
  def have(n, corners)
    hist = mask(corners).histogram max + 1
    msk = hist.eq n
    if msk.inject :or
      id = argmax { |i| msk.to_ubyte[i] }.first
      eq id
    else
      nil
    end
  end
  def corners(sigma = 5.0, size = 21)
    size2 = size / 2
    f1 = finalise(size, size) do |i,j|
      x, y = i - size2, j - size2
      x * y * Math.exp( -(x ** 2 + y ** 2) / sigma ** 2)
    end
    f2 = finalise(size, size) do |i,j|
      x, y = i - size2, j - size2
      0.5 * (x ** 2 - y ** 2) * Math.exp( -(x ** 2 + y ** 2) / sigma ** 2)
    end
    Math.hypot convolve(f1), convolve(f2)
  end
end
CORNERS = 0.6
THRESHOLD = 90
W, H = 8, 5
W2, H2 = 0.5 * (W - 1), 0.5 * (H - 1)
N = W * H
GRID = 7
input = AVInput.new 'calibration.avi'
width, height = input.width, input.height
X11Display.show do
  img = input.read_ubytergb
  grey = img.to_ubyte
  corners = grey.corners
  nms = corners.nms CORNERS * corners.max
  binary = grey > THRESHOLD
  edges = binary.dilate(GRID).and binary.erode(GRID).not
  components = edges.components
  grid = components.have N, nms
  if grid
    grid.and(nms).dilate.conditional RGB(255, 255, 0), img
  else
    nms.dilate.conditional RGB(255, 0, 0), img
  end
end