Direction line on image by CImg [image processing]

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C++

image-processing

I've tried to write code about draw line on picture, but I don't know how can I start.
1. Open picture
2. click many points on picture. For example, point 1 to point2 to point3 to bla vla bla , the line between point will appear.

3. when you press keyC from keyboard. The line between point 1 to the last point will appear

Here's my code

#include <cimg.h>
   #include<math.h>
   using namespace cimg_library;
   int main()
   {
       int n=1;
       int a,b,c,d;

       CImg<unsigned char> img("stupid.jpg");
       CImgDisplay img2(img,"Display Image");
       const unsigned char color[]  = {255,128,128},
                           color1[] = {128,255,255};
       while(!img2.is_closed())
       {
           if(img2.key() == cimg::keyESC)
               img2.close();
           if(img2.button()&1)
           {
               if(n==1){
                   a=img2.mouse_x();
                   b=img2.mouse_y();
                   img.draw_line(a,b,img2.mouse_x(),img2.mouse_y(),color,1);
                   img2.display(img);
                   c=img2.mouse_x();
                   d=img2.mouse_y();
                   n++;
                   }
               else{
                   img.draw_line(c,d,img2.mouse_x(),img2.mouse_y(),color,1);
                   img2.display(img);
                   c=img2.mouse_x();
                   d=img2.mouse_y();
                   }
           }
       }
       if(img2.key()==cimg::keyC)
               {
                   img.draw_line(c,d,a,b,color,1);
                   img2.display(img);
                   n=1;
               }
   return 0;
   }

I use Cimg library then I don't know when I press keyC on keyboard, the line don't appear.