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# Program to fill a Polygon using Scan Line Polygon Fill Algorithm

Posted By: Easy Tutor     Category: C++ Programming     Views: 45764

## Code for Program to fill a Polygon using Scan Line Polygon Fill Algorithm in C++ Programming

``` # 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(constfloat x1,constfloat y1,
constfloat x2,constfloat y2)
{
x_1=x1;
y_1=y1;
x_2=x2;
y_2=y2;
}
};

/*************************************************************************//*************************************************************************///-----------------------  Function Prototypes  -------------------------///*************************************************************************//*************************************************************************/void show_screen( );

void Fill_polygon(constint,constint [],constint);

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

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

void Polygon(constint,constint []);
void Line(constint,constint,constint,constint);

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(constint n,constint ppts[],constint 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( )  ----------------------------///*************************************************************************/constint yNext(constint k,constint 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,
constint 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(constint 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(constint 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(constint scan,const Edge *active,constint 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(constint 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(constint n,constint 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(constint x_1,constint y_1,constint x_2,constint 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.");
}

```
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