A sample code of CvMat#find_contours to get multiple polygons in an image.

find_contours_sample.rb

require 'opencv'

# Prepare a sample image (I used an image in this article: http://opencv-srf.blogspot.se/2011/09/object-detection-tracking-using-contours.html)
# $ wget http://4.bp.blogspot.com/-r36OpIdjPWE/UDPMJ2kPFZI/AAAAAAAAAPs/-eO4W_XeDo0/s1600/FindingContours.png
cvMat = OpenCV::CvMat.load('FindingContours.png', 0)
img = cvMat.GRAY2BGR

polygons = []

# CvMat#find_contours returns CvChain when the mode is CV_CHAIN_CODE, or returns CvContour when the mode is one of the others.
# This method never returns CvSeq, but it is not a problem because CvContour extends CvSeq.
find_contours_result = cvMat.find_contours(:mode => OpenCV::CV_RETR_CCOMP, :method => OpenCV::CV_CHAIN_APPROX_SIMPLE)

puts find_contours_result.class #=> OpenCV::CvContour
while find_contours_result
  polygons << find_contours_result.approx_poly(:recursive => true)

  # Obtain the next contour
  # You can get multiple polys in this way.
  find_contours_result = find_contours_result.h_next
end

# Show each polygon
polygons.each_with_index do |polygon, i|
  points = polygon.map { |p| "(#{p.x}, #{p.y})" }.join(', ')
  puts "#{i}: #{points}"
  img.poly_line!([polygon.to_a], :color => OpenCV::CvColor::Red, :thickness => 4, :is_closed => true)
end

w = OpenCV::GUI::Window.new('Result')
w.show img
OpenCV::GUI::wait_key