Program to fill a Polygon using Scan Line Polygon Fill Algorithm

 # include <iostream.h>
 # include <graphics.h>
 # include    <conio.h>
 # include     <math.h>



 //---------------------------------  Edge  ------------------------------//


 class Edge
    {
       public:
      int yUpper;

      float xIntersect;
      float dxPerScan;

      Edge *next;
    };


 //--------------------------  PointCoordinates  -------------------------//


 class PointCoordinates
    {
       public:
      float x;
      float y;

      PointCoordinates( )
         {
        x=0;
        y=0;
         }
    };


 //---------------------------  LineCoordinates  -------------------------//


 class LineCoordinates
    {
       public:
      float x_1;
      float y_1;
      float x_2;
      float y_2;

      LineCoordinates( )
         {
        x_1=0;
        y_1=0;
        x_2=0;
        y_2=0;
         }

      LineCoordinates(const float x1,const float y1,
                          const float x2,const float y2)
         {
        x_1=x1;
        y_1=y1;
        x_2=x2;
        y_2=y2;
         }
    };



 //-----------------------  Function Prototypes  -------------------------//



 void show_screen( );

 void Fill_polygon(const int,const int [],const int);

 void insertEdge(Edge *,Edge *);
 void makeEdgeRec(const PointCoordinates,const PointCoordinates,
                         const int,Edge *,Edge *[]);
 void buildEdgeList(const int,const PointCoordinates [],Edge *[]);
 void buildActiveList(const int,Edge *,Edge *[]);
 void fillScan(const int,const Edge *,const int);
 void deleteAfter(Edge []);
 void updateActiveList(const int,Edge []);
 void resortActiveList(Edge []);

 const int yNext(const int,const int,const PointCoordinates []);

 void Polygon(const int,const int []);
 void Line(const int,const int,const int,const int);


 int main( )
    {
       int driver=VGA;
       int mode=VGAHI;

       initgraph(&driver,&mode,\"..\\\\Bgi\");

       show_screen( );

       int n=10;
       int polygon_points[20]={ 220,340 , 220,220 , 250,170 , 270,200 ,
                300,140 , 320,240 , 320,290 , 420,220 ,
                420,340 , 220,340 };

       setcolor(15);
     Polygon(10,polygon_points);

       Fill_polygon(n,polygon_points,9);

       getch( );
       return 0;
    }


 //----------------------------  Fill_polygon( )  ------------------------//


 void Fill_polygon(const int n,const int ppts[],const int fill_color)
    {
       Edge *edges[480];
       Edge *active;

       PointCoordinates *pts=new PointCoordinates[n];

       for(int count_1=0;count_1<n;count_1++)
      {
         pts[count_1].x=(ppts[(count_1*2)]);
         pts[count_1].y=(ppts[((count_1*2)+1)]);
      }

       for(int count_2=0;count_2<640;count_2++)
      {
         edges[count_2]=new Edge;
         edges[count_2]->next=NULL;
      }

       buildEdgeList(n,pts,edges);

       active=new Edge;
       active->next=NULL;

       for(int count_3=0;count_3<480;count_3++)
      {
         buildActiveList(count_3,active,edges);

         if(active->next)
        {
           fillScan(count_3,active,fill_color);
           updateActiveList(count_3,active);
           resortActiveList(active);
        }
      }

       Polygon(n,ppts);

       delete pts;
    }


 //-------------------------------  yNext( )  ----------------------------//


 const int yNext(const int k,const int cnt,const PointCoordinates pts[])
    {
       int j;

       if((k+1)>(cnt-1))
      j=0;

       else
      j=(k+1);

       while(pts[k].y==pts[j].y)
      {
         if((j+1)>(cnt-1))
        j=0;

         else
        j++;
      }

       return (pts[j].y);
    }


 //-----------------------------  insertEdge( )  -------------------------//


 void insertEdge(Edge *list,Edge *edge)
    {
       Edge *p;
       Edge *q=list;

       p=q->next;

       while(p!=NULL)
      {
         if(edge->xIntersect<p->xIntersect)
        p=NULL;

         else
        {
           q=p;
           p=p->next;
        }
      }

       edge->next=q->next;
       q->next=edge;
    }


 //---------------------------  makeEdgeRec( )  --------------------------//


 void makeEdgeRec(const PointCoordinates lower,const PointCoordinates upper,
                   const int yComp,Edge *edge,Edge *edges[])
    {
       edge->dxPerScan=((upper.x-lower.x)/(upper.y-lower.y));
       edge->xIntersect=lower.x;

       if(upper.y<yComp)
      edge->yUpper=(upper.y-1);

       else
      edge->yUpper=upper.y;

       insertEdge(edges[lower.y],edge);
    }


 //---------------------------  buildEdgeList( )  ------------------------//


 void buildEdgeList(const int cnt,const PointCoordinates pts[],Edge *edges[])
    {
       Edge *edge;
       PointCoordinates v1;
       PointCoordinates v2;

       int yPrev=(pts[cnt-2].y);

       v1.x=pts[cnt-1].x;
       v1.y=pts[cnt-1].y;

