Added example: extract foreground via GrabCut algorithm

examples/grabcut.rb

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+# cv::CamelCase - class or constant
+# cv::snake_case - function matched to snakeCase
+# http://www.ropencv.aduda.eu/doc/
+
+require 'ropencv'
+include OpenCV
+
+rgb_image = cv::imread('grabcut_fgd.jpg')
+if rgb_image.data.null?
+  raise "Error in reading image file"
+end
+
+# resize image for faster analysis
+NEW_SIZE_WIDTH = 250
+new_size = cv::Size.new(NEW_SIZE_WIDTH, rgb_image.rows * NEW_SIZE_WIDTH / rgb_image.cols)
+rgb_image_resized = cv::Mat.new(new_size.width, new_size.height, cv::CV_8UC3)
+cv::resize(rgb_image, rgb_image_resized, new_size)
+
+# show resized image
+cv::imshow("original image (resized)", rgb_image_resized)
+cv::wait_key(-1)
+
+# create mask of all zeros (grayscale)
+mask = cv::Mat::zeros(rgb_image_resized.rows, rgb_image_resized.cols, cv::CV_8U)
+
+# define vectors internally used by grabcut algorithm
+bgd_model_tmp = cv::Mat::zeros(1, 13*5, cv::CV_64FC1)
+fgd_model_tmp = cv::Mat::zeros(1, 13*5, cv::CV_64FC1)
+
+# define rectangle of initial approximation
+# everything outside is treated as probable background
+PERCENTAGE_CUT_X = 0.10
+PERCENTAGE_CUT_Y = 0.05
+left_offset = PERCENTAGE_CUT_X * new_size.width
+right_offset = (1 - PERCENTAGE_CUT_X) * new_size.width
+offset_width = right_offset - left_offset
+
+top_offset = PERCENTAGE_CUT_Y * new_size.height
+bottom_offset = (1 - PERCENTAGE_CUT_Y) * new_size.height
+offset_height = bottom_offset - top_offset
+
+rectangle = cv::Rect.new(left_offset, top_offset, offset_width, offset_height)
+# the same as
+# rectangle = cv::Rect.new
+# rectangle.set_x left_offset
+# rectangle.set_y top_offset
+# rectangle.set_width offset_width
+# rectangle.set_height offset_height
+
+
+# run grabcut algorithm
+GRAB_CUT_ITERATIONS = 10
+cv::grab_cut(rgb_image_resized, mask, rectangle, bgd_model_tmp, fgd_model_tmp, GRAB_CUT_ITERATIONS, cv::GC_INIT_WITH_RECT)
+
+
+# set mask matrix to binary format 0 for background and 255 for foreground
+cmp_elem = cv::Mat.ones(1, 1, cv::CV_8U)*cv::GC_PR_FGD # single element for comparison with Mat should be Mat too (not Scalar)
+cv::compare(mask, cmp_elem, mask, cv::CMP_EQ) # 0 - false, 255 - true
+
+# add initial rectangle to the mask (just for demo)
+left_up_corner = cv::Point.new(left_offset, top_offset)
+right_bottom_corner = cv::Point.new(right_offset, bottom_offset)
+cv::rectangle(mask, left_up_corner, right_bottom_corner, cv::Scalar.new(255))
+
+# show mask
+cv::imshow("rectangle mask",mask)
+cv::wait_key(-1)
+
+
+# apply mask to image
+rgb_masked_image_resized = cv::Mat::zeros(rgb_image_resized.rows, rgb_image_resized.cols, cv::CV_8UC3)
+rgb_image_resized.copy_to(rgb_masked_image_resized, mask) # copy only where mask is non-zero
+
+# show final image
+cv::imshow("image with foreground only", rgb_masked_image_resized)
+cv::wait_key(-1)