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Help:I am puzzled by cvCalcHist function,can you help me?Thank you

asked 2013-04-22 08:39:55 -0500

btdan gravatar image

Hello, I am using cvCalcHist function. The defination is : void cvCalcHist( IplImage* image, CvHistogram hist,int accumulate=0, const CvArr* mask=NULL ); However, I don't know how to use the parameter mask, Can somebody give me an sample source code? Thanks very much. Best wishes to you.

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Do you want to use mask? mask is used to find histogram of a specific region of image. If you want to find histogram of full image, just ignore it.

Abid Rahman K gravatar imageAbid Rahman K ( 2013-04-23 01:39:03 -0500 )edit

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answered 2018-09-06 14:16:24 -0500

MikeTronix gravatar image

updated 2018-09-06 14:16:51 -0500

Here is some sample C++ code extracted from a current throwaway project using openCV 3.4.1. Hope this helps.

 // input floating point image fdata of size nrows x ncols
    cv::Mat fdata = cv::Mat::zeros(nrows,ncols,CV_32FC1);
    // load data into fdata here
    // set up to find maximum and minimum values
    double minv, maxv;
    cv::minMaxIdx(fdata,&minv,&maxv);
    // mask is from thresholding all data above 31 counts
    cv::Mat mask8 = cv::Mat::zeros(nrows,ncols,CV_8UC1);
    // mask8 values are 0 or 255
    cv::threshold(fdata,mask8,31,255,cv::THRESH_BINARY);
    // size of histogram to create
    int hist_sz = 10;
    // create "tree" of pointers for binning histogram, contains max and min values
    float rngs[2];
    rngs[0] = static_cast<float>(minv);
    rngs[1] = static_cast<float>(maxv);
    const float* prng = { &rngs[0] };
    const float** ranges = { &prng };
    // print out values to see if tree is correct so that histogram function will not assert an error
    std::cout << "ranges[0]=" << ranges[0] << " ranges[0][0]=" << ranges[0][0] <<
             " ranges[0][1]=" << ranges[0][1] << std::endl;
    // compute histogram on first channel
    int chan = 0;
    cv::MatND hist;
    cv::calcHist(&fdata,1,&chan,mask8,hist,1,&hist_sz,ranges,true,false);
    // histogram now only contains data counted from pixels with mask8 > 0
    // note that histogram is of type float and is a cv::Mat vector with 10 rows and 1 column
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Asked: 2013-04-22 08:39:55 -0500

Seen: 172 times

Last updated: Sep 06 '18