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296

B. Bustos and I. Sipiran

Table 7.6 Performance using bag of features with vocabulary size 48. Table reproduced from Ovsjanikov et al. [83]

Transformation

EER

FPR @ FNR = 1 %

FPR @ FNR = 0.1 %

Null

0.97 %

0.90 %

6.47 %

Isometry

1.34 %

1.56 %

11.13 %

Topology

1.12 %

2.49 %

14.41 %

Isometry+Topology

1.82 %

2.38 %

13.90 %

Triangulation

2.29 %

4.26 %

14.66 %

Partiality

3.81 %

5.68 %

17.28 %

All

1.44 %

1.79 %

11.09 %

 

 

 

 

7.3.4.1 Evaluation

The main goal of this technique is to be robust in large scale databases, so the authors composed a database with models from the TOSCA dataset, the Princeton shape benchmark and the Sumner dataset. The performance was measured using ROC curves. It is important to note that the shapes in the TOSCA dataset were used as positive examples. These shapes contain transformed versions of the original shapes (null shapes), so the experiments were performed to assess the ability of the algorithm to retrieve shapes under the transformations (isometry, topology, triangulation, partiality).

The parameters used in computing the final descriptor were t0 = 1024 and α = 1.32, the size of vocabulary was 48, σ for soft quantization was set to twice the median size of the clusters in the geometric vocabulary. In addition, only 200 eigenvalues were used to compute the heat kernel signatures in Eq. (7.44).

The authors used three criteria to evaluate their method:

Equal error rate (EER), the value of false positive rate (FPR) at which it equals the false negative rate (FNR).

FPR at 1 % FNR.

FPR at 0.1 % FNR.

In terms of EER, when null shapes were used as queries, the method obtained 0.97 %. With respect to the transformations, ‘topology’ gave the best performance with 1.12 %, followed by ‘isometry’ with 1.34 %. These results show that this method is robust in the presence of transformations such as topology changes and isometry. We can conjecture that the formulation of the heat kernel signatures, over which the algorithm is based, largely supports these issues. This fact is also noted in the performance of the partiality transformation (3.81 % in terms of EER). As the heat kernel signature is based on the Laplace-Beltrami operator, the partiality transformation reduces the chance of correctly retrieving similar shapes because with partial shapes the intrinsic geometry changes considerably. Tables 7.6 and 7.7 show the complete results.