       for(int count=0;count<cnt;count++)
      {
         v2=pts[count];

         if(v1.y!=v2.y)
        {
           edge=new Edge;

           if(v1.y<v2.y)
              makeEdgeRec(v1,v2,yNext(count,cnt,pts),edge,edges);

           else
              makeEdgeRec(v2,v1,yPrev,edge,edges);
        }

         yPrev=v1.y;
         v1=v2;
      }
    }


 //------------------------  buildActiveList( )  -------------------------//


 void buildActiveList(const int scan,Edge *active,Edge *edges[])
   {
    Edge *p;
    Edge *q;

    p=edges[scan]->next;

    while(p)
      {
         q=p->next;

         insertEdge(active,p);

         p=q;
      }
    }


 //----------------------------  fillScan( )  ----------------------------//


 void fillScan(const int scan,const Edge *active,const int fill_color)
    {
       Edge *p1;
       Edge *p2;

       p1=active->next;

       while(p1)
      {
         p2=p1->next;

         for(int count=p1->xIntersect;count<=p2->xIntersect;count++)
        putpixel(count,scan,fill_color);

         p1=p2->next;
      }
    }


 //-------------------------  deleteAfter( )  ----------------------------//


 void deleteAfter(Edge * q)
    {
       Edge *p=q->next;

       q->next=p->next;

       delete p;
    }


 //-------------------------  updateActiveList( )  -----------------------//


 void updateActiveList(const int scan,Edge *active)
    {
       Edge *q=active;
       Edge *p=active->next;

       while(p)
      {
         if(scan>=p->yUpper)
        {
           p=p->next;

           deleteAfter(q);
        }

         else
        {
           p->xIntersect=(p->xIntersect+p->dxPerScan);
           q=p;
           p=p->next;
        }
      }
    }


 //-------------------------  resortActiveList( )  -----------------------//


 void resortActiveList(Edge *active)
    {
       Edge *q;
       Edge *p=active->next;

       active->next=NULL;

       while(p)
      {
         q=p->next;

        insertEdge(active,p);

         p=q;
      }
    }


 //-----------------------------  Polygon( )  ----------------------------//


 void Polygon(const int n,const int coordinates[])
    {
       if(n>=2)
      {
         Line(coordinates[0],coordinates[1],
                         coordinates[2],coordinates[3]);

         for(int count=1;count<(n-1);count++)
        Line(coordinates[(count*2)],coordinates[((count*2)+1)],
                        coordinates[((count+1)*2)],
                        coordinates[(((count+1)*2)+1)]);
      }
    }


 //-------------------------------  Line( )  -----------------------------//


 void Line(const int x_1,const int y_1,const int x_2,const int y_2)
    {
       int color=getcolor( );

       int x1=x_1;
       int y1=y_1;

       int x2=x_2;
       int y2=y_2;

       if(x_1>x_2)
      {
         x1=x_2;
         y1=y_2;

         x2=x_1;
         y2=y_1;
      }

       int dx=abs(x2-x1);
       int dy=abs(y2-y1);
       int inc_dec=((y2>=y1)?1:-1);

       if(dx>dy)
      {
         int two_dy=(2*dy);
         int two_dy_dx=(2*(dy-dx));
         int p=((2*dy)-dx);

         int x=x1;
         int y=y1;

         putpixel(x,y,color);

         while(x<x2)
        {
           x++;

           if(p<0)
              p+=two_dy;

           else
              {
             y+=inc_dec;
             p+=two_dy_dx;
              }

           putpixel(x,y,color);
        }
      }

       else
      {
         int two_dx=(2*dx);
         int two_dx_dy=(2*(dx-dy));
         int p=((2*dx)-dy);

         int x=x1;
         int y=y1;

         putpixel(x,y,color);

         while(y!=y2)
        {
           y+=inc_dec;

           if(p<0)
              p+=two_dx;

           else
              {
             x++;
             p+=two_dx_dy;
              }

           putpixel(x,y,color);
        }
      }
    }


 //--------------------------  show_screen( )  ---------------------------//


 void show_screen( )
    {
       setfillstyle(1,1);
     bar(178,26,450,38);

       settextstyle(0,0,1);
     setcolor(15);
       outtextxy(5,5,\"******************************************************************************\");
       outtextxy(5,17,\"*-**************************************************************************-*\");
       outtextxy(5,29,\"*-------------------                                     --------------------*\");
       outtextxy(5,41,\"*-**************************************************************************-*\");
       outtextxy(5,53,\"*-**************************************************************************-*\");

     setcolor(11);
       outtextxy(185,29,\"Scan Line Polygon Fill Algorithm\");

     setcolor(15);

       for(int count=0;count<=30;count++)
          outtextxy(5,(65+(count*12)),\"*-*                                                                        *-*\");

       outtextxy(5,438,\"*-**************************************************************************-*\");
       outtextxy(5,450,\"*-------------------------                          -------------------------*\");
       outtextxy(5,462,\"******************************************************************************\");

     setcolor(12);
       outtextxy(229,450,\"Press any Key to exit.\");
    }