Sunday 20 October 2013

openCV and color quantization--01 : kmeans

   color quantization is a powerful weapon for image segmentation.

    At first, I separate the image by thresholding graph_00, but the results are far from satisfy.


void separate_by_otsu(cv::Mat const &src)
    cv::Mat result = src.clone();

    cv::threshold(result, result, 0, 255,      
                  cv::THRESH_BINARY + cv::THRESH_OTSU);

graph_01(process by otsu)

Adaptive Threshold

void separate_adaptive_threshold(cv::Mat const &src)
    cv::Mat result = src.clone();

    cv::adaptiveThreshold(result, result, 255,

    cv::adaptiveThreshold(result, result, 255,

graph_02(process by adaptive gaussian)

graph_03(process by adaptive mean)


   As you can see, the results(graph_01~graph_03) can't cut out the second card.Luckily, the colors of the cards and the background are different, a fairly good example for kmeans.

  To adopt the cv::kmeans, we need to transfer the image into a samples, each data set of the samples should consist a pixel groups(ex : bgr, rgb, hsv and so on).To generate the image after color segmentation, we have to map the centers generated by the cv::kmeans to the image.

#include <iostream>

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

int main()
    cv::Mat src = cv::imread(Folder + "perspective05.jpg");
        std::cerr<"can't read the image"<std::endl;
        return -1;

    //step 1 : map the src to the samples
    cv::Mat samples(, 3, CV_32F);
    auto samples_ptr = samples.ptr<float>(0);
    for( int row = 0; row != src.rows; ++row){
        auto src_begin = src.ptr<uchar>(row);
        auto src_end = src_begin + src.cols * src.channels();
        //auto samples_ptr = samples.ptr<float>(row * src.cols);
        while(src_begin != src_end){
            samples_ptr[0] = src_begin[0];
            samples_ptr[1] = src_begin[1];
            samples_ptr[2] = src_begin[2];
            samples_ptr += 3; src_begin +=3;

    //step 2 : apply kmeans to find labels and centers
    int clusterCount = 3;
    cv::Mat labels;
    int attempts = 5;
    cv::Mat centers;
    cv::kmeans(samples, clusterCount, labels,
               cv::TermCriteria(CV_TERMCRIT_ITER | CV_TERMCRIT_EPS, 
                                10, 0.01),
               attempts, cv::KMEANS_PP_CENTERS, centers);

    //step 3 : map the centers to the output
    cv::Mat new_image(src.size(), src.type());
    for( int row = 0; row != src.rows; ++row){
        auto new_image_begin = new_image.ptr<uchar>(row);
        auto new_image_end = new_image_begin + new_image.cols * 3;
        auto labels_ptr = labels.ptr<int>(row * src.cols);

        while(new_image_begin != new_image_end){
            int const cluster_idx = *labels_ptr;
            auto centers_ptr = centers.ptr<float>(cluster_idx);
            new_image_begin[0] = centers_ptr[0];
            new_image_begin[1] = centers_ptr[1];
            new_image_begin[2] = centers_ptr[2];
            new_image_begin += 3; ++labels_ptr;

    cv::Mat binary;    
    cv::Canny(new_image, binary, 30, 90);

    cv::imshow("original", src);
    cv::imshow("binary", binary);
    cv::imshow( "clustered image", new_image );


    return 0;

   In step 1, we rearrange src to sample as following

src :
(b00,g00,r00) (b01,g01,r01)  (b02,g02,r02).......
(b10,g10,r10) (b11,g11,r11)  (b12,g12,r12).......


sample :

   The results of the labels are associated with the samples generated by cv::kmeans.The order of reading the labels should be the same as you generated the samples.

graph_04(process by kmeans)

graph_05(apply canny on graph_04)

  Now we could cut the cards from graph_04 without much troubles.To simplify the procedures of kmeans segmentation, I encapsulate it by class.The codes can download from github.

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