## 7 Performance Evaluation
### Experimental Results
However, an increase in distortion results in a decrease in precision to 45% for M-distortion and 5% for H-distortion. For the 32- and 23-inch monitors (32, 23-SIFT-N-L), there is a decrease in precision to 85% and 0%. In addition, when there is an increase in distortion for M and to H (32, 23-SIFT-N-M, H), the precision is less than 25%. The second robust feature that contrasts distortion of the 360-degree image is the SURF. Precision for a 79-inch monitor at the L-distortion position (79-SURF-N-L) is 55%. However, the precision decreases to below 25% when there is either an increase in distortion or a decrease in artwork size. The precision for the other five features was less than 25% under all conditions. The five features are vulnerable to distortion of the 360-degree image. Thus, they are not suitable for identifying artwork inside the 360-degree images. After applying the polyhedron-based rectilinear projection to the 360-degree images for the 79-inch monitor, there was a 50% increase in the precision of the SIFT for M-distortion (79-SIFT-O, D, S-M). For H-distortion (79-SIFT-O, D, S-H), there were 25%, 45%, and 30% increases in precision after applying the octahedron, dodecahedron, and 32-hedron, respectively. For the SURF features, there were increases in the precision for H, M, and L-distortions of 5%, 25%, and 10% after using the octahedron (79-SURF-O-H, M, L), and 25%, 40%, and 20% after using the dodecahedron (79-SURF-D-H, M, L). However, the 32-hedron-based rectilinear projection did not significantly improve the SURF's precision for the 79-inch monitor. For the FAST features, after using the octahedron and dodecahedron, there were 10% and 25% increases in precision for the L-distortion (79-FAST-O, D-L). Summarizing the experiments from the 79-inch monitor, the dodecahedron-based method has the largest influence on improving precision, followed by the octahedron-based method. After applying the polyhedron-based methods to SIFT for the 32-inch monitor, there were 15%, 25%, and 20% increases in precision for the M-distortion (32-SIFT-O, D, S-M), whereas the precision was 0% without the proposed methods. For the SURF features, there were 45%, 40%, and 70% increases in precision for the L-distortion (32-SURF-O, D, S-L). The 32-hedron-based method improves the precision of the SURF for the 32-inch monitor, but does not improve that of the SURF for the 79-inch monitor. This is because the polygon sizes in the 32-hedron are small. There is an increased probability of segmenting the artwork into more than one projected image when the artwork size is large and the polygon sizes are small. After applying the proposed methods to the SIFT for the 23-inch monitor, there are 30%, 85% and 55% increases in precision for the L-distortion (23-SIFT-O, D, S-L), whereas the precision was 0% without using the proposed methods. After summarizing the experiments for the three types of monitors and distortions, the performance of the dodecahedron-based rectilinear projection is the best. However, there were only minimal increases in precision for the M- and H-distortions in the  
Figure 16: Artwork identification precision for the seven types of features for the three types of monitors. (a) 79-SIFT; (b) 32-SIFT; (c) 23-SIFT; (d) 79-SURF; (e) 32-SURF; (f) 23-SURF; (g) 79-FAST; (h) 32-FAST; (i) 23-FAST; (j) 79-BRISK; (k) 32-BRISK; (l) 23-BRISK; (m) 79-MSER; (n) 32-MSER; (o) 23-MSER; (p) 79-HOG; (q) 32-HOG; (r) 23-HOG; (s) 79-FREAK; (t) 32-FREAK; (u) 23-FREAK. 32 and 23-inch monitors using the proposed methods because the M- and H-distortions for 32 and 23-inch monitors are extremely challenging. Thus, there is serious damage to the artwork inside the 360-degree image. To further improve the identification precision of the artwork that is located in the highly distorted position, we applied DSK to the matched features. For reliable identification, there should be at least three matched features [10]. However, we increased the recommended number of matched features to five to exclude increases in false matches.