Find the corners of a polygon represented by a region mask

Question

BW = poly2mask(x, y, m, n) computes a binary region of interest (ROI) mask, BW, from an ROI polygon, represented by the vectors x and y. Th

Answer 1

Here's an example using Ruby and HornetsEye. Basically the program creates a histogram of the quantised Sobel gradient orientation to find dominant orientations. If four dominant orientations are found, lines are fitted and the intersections between neighbouring lines are assumed to be the corners of the projected rectangle.

#!/usr/bin/env ruby
require 'hornetseye'
include Hornetseye
Q = 36
img = MultiArray.load_ubyte 'http://imgur.com/oxyjj.png'
dx, dy = 8, 6
box = [ dx ... 688, dy ... 473 ]
crop = img[ *box ]
crop.show
s0, s1 = crop.sobel( 0 ), crop.sobel( 1 )
mag = Math.sqrt s0 ** 2 + s1 ** 2
mag.normalise.show
arg = Math.atan2 s1, s0
msk = mag >= 500
arg_q = ( ( arg.mask( msk ) / Math::PI + 1 ) * Q / 2 ).to_int % Q
hist = arg_q.hist_weighted Q, mag.mask( msk )
segments = ( hist >= hist.max / 4 ).components
lines = arg_q.map segments
lines.unmask( msk ).normalise.show
if segments.max == 4
  pos = MultiArray.scomplex *crop.shape
  pos.real = MultiArray.int( *crop.shape ).indgen! % crop.shape[0]
  pos.imag = MultiArray.int( *crop.shape ).indgen! / crop.shape[0]
  weights = lines.hist( 5 ).major 1.0
  centre = lines.hist_weighted( 5, pos.mask( msk ) ) / weights
  vector = pos.mask( msk ) - lines.map( centre )
  orientation = lines.hist_weighted( 5, vector ** 2 ) ** 0.5
  corner = Sequence[ *( 0 ... 4 ).collect do |i|
    i1, i2 = i + 1, ( i + 1 ) % 4 + 1
    l1, a1, l2, a2 = centre[i1], orientation[i1], centre[i2], orientation[i2]
    ( l1 * a1.conj * a2 - l2 * a1 * a2.conj -
      l1.conj * a1 * a2 + l2.conj * a1 * a2 ) /
      ( a1.conj * a2 - a1 * a2.conj )
  end ] 
  result = MultiArray.ubytergb( *img.shape ).fill! 128
  result[ *box ] = crop
  corner.to_a.each do |c|
    result[ c.real.to_i + dx - 1 .. c.real.to_i + dx + 1,
            c.imag.to_i + dy - 1 .. c.imag.to_i + dy + 1 ] = RGB 255, 0, 0
  end
  result.show
end