Fill histogram - Boost.GIL documentation

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# Fill histogram

## 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)];
   >     });