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Computer Graphics

5.2 Region Filling

The process of coloring the object or image area or region is called region filling. The region of the
image or the object is defined at the pixel level. At pixel level, the region is defined in terms of bounding
pixels that outline the entire image or interior pixels that define the object on the screen. Based on this,
two algorithms are defined for filling the object. They are:
1. Boundary-Defined Region: Figure 5.3 (a) shows the boundary defined region of a polygon. The
algorithm used to define this is called the boundary-fill algorithm. This algorithm begins with a
starting pixel or seed from which the filling starts in side the region. The algorithm continuously
checks for the image’s boundary. The boundary acts as the end of filling. All the pixels that lie
within the boundary are filled. The algorithm works on an arbitrary region by checking and filling
the non-boundary pixels. All the pixels that are not the part of the boundary are filled. The filling
seed is considered as the reference point or start pixel, then the neighboring pixel is processed by
the algorithm to check whether it is the boundary pixel or not. If the neighboring pixel is a non-
boundary pixel it is filled and acts as the seed for next filling. If it is the boundary then filling stops
for the current pixel. Thus, the algorithm processes and checks for non-boundary pixels and fills
them.

Figure 5.3 (a): Boundary-Defined Region

When the filling of the pixel takes place the pixel acquires a color that is defined by the
algorithm.

2. Interior-defined Region: Figure 5.3(b) shows the interior-defined region of a polygon. The
algorithm used to define this is called flood-fill algorithm. Even this algorithm begins with a seed
inside the polygon. The pixel inside the polygon region has the region's original color i.e., the
interior-deigned region of the polygon has a pre-defined color, when the algorithm finds the
original color then the pixel is changed to a new color. This pixel acts as the seed for the next pixel
and processes it. If the pixel does not has the region's color then that pixel is consider as the end of
the polygon. The process continues till the colors of all the pixels are changed from the original to
the new color. Thus, in a flood-fill algorithm the filling is done by chaining the original color to the
new color.

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