Write C++ program to draw a concave polygon and fill it with desired color using scan fill algorithm.

// Write C++ program to draw a concave polygon and fill it with desired color using scan fill algorithm. 

#include<iostream>
#include<graphics.h> //graphics.h library is used to include  graphical operations in a program.
#include<math.h>
using namespace std;

class scan
{
    public:
    int x[20],y[20],k;
    float slope[20],x_int[20];
    void polygon(int n);
};

void scan::polygon(int n)
{
    int i;
    float dx,dy;
   
    x[n]=x[0];
    y[n]=y[0];
    for(int i=0;i<n;i++)  //draw all lines (edges of polygon)
    {   
       
            line(x[i],y[i],x[i+1],y[i+1]);  // line cordinates x1,y1,x2,y2  
    }
    
    for(i=0;i<n;i++)   // finding slope of all lines
    {
        dy=y[i+1]-y[i];  // dy=y2-y1
        dx=x[i+1]-x[i];  // dx=x2-x1
        if(dy==0)
        slope[i]=1;
        else if(dx==0)
        slope[i]=0;
        else
        slope[i]=dx/dy;
    }
        // finding intersection points
    for(int p=0;p<480;p++)  // consider 480 horizontal lines on screen
    {
        k=0;
        for(i=0;i<n;i++)
        {
            if(( (y[i]<=p) && (y[i+1]>p)) || ((y[i]>p) && (y[i+1]<=p) ))
              {
                  x_int[k]=x[i]+slope[i]*(p-y[i]);       // find out intersection points using formula
                  k++;
              }
          }
          
          for(int j=0;j<k-1;j++)   // perform sorting of intersection points on x direction
          {
              for(int i=0;i<k-1;i++)
              {
                  if(x_int[i]>x_int[i+1])   
                  {
                      int temp = x_int[i];
                      x_int[i] = x_int[i+1];
                      x_int[i+1] = temp;
                  }
              }
          }
         
        
          for(int i=0;i<k;i=i+2)   //fill points of line that are interior to polygon
          {
               setcolor(YELLOW);

              line(x_int[i], p ,x_int[i+1], p);  // x1,y1,x2,y2
              delay(10);      
          }
          
          }
  }
         int main()
  {
      int n,i;
      scan p;
      cout<<"Enter edge : \t";
      cin>>n;
      cout<<"\n\nEnter Coordinates : \t";
      for(i=0;i<n;i++)
      {
          cin>>p.x[i]>>p.y[i];
      }
      int gd,gm;
      gd=DETECT;
      initgraph(&gd,&gm,NULL);
      
      p.polygon(n);
      getch();
        closegraph();
        return 0;
 }

/* Enter edge :     4
Enter Coordinates :     
50 100
100 50
300 200
150 120               Enter edge :     4


Enter Coordinates :     200 300
300 400
200 400
300 300
  Enter edge :     4
200 300
300 200
300 100
150 120
 
*/