Image Segmentation with Distance Transform and Watershed Algorithm

Goal

In this tutorial you will learn how to:

• Use the OpenCV function cv::filter2D in order to perform some laplacian filtering for image sharpening
• Use the OpenCV function cv::distanceTransform in order to obtain the derived representation of a binary image, where the value of each pixel is replaced by its distance to the nearest background pixel
• Use the OpenCV function cv::watershed in order to isolate objects in the image from the background

Theory

Code

This tutorial code's is shown lines below. You can also download it from here.

#include <opencv2/opencv.hpp>
#include <iostream>

using namespace std;
using namespace cv;

int main()
{
    // Load the image
    Mat src = imread("../data/cards.png");

    // Check if everything was fine
    if (!src.data)
        return -1;

    // Show source image
    imshow("Source Image", src);

    // Change the background from white to black, since that will help later to extract
    // better results during the use of Distance Transform
    for( int x = 0; x < src.rows; x++ ) {
      for( int y = 0; y < src.cols; y++ ) {
          if ( src.at<Vec3b>(x, y) == Vec3b(255,255,255) ) {
            src.at<Vec3b>(x, y)[0] = 0;
            src.at<Vec3b>(x, y)[1] = 0;
            src.at<Vec3b>(x, y)[2] = 0;
          }
        }
    }

    // Show output image
    imshow("Black Background Image", src);

    // Create a kernel that we will use for accuting/sharpening our image
    Mat kernel = (Mat_<float>(3,3) <<
            1,  1, 1,
            1, -8, 1,
            1,  1, 1); // an approximation of second derivative, a quite strong kernel

    // do the laplacian filtering as it is
    // well, we need to convert everything in something more deeper then CV_8U
    // because the kernel has some negative values,
    // and we can expect in general to have a Laplacian image with negative values
    // BUT a 8bits unsigned int (the one we are working with) can contain values from 0 to 255
    // so the possible negative number will be truncated
    Mat imgLaplacian;
    Mat sharp = src; // copy source image to another temporary one
    filter2D(sharp, imgLaplacian, CV_32F, kernel);
    src.convertTo(sharp, CV_32F);
    Mat imgResult = sharp - imgLaplacian;

    // convert back to 8bits gray scale
    imgResult.convertTo(imgResult, CV_8UC3);
    imgLaplacian.convertTo(imgLaplacian, CV_8UC3);

    // imshow( "Laplace Filtered Image", imgLaplacian );
    imshow( "New Sharped Image", imgResult );

    src = imgResult; // copy back

    // Create binary image from source image
    Mat bw;
    cvtColor(src, bw, CV_BGR2GRAY);
    threshold(bw, bw, 40, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);
    imshow("Binary Image", bw);

    // Perform the distance transform algorithm
    Mat dist;
    distanceTransform(bw, dist, CV_DIST_L2, 3);

    // Normalize the distance image for range = {0.0, 1.0}
    // so we can visualize and threshold it
    normalize(dist, dist, 0, 1., NORM_MINMAX);
    imshow("Distance Transform Image", dist);

    // Threshold to obtain the peaks
    // This will be the markers for the foreground objects
    threshold(dist, dist, .4, 1., CV_THRESH_BINARY);

    // Dilate a bit the dist image
    Mat kernel1 = Mat::ones(3, 3, CV_8UC1);
    dilate(dist, dist, kernel1);
    imshow("Peaks", dist);

    // Create the CV_8U version of the distance image
    // It is needed for findContours()
    Mat dist_8u;
    dist.convertTo(dist_8u, CV_8U);

    // Find total markers
    vector<vector<Point> > contours;
    findContours(dist_8u, contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

    // Create the marker image for the watershed algorithm
    Mat markers = Mat::zeros(dist.size(), CV_32SC1);

    // Draw the foreground markers
    for (size_t i = 0; i < contours.size(); i++)
        drawContours(markers, contours, static_cast<int>(i), Scalar::all(static_cast<int>(i)+1), -1);

    // Draw the background marker
    circle(markers, Point(5,5), 3, CV_RGB(255,255,255), -1);
    imshow("Markers", markers*10000);

    // Perform the watershed algorithm
    watershed(src, markers);

    Mat mark = Mat::zeros(markers.size(), CV_8UC1);
    markers.convertTo(mark, CV_8UC1);
    bitwise_not(mark, mark);
//    imshow("Markers_v2", mark); // uncomment this if you want to see how the mark
                                  // image looks like at that point

    // Generate random colors
    vector<Vec3b> colors;
    for (size_t i = 0; i < contours.size(); i++)
    {
        int b = theRNG().uniform(0, 255);
        int g = theRNG().uniform(0, 255);
        int r = theRNG().uniform(0, 255);

        colors.push_back(Vec3b((uchar)b, (uchar)g, (uchar)r));
    }

    // Create the result image
    Mat dst = Mat::zeros(markers.size(), CV_8UC3);

    // Fill labeled objects with random colors
    for (int i = 0; i < markers.rows; i++)
    {
        for (int j = 0; j < markers.cols; j++)
        {
            int index = markers.at<int>(i,j);
            if (index > 0 && index <= static_cast<int>(contours.size()))
                dst.at<Vec3b>(i,j) = colors[index-1];
            else
                dst.at<Vec3b>(i,j) = Vec3b(0,0,0);
        }
    }

    // Visualize the final image
    imshow("Final Result", dst);

    waitKey(0);
    return 0;
}

Explanation / Result

1. Load the source image and check if it is loaded without any problem, then show it:

       // Load the image
       Mat src = imread("../data/cards.png");
   
