Geometric Model Reconstruction - Computational Hemodynamics: Theory, Modelling and Applications - page 51

Region growing of seed pixels of th e following colour
Seed pixels

Region growing Completed segmentation

Region growing of seed pixels of th e following colour
Seed pixels

Region growing Completed segmentation

Fig. 3.7 Region growing approach using a single criterion each time, firstly by selecting the yel-
low flower petal colour, and secondly by selecting the background blue colour. The first criterion
segments the flower petals, as shown in black , while the second criterion is based on the blue
background pixels

region based segmentation is presented which involves finding the region of inter-
est directly. Unlike the edge based methods that detect boundaries, region-based
segmentation determines the regions directly working from the inside and growing
outwards, instead of outside in. The difficulties lie in the criteria for region classifi-
cation which are typically more difficult than edge detection methods. In addition it
generally can't find objects that span multiple disconnected regions.
Region growing is a group of pixels or sub-regions collected together with some
predefined criteria, based on greyscale intensity and/or edge information, in order to
form a larger region. The process begins by manually defining seed pixels in a way
that the local group of seed pixels grows by appending to each all pixels connected
to the initial seed pixel that satisfies some predefined criteria (greyscale, texture,
colour or shape) are included, thus the region grows. Such criteria include specific
range of grey level (average intensity or variance), colour, or shape. Region grow-
ing is advantageous over edge detection in noisy images, where edges are difficult
to detect.
An example of region growing is shown in Fig. 3.7 on a yellow flower with a
blue background. To demonstrate the region growing we firstly define the seed pix-
els as those pixels matching a shade of yellow. The selected pixels are shown as a
black colour where the initially grouped seeded pixels grow with connected neigh-
bouring pixels until the entire flower petals are selected. In the second criterion,
the background is segmented based on a selection of blue seed pixels. The region
growing is able to be constrained within the blue background only.