Image Processing Reference
Number of graphs (connected edges)
Number of graphs with a single edge
Number of graphs with two edges
Number of graphs whose edges form a cycle
Number of line normals
Average length of edges in all segments
Sum of lengths of all edges
Maximum length of an edge
4.4 Corner Features
Corner points are considered as one of the uniquely recognizable features in an image. A
corner is the intersection of two edges where the variation between two perpendicular direc-
change in intensity of a pixel ( x , y ) for a displacement of a search window in all directions. We
apply Harris corner detection and count the number of corners as the image feature.
4.5 Hough Line Features
Hough transform is a very popular technique in computer vision for detecting certain class of
age. Once the lines are detected, we extract the following two line features—number of Hough
lines and the average length of line.
5 Experimental results
Our experimental dataset consists of 212 expert labeled images. The images are hand-labeled
by an expert into four different categories: needles, small crystals, large crystals, and other
crystals. The proportion of these classes are 24%, 20%, 35% and 21% respectively. For each im-
age, we apply green percentile binarization with p = 95 and p = 99. From each binary image,
we extract 17 region features. Likewise, we extract 8 edge related features, 1 Harris corner fea-
ture, and 2 Hough line features. Therefore, we extract a total of 2 × 17 + 8 + 1 + 2 = 45 features
per image. On a Windows 7 Intel Core i7 CPU @2.4 GHz system with 12 GB memory, it takes
around 232 s to extract features for 212 images. Thus the time for feature extraction is around
1.1 s per image.
We group the feature sets into three categories—region features, edge/corner/line features,
and combined features. For the classification, we test using decision tree and random forest
classiier. Table 3 shows the classification accuracy using the selected classifiers and feature
sets. The values are computed as the average accuracy over 10 runs of 10-fold cross validation.