boundary-fill algorithm or flood-fill algorithm

As you saw the implementation of scan line polygon fill requires that boundaries should be straight line segments.  The seed fill algorithms do not require any such constraints.  You only need to know an interior point of the closed boundary object to fill it.  This interior point is called a seed point.

However, determination of interior point for complex polygons such as the one shown in figure itself is a challenging task.The following methods are used to determine an interior point of the area to be filled. 

1.  Odd-even rule,

2.  Nonzero winding number rule.

Once an interior point of the object is determined, the boundary-fill algorithm or, flood-fill algorithm may be applied to fill the given area.