Boundary fill is the algorithm used frequently in computer graphics to fill a desired color inside a closed polygon having the same boundary color for all of its sides.

The most approached implementation of the algorithm is a stack-based recursive function.

How it works:

The problem is pretty simple and usually follows these steps:

  1. Take the position of the starting point and the boundary color.
  2. Decide wether you want to go in 4 directions (N, S, W, E) or 8 directions (N, S, W, E, NW, NE, SW, SE).
  3. Choose a fill color.
  4. Travel in those directions.
  5. If the pixel you land on is not the fill color or the boundary color , replace it with the fill color.

Repeat 4 and 5 until you’ve been everywhere within the boundaries.

Certain Restrictions:

The boundary color should be the same for all the edges of the polygon.

The starting point should be within the polygon.

Code Snippet:

void boundary_fill(int pos_x, int pos_y, int boundary_color, int fill_color)
{  
current_color= getpixel(pos_x,pos_y);  //get the color of the current pixel position
if( current_color!= boundary_color || currrent_color != fill_color) // if pixel not already filled or part of the boundary then
{    
 putpixel(pos_x,pos_y,fill_color);  //change the color for this pixel to the desired fill_color
 boundary_fill(pos_x + 1, pos_y,boundary_color,fill_color);  // perform same function for the east pixel
 boundary_fill(pos_x - 1, pos_y,boundary_color,fill_color);  // perform same function for the west pixel
 boundary_fill(pos_x, pos_y + 1,boundary_color,fill_color);  // perform same function for the north pixel
 boundary_fill(pos_x, pos_y - 1,boundary_color,fill_color);  // perform same function for the south pixel
}
}

From the given code you can see that for any pixel that you land on, you first check whether it can be changed to the fill_color and then you do so for its neighbours till all the pixels within the boundary have been checked.