Flood Fill:There are some cases where we want to fill an area which is not defined
by a single boundary color.For situations like these, Flood fill algorithm is used. 
Here the process is started in a similar way by examining the colors of neighboring pixels. 
But instead of matching it with a boundary color a specified color is matched.

Algorithm:Flood Fill

Steps:

Enter the old color code(old_color) and new color(new_color) code.
Call the procedure flood_fill.
flood_ fill (x,y, old_color, new_color)
{
putpixel(x,y,new_color)
flood_ fill (x+1, y, old_color, new_color)
flood_ fill (x-1, y, old_color, new_color)
flood_ fill (x, y+1, old_color, new_color)
flood_ fill (x, y-1, old_color, new_color)
flood_ fill (x+1, y+1, old_color, new_color)
flood_ fill (x-1, y-1, old_color, new_color)
flood_ fill (x+1, y-1, old_color, new_color)
flood_ fill (x-1, y+1, old_color, new_color)
}
}


C Program Flood Fill Algorithm

Source Code:


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



-----------------------  Function Prototypes  -------------------------



 void show_screen( );

 void Flood_fill(const int,const int,const int,const int);

 void Circle(const int,const int,const int);
 void Triangle(const int,const int,const int,const int,const int,const int);
 void Rectangle(const int,const int,const int,const int);
 void Polygon(const int,const int []);
 void Line(const int,const int,const int,const int);



------------------------------  main( )  ------------------------------


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

       initgraph(&driver,&mode,"c:/tc/bgi");

       show_screen( );

       setcolor(15);
	 Circle(175,175,40);

	 Flood_fill(175,175,10,0);

       setcolor(15);
       settextstyle(0,0,1);
	 outtextxy(150,225,"Circle");

       setcolor(15);
	 Rectangle(375,145,475,205);

	 Flood_fill(400,175,9,0);

       setcolor(15);
       settextstyle(0,0,1);
	 outtextxy(390,215,"Rectangle");

       setcolor(15);
	 Triangle(135,360,215,360,175,290);

	 Flood_fill(175,325,8,0);

       setcolor(15);
       settextstyle(0,0,1);
	 outtextxy(145,370,"Triangle");

       int polygon_points[14]={ 365,325, 400,290, 450,290, 485,325,
				450,360, 400,360, 365,325 };

       setcolor(15);
	 Polygon(7,polygon_points);

	 Flood_fill(425,325,12,0);

       setcolor(15);
       settextstyle(0,0,1);
	 outtextxy(395,370,"Polygon");

       getch( );
       return 0;
    }



------------------------  Funcion Definitions  ------------------------




---------------------------  Flood_fill( )  ---------------------------


 void Flood_fill(const int x,const int y,
				   const int fill_color,const int old_color)
    {
       if(getpixel(x,y)==old_color)
	  {
	     putpixel(x,y,fill_color);

	     Flood_fill((x+1),y,fill_color,old_color);
	     Flood_fill((x-1),y,fill_color,old_color);
	     Flood_fill(x,(y+1),fill_color,old_color);
	     Flood_fill(x,(y-1),fill_color,old_color);
	  }
    }


------------------------------  Circle( )  ----------------------------


 void Circle(const int h,const int k,const int r)
    {
       int color=getcolor( );

       int x=0;
       int y=r;
       int p=(1-r);

       do
	  {
	     putpixel((h+x),(k+y),color);
	     putpixel((h+y),(k+x),color);
	     putpixel((h+y),(k-x),color);
	     putpixel((h+x),(k-y),color);
	     putpixel((h-x),(k-y),color);
	     putpixel((h-y),(k-x),color);
	     putpixel((h-y),(k+x),color);
	     putpixel((h-x),(k+y),color);

	     x++;

	     if(p<0)
		p+=((2*x)+1);

	     else
		{
		   y--;
		   p+=((2*(x-y))+1);
		}
	  }
       while (x<=y);
       }

----------------------------  Triangle( )  ----------------------------


 void Triangle(const int x_1,const int y_1,const int x_2,const int y_2,
					const int x_3,const int y_3)
    {
       Line(x_1,y_1,x_2,y_2);
       Line(x_2,y_2,x_3,y_3);
       Line(x_3,y_3,x_1,y_1);
    }


---------------------------  Rectangle( )  ----------------------------


 void Rectangle(const int x_1,const int y_1,const int x_2,const int y_2)
    {
       Line(x_1,y_1,x_2,y_1);
       Line(x_2,y_1,x_2,y_2);
       Line(x_2,y_2,x_1,y_2);
       Line(x_1,y_2,x_1,y_1);
    }


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


 void show_screen( )
    {
       setfillstyle(1,1);
	 bar(230,26,405,38);

       settextstyle(0,0,1);
	 setcolor(15);

	 setcolor(11);
	   outtextxy(237,29,"Flood Fill Algorithm");

	 setcolor(15);

	   for(int count=0;count<=30;count++)

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