Overview

We will demonstrate the available options for filling an instance of the histogram class with values that cater from the most simplest to the complex needs that might arise.

Basic

1. Use operator()

   Task - Add value to a particular cell / key / bin in histogram

   histogram<int, int> h;
   h(1, 2) = 1;
   

2. Use operator[]

   This requires to input the indices in a format the histogram internally stores its keys, which is of std::tuple due to its simple interface.

   Task - Output value of a bin

   histogram<int, int> h;
   h(1, 2) = 1;
   h[{1, 2}] += 1; // Note the curly braces reqd. to construct a tuple
   std::cout<<h[{1, 2}]; // Output - 2
   

3. Use another histogram (copy constructor or assignment)

   Task - Fill a histogram using another

   histogram<int, int> A;
   /*
   Fill value in A
   */
   histogram<int, int> B(A), C;
   C = A;
   

4. Use a GIL image view

   You can also use GIL images to directly fill histograms.

   Task - Fill histogram using GIL image view

   gil::gray8_image_t img;
   /*
   Fill img ...
   */
   histogram<int> h;
   h.fill(view(img));
   // OR
   gil::fill_histogram(view(img), h, false); // false if histogram needs to be cleared before filling
   

Advanced

1. Fill histogram using only a few dimensions of image

   Task - Make an histogram over Red and Blue channel of an rgb image

   gil::rgb8_image_t img;
   /*
   Fill img ...
   */
   histogram<int, int> h;
   fill_histogram<0, 2>(view(img), h, false); // 0 - red, 1 - green, 2 - blue
   

2. Fill histogram using GIL pixel

   Task - Fill histogram bin using pixel construct in GIL

   gil::gray8_image_t img;
   /*
   Fill img ...
   */
   histogram<int> h;
   gil::for_each_pixel(view(img), [](gil::gray8_pixel_t const& p){
       ++h[h.key_from_pixel(p)];
   });