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}