Content-Based Retrieval of Aurora Images Based on the Hierarchical Representation - Advanced Concepts for Intelligent Vision Systems - page 257

4.3 Determination of Type3 Aurora
In a Type3 aurora image, the night side of the aurora is visible and most of the day
side of the aurora is not visible. The edge image of a typical Type3 aurora usually
consists of one closed curve with a large and wide concavity as shown in Fig. 8 (b).
The method used to identify a Type3 aurora is given below.
(a) (b) (c) (d)
Fig. 8. Illustration of the determination of Type3 aurora: (a) Aurora image, (b) Edge image, (c)
Convex hull of the edge image, and (d) Minimum bounding rectangle
Step 1. The length of the convex hull edge that bridges the widest concavity is
determined. The hull edge for the edge image in Fig. 8 (b) is given in Fig.8 (c).
Step 2. The extent of the aurora oval in a direction of the hull edge determined in
Step 1 is found. The algorithm that finds the minimum bounding rectangle given in
Section 3.4 is used for this purpose. This is illustrated in Fig.8 (d).
Step 3. If the ratio of the length of the hull edge in Step 1 to the extent determined in
Step 2 is greater than a predetermined threshold, then the image is taken as a Type3
aurora image.
5 Simulation Results
Precision and recall [7] are used in a content-based retrieval system to measure the
performance of the retrieval. Precision is the ratio of the number of the relevant im-
ages retrieved ( N r ) to the total number of images retrieved ( N t ). Recall is the ratio of
the number of the relevant images retrieved ( N r ) to the total number of relevant im-
ages in the database ( N d ).
Forty sample aurora images, ten for each type of aurora including the standard au-
rora, are analyzed to obtain the parameters for the decision of types. For Type1 auro-
ra, the ratio of the depth of the concavity to the length of the side of the minimum
bounding rectangle of the inner boundary that is roughly oriented in the direction of
the concavity is found to be greater than 0.35. For Type2 aurora, the value of the
thickness measured in pixels is found to be greater than 45. For Type3 aurora, the
ratio of the length of the hull edge that bridges the widest concavity to the extent of
the minimum bounding rectangle in the direction of the hull edge is found to be great-
er than 0.65.