Question

I am trying to use opencv EM algorithm to do color extraction.I am using the following code based on example in opencv documentation:

cv::Mat capturedFrame ( height, width, CV_8UC3 );
int i, j;
int nsamples = 1000;
cv::Mat samples ( nsamples, 2, CV_32FC1 );
cv::Mat labels;
cv::Mat img = cv::Mat::zeros ( height, height, CV_8UC3 );
img = capturedFrame;
cv::Mat sample ( 1, 2, CV_32FC1 );
CvEM em_model;
CvEMParams params;
samples = samples.reshape ( 2, 0 );

    for ( i = 0; i < N; i++ )
    {           
        //from the training samples
        cv::Mat samples_part = samples.rowRange ( i*nsamples/N, (i+1)*nsamples/N);

        cv::Scalar mean (((i%N)+1)*img.rows/(N1+1),((i/N1)+1)*img.rows/(N1+1));
        cv::Scalar sigma (30,30);
        cv::randn(samples_part,mean,sigma);                     

    }       

    samples = samples.reshape ( 1, 0 );

    //initialize model parameters
    params.covs         = NULL;
    params.means        = NULL;
    params.weights      = NULL;
    params.probs        = NULL;
    params.nclusters    = N;
    params.cov_mat_type = CvEM::COV_MAT_SPHERICAL;
    params.start_step   = CvEM::START_AUTO_STEP;
    params.term_crit.max_iter = 300;
    params.term_crit.epsilon  = 0.1;
    params.term_crit.type   = CV_TERMCRIT_ITER|CV_TERMCRIT_EPS;     
    //cluster the data
    em_model.train ( samples, Mat(), params, &labels );     

    cv::Mat probs;
    probs = em_model.getProbs();

    cv::Mat weights;
    weights = em_model.getWeights();

cv::Mat modelIndex = cv::Mat::zeros ( img.rows, img.cols, CV_8UC3 );

for ( i = 0; i < img.rows; i ++ )
{
    for ( j = 0; j < img.cols; j ++ )
    {
        sample.at<float>(0) = (float)j;
    sample.at<float>(1) = (float)i;     

    int response = cvRound ( em_model.predict ( sample ) ); 
    modelIndex.data [ modelIndex.cols*i + j] = response;

    }
}

My question here is:

Firstly, I want to extract each model, here totally five, then store those corresponding pixel values in five different matrix. In this case, I could have five different colors seperately. Here I only obtained their indexes, is there any way to achieve their corresponding colors here? To make it easy, I can start from finding the dominant color based on these five GMMs.

Secondly, here my sample datapoints are “100”, and it takes about nearly 3 seconds for them. But I want to do all these things in no more than 30 milliseconds. I know OpenCV background extraction, which is using GMM, performs really fast, below 20ms, that means, there must be a way for me to do all these within 30 ms for all 600×800=480000 pixels. I found predict function is the most time consuming one.

Answer 1

First Question:

In order to do color extraction you first need to train the EM with your input pixels. After that you simply loop over all the input pixels again and use predict() to classify each of them. I’ve attached a small example that utilizes EM for foreground/background separation based on colors. It shows you how to extract the dominant color (mean) of each gaussian and how to access the original pixel color.

#include <opencv2/opencv.hpp>

int main(int argc, char** argv) {

    cv::Mat source = cv::imread("test.jpg");

    //ouput images
    cv::Mat meanImg(source.rows, source.cols, CV_32FC3);
    cv::Mat fgImg(source.rows, source.cols, CV_8UC3);
    cv::Mat bgImg(source.rows, source.cols, CV_8UC3);

    //convert the input image to float
    cv::Mat floatSource;
    source.convertTo(floatSource, CV_32F);

    //now convert the float image to column vector
    cv::Mat samples(source.rows * source.cols, 3, CV_32FC1);
    int idx = 0;
    for (int y = 0; y < source.rows; y++) {
        cv::Vec3f* row = floatSource.ptr<cv::Vec3f > (y);
        for (int x = 0; x < source.cols; x++) {
            samples.at<cv::Vec3f > (idx++, 0) = row[x];
        }
    }

    //we need just 2 clusters
    cv::EMParams params(2);
    cv::ExpectationMaximization em(samples, cv::Mat(), params);

    //the two dominating colors
    cv::Mat means = em.getMeans();
    //the weights of the two dominant colors
    cv::Mat weights = em.getWeights();

    //we define the foreground as the dominant color with the largest weight
    const int fgId = weights.at<float>(0) > weights.at<float>(1) ? 0 : 1;

    //now classify each of the source pixels
    idx = 0;
    for (int y = 0; y < source.rows; y++) {
        for (int x = 0; x < source.cols; x++) {

            //classify
            const int result = cvRound(em.predict(samples.row(idx++), NULL));
            //get the according mean (dominant color)
            const double* ps = means.ptr<double>(result, 0);

            //set the according mean value to the mean image
            float* pd = meanImg.ptr<float>(y, x);
            //float images need to be in [0..1] range
            pd[0] = ps[0] / 255.0;
            pd[1] = ps[1] / 255.0;
            pd[2] = ps[2] / 255.0;

            //set either foreground or background
            if (result == fgId) {
                fgImg.at<cv::Point3_<uchar> >(y, x, 0) = source.at<cv::Point3_<uchar> >(y, x, 0);
            } else {
                bgImg.at<cv::Point3_<uchar> >(y, x, 0) = source.at<cv::Point3_<uchar> >(y, x, 0);
            }
        }
    }

    cv::imshow("Means", meanImg);
    cv::imshow("Foreground", fgImg);
    cv::imshow("Background", bgImg);
    cv::waitKey(0);

    return 0;
}

I’ve tested the code with the following image and it performs quite good.

Second Question:

I’ve noticed that the maximum number of clusters has a huge impact on the performance. So it’s better to set this to a very conservative value instead of leaving it empty or setting it to the number of samples like in your example. Furthermore the documentation mentions an iterative procedure to repeatedly optimize the model with less-constrained parameters. Maybe this gives you some speed-up. To read more please have a look at the docs inside the sample code that is provided for train() here.