How to fill contours that touch the image border?

user_476920 0
Viewed 8894

Say I have the following binary image created from the output of cv::watershed():

enter image description here

Now I want to find and fill the contours, so I can separate the corresponding objects from the background in the original image (that was segmented by the watershed function).

To segment the image and find the contours I use the code below:

cv::Mat bgr = cv::imread("test.png");

// Some function that provides the rough outline for the segmented regions.
cv::Mat markers = find_markers(bgr); 

cv::watershed(bgr, markers);

cv::Mat_<bool> boundaries(bgr.size());
for (int i = 0; i < bgr.rows; i++) {
    for (int j = 0; j < bgr.cols; j++) {
        boundaries.at<bool>(i, j) = (markers.at<int>(i, j) == -1);
    }
}

std::vector<std::vector<cv::Point> > contours;
std::vector<cv::Vec4i> hierarchy;

cv::findContours(
    boundaries, contours, hierarchy,
    CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE
);

So far so good. However, if I pass the contours acquired above to cv::drawContours() as below:

cv::Mat regions(bgr.size(), CV_32S);
cv::drawContours(
    regions, contours, -1, cv::Scalar::all(255),
    CV_FILLED, 8, hierarchy, INT_MAX
);

This is what I get:

enter image description here

The leftmost contour was left open by cv::findContours(), and as a result it is not filled by cv::drawContours().

Now I know this is a consequence of cv::findContours() clipping off the 1-pixel border around the image (as mentioned in the documentation), but what to do then? It seems an awful waste to discard a contour just because it happened to brush off the image's border. And anyway how can I even find which contour(s) fall in this category? cv::isContourConvex() is not a solution in this case; a region can be concave but "closed" and thus not have this problem.

Edit: About the suggestion to duplicate the pixels from the borders. The problem is that my marking function is also painting all pixels in the "background", i.e. those regions I'm sure aren't part of any object:

enter image description here

This results in a boundary being drawn around the output. If I somehow avoid cv::findContours() to clip off that boundary:

enter image description here

The boundary for the background gets merged with that leftmost object:

enter image description here

Which results in a nice white-filled box.

opencv c++
2 Answers

Solution number 1: use image extended by one pixel in each direction:

Mat extended(bgr.size()+Size(2,2), bgr.type());
Mat markers = extended(Rect(1, 1, bgr.cols, bgr.rows));

// all your calculation part

std::vector<std::vector<Point> > contours;
findContours(boundaries, contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

Mat regions(bgr.size(), CV_8U);
drawContours(regions, contours, -1, Scalar(255), CV_FILLED, 8, Mat(), INT_MAX, Point(-1,-1));

Note that contours were extracted from extended image, i.e. their x and y values are bigger by 1 from what they should be. This is why I use drawContours with (-1,-1) pixel offset.

Solution number 2: add white pixels from boundary of image to the neighbor row/column:

bitwise_or(boundaries.row(0), boundaries.row(1), boundaries.row(1));
bitwise_or(boundaries.col(0), boundaries.col(1), boundaries.col(1));
bitwise_or(boundaries.row(bgr.rows()-1), boundaries.row(bgr.rows()-2), boundaries.row(bgr.rows()-2));
bitwise_or(boundaries.col(bgr.cols()-1), boundaries.col(bgr.cols()-2), boundaries.col(bgr.cols()-2));

Both solution are half-dirty workarounds, but this is all I could think about.

user_28894310
Related