# Algorithms -- Boundary Fill

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.

### Working:

The problem is pretty simple and usually follows these steps:

1. Take the position of the starting point and the boundary color.
2. Decide whether 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.
6. Repeat 4 and 5 until youâ€™ve been everywhere within the boundaries.

### Certain Restrictions:

1. The boundary color should be the same for all the edges of the polygon.
2. 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.