C++ Boost


Making a sub-histogram

Overview

Sub-histogram is a subset of a histogram or one that is formed by masking out a few axis of the parent histogram.

GIL class histogram provides these functions as members and returns an instance of the desired sub-class.

  1. Histogram over fewer axes

    Task - Get a 2D histogram from a 3D RGB histogram over red and green axes

    histogram<int, int, int> h;
    gil::rgb8_image_t img;
    /*
    Fill img ...
    */
    fill_histogram(view(img), h, false);
    auto sub_h = h.sub_histogram<0, 1>(); // sub_h is a 2D histogram
    

    Demo output:

    h is {{1, 2, 3} : 1,
          {1, 4, 3} : 2,
          {1, 2, 5} : 3}
    sub_h would be {{1, 2} : 4, {1, 4} : 2}
    
  2. Histogram using a particular key range

    Task - Get a 2D histogram from a 3D RGB histogram for bins whose red color lie between 10 - 20 and blue color lie between 2 - 10

    histogram<int, int, int> h;
    gil::rgb8_image_t img;
    /*
    Fill img ...
    */
    fill_histogram(view(img), h, false);
    std::tuple<int, int, int> low, high;
    low = {10, 0, 2} // Since no check over blue channel pass any dummy value
    high = {20, 0, 10} // Since no check over blue channel pass any dummy value
    auto sub_h = h.sub_histogram<0, 2>(low, high); // In angle brackets pass the relevant dimensions in order
    

    Demo Output:

    h is {{11, 2, 3 } : 1,
          {1 , 4, 11} : 2,
          {1 , 2, 1 } : 3,
          {11, 3, 3 } : 4}
    sub_h would be {{11, 2, 3} : 1, {11, 3, 3} : 4}