From f87ee565c1c4157b880c729d04b1d21d1f6dcc2f Mon Sep 17 00:00:00 2001
From: Olzhas Zhumabek <anonymous.from.applecity@gmail.com>
Date: Wed, 18 May 2022 02:03:36 +0600
Subject: [PATCH] In depth explanation for Hough line

Added in depth explanation for
Hough line transform in the
associated markdown file
---
 example/histogram_equalization.md | 46 +++++++++++++++++++++++++++++++
 example/hough_transform_line.cpp  |  2 +-
 2 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/example/histogram_equalization.md b/example/histogram_equalization.md
index c3dcd57832..eb71568dac 100644
--- a/example/histogram_equalization.md
+++ b/example/histogram_equalization.md
@@ -7,6 +7,52 @@ Histogram equalization capabilities in GIL are demonstrated by the program `hist
 
 The program doesn't take any argument on the command line.
 
+Hough line transform solves the equation of a line in reverse, but in *polar coordinates*! The implementation will make each pixel vote on all possible lines it could be part of, limited by input Hough parameters.
+
+A line in polar coordinates is represented by normal to it in polar coordinates, as shown in example here 
+https://docs.opencv.org/3.4/d9/db0/tutorial_hough_lines.html
+
+Since real line is the diagonal throughout the image, the angle of normal to it will 45 degrees. (1s represent real line, 5s represent normal to it).
+
+```
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 
+0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 
+0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 
+0 0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 
+0 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 
+0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 
+0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 
+0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 
+5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 
+```
+
+It is obvious that the angle for the expected Hough transform is 45 degrees (angle between 5s and bottom 0s). Now we 
+need to find the length of 5s. Since 5s, 1s and bottom 0s form a right triangle, we know that bottom 0s are 32 in length
+and is a hypotenuse! Using trivial trigonometry we know that the length we are searching for is 32 * cos(45).  
+
 `histogram_equalization` expects to find an image called `test_adaptive.png` in the current directory, and produces an image in return, where the equalization have been applied: `histogram_gray_equalized.png`.
 
 ## Specific requirements
diff --git a/example/hough_transform_line.cpp b/example/hough_transform_line.cpp
index 0d9a14f943..d67fff5b59 100644
--- a/example/hough_transform_line.cpp
+++ b/example/hough_transform_line.cpp
@@ -25,7 +25,7 @@ namespace gil = boost::gil;
 
 int main()
 {
-    std::ptrdiff_t size = 32;
+    const std::ptrdiff_t size = 32;
     gil::gray16_image_t input_image(size, size);
     auto input_view = gil::view(input_image);