#!/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 anchor (see image here: 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")