Ignore connected component inside another one

Question

I'm trying to separate the background (green field and light green towel) from the objects using OpenCV so I segmented the following image manually:

By bordering the objects in red and coloring blue the connected components which should not be taken into consideration as you can see in the bottom right of the image:

After threasholding on 254 the channels R and B, I got the following:

1. Channel Red

2. Channel Blue

If I fulfill the all contours of red channel using

findContours( bordersRed, contoursRedChannel, hierarchyRedChannel, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, cv::Point(0, 0) );
for (int index = 0; index < contoursRedChannel.size(); index ++)
    {
        drawContours( bordersRed, contoursRedChannel, index, colorForMask, CV_FILLED, 8, hierarchyRedChannel, 0, cv::Point() );
    }

the bottom right corner will be like:

But what I need is to ignore the contours that contains only blue points in order to have something like:

so I have to combine the red with blue channels to get it but don't know how yet. Any advice would be appreciated.

Thanks.

Answer 1

You can do that using floodFill, assuming you know a point inside the shape you want to fill.

Result starting from your "Channel red":

Code:

#include <opencv2/opencv.hpp>
#include <iostream>
using namespace std;
using namespace cv;

int main()
{
    // Your image
    Mat1b img = imread("path_to_image", IMREAD_GRAYSCALE);

    // Assume you know a point inside the shape
    Point seed(930, 370); 

    // Apply floodfill
    floodFill(img, seed, Scalar(255));

    // Show image
    imshow("Result", img);
    waitKey();

    return 0;
}

---

UPDATE

Once you fill the contours in both masks with drawContours(... CV_FILLED), you can simply XOR the two mask:

Code:

#include <opencv2\opencv.hpp>
#include <vector>
#include <algorithm>

using namespace std;
using namespace cv;

int main()
{
    // Load the two mask
    Mat1b channel_red_mask = imread("channel_red.png", IMREAD_GRAYSCALE);
    Mat1b channel_blue_mask = imread("channel_blue.png", IMREAD_GRAYSCALE);

    // Use just the bottom right part
    Rect roi(Point(800, 270), Point(channel_red_mask.cols, channel_red_mask.rows));
    channel_red_mask = channel_red_mask(roi).clone();
    channel_blue_mask = channel_blue_mask(roi).clone();


    // Fill all contours, in both masks
    {
        vector<vector<Point>> contours;
        findContours(channel_red_mask.clone(), contours, RETR_LIST, CHAIN_APPROX_SIMPLE);
        for (int i = 0; i < contours.size(); ++i)
        {
            drawContours(channel_red_mask, contours, i, Scalar(255), CV_FILLED);
        }
    }
    {
        vector<vector<Point>> contours;
        findContours(channel_blue_mask.clone(), contours, RETR_LIST, CHAIN_APPROX_SIMPLE);
        for (int i = 0; i < contours.size(); ++i)
        {
            drawContours(channel_blue_mask, contours, i, Scalar(255), CV_FILLED);
        }
    }

    // XOR the masks
    Mat1b xored = channel_red_mask ^ channel_blue_mask;

    imshow("XOR", xored);
    waitKey();

    return 0;
}