       // Check if everything was fine
       if (!src.data)
           return -1;
   
       // Show source image
       imshow("Source Image", src);

   
2. Then if we have an image with white background, it is good to tranform it black. This will help us to desciminate the foreground objects easier when we will apply the Distance Transform:

       // Change the background from white to black, since that will help later to extract
       // better results during the use of Distance Transform
       for( int x = 0; x < src.rows; x++ ) {
         for( int y = 0; y < src.cols; y++ ) {
             if ( src.at<Vec3b>(x, y) == Vec3b(255,255,255) ) {
               src.at<Vec3b>(x, y)[0] = 0;
               src.at<Vec3b>(x, y)[1] = 0;
               src.at<Vec3b>(x, y)[2] = 0;
             }
           }
       }
   
       // Show output image
       imshow("Black Background Image", src);

   
3. Afterwards we will sharp our image in order to acute the edges of the foreground objects. We will apply a laplacian filter with a quite strong filter (an approximation of second derivative):

       // Create a kernel that we will use for accuting/sharpening our image
       Mat kernel = (Mat_<float>(3,3) <<
               1,  1, 1,
               1, -8, 1,
               1,  1, 1); // an approximation of second derivative, a quite strong kernel
   
       // do the laplacian filtering as it is
       // well, we need to convert everything in something more deeper then CV_8U
       // because the kernel has some negative values,
       // and we can expect in general to have a Laplacian image with negative values
       // BUT a 8bits unsigned int (the one we are working with) can contain values from 0 to 255
       // so the possible negative number will be truncated
       Mat imgLaplacian;
       Mat sharp = src; // copy source image to another temporary one
       filter2D(sharp, imgLaplacian, CV_32F, kernel);
       src.convertTo(sharp, CV_32F);
       Mat imgResult = sharp - imgLaplacian;
   
       // convert back to 8bits gray scale
       imgResult.convertTo(imgResult, CV_8UC3);
       imgLaplacian.convertTo(imgLaplacian, CV_8UC3);
   
       // imshow( "Laplace Filtered Image", imgLaplacian );
       imshow( "New Sharped Image", imgResult );

   
   
4. Now we tranfrom our new sharped source image to a grayscale and a binary one, respectively:

       // Create binary image from source image
       Mat bw;
       cvtColor(src, bw, CV_BGR2GRAY);
       threshold(bw, bw, 40, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);
       imshow("Binary Image", bw);

   
5. We are ready now to apply the Distance Tranform on the binary image. Moreover, we normalize the output image in order to be able visualize and threshold the result:

       // Perform the distance transform algorithm
       Mat dist;
       distanceTransform(bw, dist, CV_DIST_L2, 3);
   
       // Normalize the distance image for range = {0.0, 1.0}
       // so we can visualize and threshold it
       normalize(dist, dist, 0, 1., NORM_MINMAX);
       imshow("Distance Transform Image", dist);

   
6. We threshold the dist image and then perform some morphology operation (i.e. dilation) in order to extract the peaks from the above image:

       // Threshold to obtain the peaks
       // This will be the markers for the foreground objects
       threshold(dist, dist, .4, 1., CV_THRESH_BINARY);
   
       // Dilate a bit the dist image
       Mat kernel1 = Mat::ones(3, 3, CV_8UC1);
       dilate(dist, dist, kernel1);
       imshow("Peaks", dist);

   
7. From each blob then we create a seed/marker for the watershed algorithm with the help of the cv::findContours function:

       // Create the CV_8U version of the distance image
       // It is needed for findContours()
       Mat dist_8u;
       dist.convertTo(dist_8u, CV_8U);
   
       // Find total markers
       vector<vector<Point> > contours;
       findContours(dist_8u, contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
   
       // Create the marker image for the watershed algorithm
       Mat markers = Mat::zeros(dist.size(), CV_32SC1);
   
       // Draw the foreground markers
       for (size_t i = 0; i < contours.size(); i++)
           drawContours(markers, contours, static_cast<int>(i), Scalar::all(static_cast<int>(i)+1), -1);
   
       // Draw the background marker
       circle(markers, Point(5,5), 3, CV_RGB(255,255,255), -1);
       imshow("Markers", markers*10000);

   
8. Finally, we can apply the watershed algorithm, and visualize the result:

       // Perform the watershed algorithm
       watershed(src, markers);
   
       Mat mark = Mat::zeros(markers.size(), CV_8UC1);
       markers.convertTo(mark, CV_8UC1);
       bitwise_not(mark, mark);
   //    imshow("Markers_v2", mark); // uncomment this if you want to see how the mark
                                     // image looks like at that point
   
       // Generate random colors
       vector<Vec3b> colors;
       for (size_t i = 0; i < contours.size(); i++)
       {
           int b = theRNG().uniform(0, 255);
           int g = theRNG().uniform(0, 255);
           int r = theRNG().uniform(0, 255);
   
           colors.push_back(Vec3b((uchar)b, (uchar)g, (uchar)r));
       }
   
       // Create the result image
       Mat dst = Mat::zeros(markers.size(), CV_8UC3);
   
       // Fill labeled objects with random colors
       for (int i = 0; i < markers.rows; i++)
       {
           for (int j = 0; j < markers.cols; j++)
           {
               int index = markers.at<int>(i,j);
               if (index > 0 && index <= static_cast<int>(contours.size()))
                   dst.at<Vec3b>(i,j) = colors[index-1];
               else
                   dst.at<Vec3b>(i,j) = Vec3b(0,0,0);
           }
       }
   
       // Visualize the final image
       imshow("Final Result", dst);