Add ArcLength, ContourArea and Minimal Area Rectangle example

.gitignore

@@ -16,4 +16,4 @@ ext/opencv/test.txt
 pkg/
 log.txt
 *.avi
-examples/rotated-boxes-with-detected-bounding-rectangles.jpg
+examples/contours/rotated-boxes-with-detected-bounding-rectangles.jpg

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

@@ -30,15 +30,35 @@ contour = canny.find_contours(:mode => OpenCV::CV_RETR_LIST, :method => OpenCV::
 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 from #{box.top_left.x},#{box.top_left.y} to #{box.bottom_right.x},#{box.bottom_right.y}"
+    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 "Saving image with bounding rectangles"
+puts "\nSaving image with bounding rectangles"
 cvmat.save_image("rotated-boxes-with-detected-bounding-rectangles.jpg")