Add ArcLength, ContourArea and Minimal Area Rectangle example

examples/contours/bounding-box-detect-canny.rb

@@ -0,0 +1,64 @@
+#!/usr/bin/env ruby
+#
+# Detects contours in an image and
+# their boundingboxes
+#
+require "opencv"
+
+# Load image
+cvmat = OpenCV::CvMat.load("rotated-boxes.jpg")
+
+# The "canny" edge-detector does only work with grayscale images
+# so to be sure, convert the image
+# otherwise you will get something like:
+# terminate called after throwing an instance of 'cv::Exception'
+#  what():  /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_ports_graphics_opencv/work/OpenCV-2.2.0/modules/imgproc/src/canny.cpp:67: error: (-210)  in function cvCanny
+cvmat = cvmat.BGR2GRAY
+
+# Use the "canny" edge detection algorithm (http://en.wikipedia.org/wiki/Canny_edge_detector)
+# Parameters are two colors that are then used to determine possible corners
+canny = cvmat.canny(50, 150)
+
+# Look for contours
+# We want them to be returned as a flat list (CV_RETR_LIST) and simplified (CV_CHAIN_APPROX_SIMPLE)
+# Both are the defaults but included here for clarity
+contour = canny.find_contours(:mode => OpenCV::CV_RETR_LIST, :method => OpenCV::CV_CHAIN_APPROX_SIMPLE)
+
+# The Canny Algorithm returns two matches for every contour (see O'Reilly: Learning OpenCV Page 235)
+# We need only the external edges so we ignore holes.
+# When there are no more contours, contours.h_next will return nil
+while contour
+  # No "holes" please (aka. internal contours)
+  unless contour.hole?
+
+    puts '-' * 80
+    puts "BOUNDING RECT FOUND"
+    puts '-' * 80
+
+    # You can detect the "bounding rectangle" which is always oriented horizontally and vertically
+    box = contour.bounding_rect
+    puts "found external contour with bounding rectangle from #{box.top_left.x},#{box.top_left.y} to #{box.bottom_right.x},#{box.bottom_right.y}"
+
+    # The contour area can be computed:
+    puts "that contour encloses an area of #{contour.contour_area} square pixels"
+
+    # .. as can be the length of the contour
+    puts "that contour is #{contour.arc_length} pixels long "
+
+    # Draw that bounding rectangle
+    cvmat.rectangle! box.top_left, box.bottom_right, :color => OpenCV::CvColor::Black
+
+    # You can also detect the "minimal rectangle" which has an angle, width, height and center coordinates
+    # and is not neccessarily horizonally or vertically aligned.
+    # The corner of the rectangle with the lowest y and x position is the achor (see imagehere: http://bit.ly/lT1XvB)
+    # The zero angle position is always straight up. 
+    # Positive angle values are clockwise and negative values counter clockwise (so -60 means 60 degree counter clockwise)
+    box = contour.min_area_rect
+    puts "found minimal rectangle with its center at (#{box.center.x.round},#{box.center.y.round}), width of #{box.size.width.round}px, height of #{box.size.height.round} and an angle of #{box.angle.round} degree"
+  end
+  contour = contour.h_next
+end 
+
+# And save the image
+puts "\nSaving image with bounding rectangles"
+cvmat.save_image("rotated-boxes-with-detected-bounding-rectangles.jpg")