## AbstractA large number of 3D models are created and available on the Web, since more and more 3D modelling and digitizing tools are developed for ever increasing applications. The techniques for content-based 3D model retrieval then become necessary. In this paper, a visual similarity-based 3D model retrieval system is proposed. This approach measures the similarity among 3D models by visual similarity, and the main idea is that if two 3D models are similar, they also look similar from all viewing angles. Therefore, one hundred orthogonal projections of an object, excluding symmetry, are encoded both by Zernike moments and Fourier descriptors as features for later retrieval. The visual similarity-based approach is robust against similarity transformation, noise, model degeneracy etc., and provides 42%, 94% and 25% better performance (precision-recall evaluation diagram) than three other competing approaches: (1)the spherical harmonics approach developed by Funkhouser et al., (2)the MPEG-7 Shape 3D descriptors, and (3)the MPEG-7 Multiple View Descriptor. The proposed system is on the Web for practical trial use (http://3d.csie.ntu.edu.tw), and the database contains more than 10,000 publicly available 3D models collected from WWW pages. Furthermore, a user friendly interface is provided to retrieve 3D models by drawing 2D shapes. The retrieval is fast enough on a server with Pentium IV 2.4GHz CPU, and it takes about 2 seconds and 0.1 seconds for querying directly by a 3D model and by hand drawn 2D shapes, respectively.## Experiment results## 1. The proposed 3D model retrieval systemThe 3D model retrieval system is on the following web site for practical trial use: http://3d.csie.ntu.edu.tw. There are 10,911 3D models in our database now, all free downloaded via the Internet. Users can query with a 3D model or drawing 2D shapes, and then search interactively and iteratively for more specific 3D models using the first retrieved results. Models are available for downloading from the hyperlink of their original downloaded path listed in the retrieval results. Figure 7 shows a typical example of a query with 2D drawing shapes, and Figure 8 shows the interactive search by selecting a 3D model from Figure 7.
The system consists of off-line feature extraction in pre-processing and on-line retrieval processes. In the off-line process, the features are extracted in a PC with a Pentium III 800MHz CPU and GeForce2 MX video card. On the average, each 3D model with 7,540 polygons takes 6.1 seconds to extract features, detailed in Table 1. Furthermore, the average time of rendering and feature extraction for a 2D shape takes 0.06 seconds. Extracting features are suitable for both 3D model and 2D shape matching. No extra effort should be done for 2D shapes. In the on-line process, the retrieval is done in a PC with two Pentium IV 2.4GHz CPUs. Only one CPU is used for the query at one time, and the retrieval takes 2 and 0.1 seconds with a 3D model and two 2D shapes as the query keys, respectively.
## 2. Performance EvaluationTraditionally, the diagram of "Precision" vs "Recall" is a common way of evaluating performance in documental and visual information retrieval. Recall measures the ability of the system to retrieve all models that are relevant. Precision measures that the ability of the system to retrieve only models that are relevant. They are defined as:To compare the performance with others systems, three major previous works are implemented as follows: (1) 3D Harmonics: This approach is proposed by Funkhouser et al., and outperforms many other approaches, such as Moments, Extended Gaussian Images, Shape Histograms and D2 Shape Distributions, which are evaluated in their paper. The source code of SpharmonicKit 2.5, also used in their implementation, is used for computing the spherical harmonics. (2) Shape 3D Descriptor: The approach is used in MPEG-7 international standard, and represents a 3D model with curvature histograms. (3) Multiple View Descriptor: This method aligns 3D objects with Principal Component Analysis (PCA), and then compares images from the primary, secondary and tertiary viewing directions of principal axes. Descriptor of the viewing directions is also recorded in MPEG-7 international standard, but does not limit the usage of image metrics. To get better performance, integration with different image metrics described in Section 2.4 are used. Furthermore, for calculating PCA correctly from vertices, each 3D model is re-sampled first to ensure that vertices are distributed evenly on the surface. Figure 9 shows the comparison of the retrieval performance of our approach, LightField Descriptors, with those of the others. Each curve plots the graph of "recall and precision" averaged over all 549 classified models in the test database. Obviously, LightField Descriptor performs better than the others. The precision values are 42%, 94% and 25% higher than those of 3D Harmonics, Shape 3D Descriptor and Multiple View Descriptor, respectively, after comparing and averaging over all the "recall" axis.
However, in our implementation of 3D Harmonics, the precision is not as good as that indicated in the original paper, shown in Table 2. Evaluating by different test database is one possible reason, and another one may lie in a small amount of different details between our implementation and original paper, even if we try to implement the same as the original paper. The test database used in the original paper is also purchased by us, and will be evaluated in the future. As for PCA applied to Multiple View Descriptor, Funkhouser et al. found that principal axes are not good while aligning orientations of different models within the same class, and also demonstrated this problem using 3 mugs. Retrieval with similar examples in our test database is shown in Figure 10. Clearly, our approach works well against this particular problem of PCA.
## 3. Robustness evaluationAll the classified 3D models in the test database are applied to the following evaluation in order to assess the robustness. Each transformed 3D model is then used for queries from the test database. The average recall and precision of all 549 classified models are used for the evaluation. The robustness is evaluated by the following transformation:(1) Similarity transformation: For each 3D model, seven random numbers are applied to x-, y-, and z-axis rotations (from 0 to 360 degree), x-, y- and z-axis translations (-10~+10 times f the length of the model's bounding box), and scaling (a factor of -10~+10). (2) Noise: Each vertex of 3D model is applied three random number to x-, y- and z-axis translation (-3%~+3% times of the length of the model's bounding box). Figure 11 (b) shows a typical example of the effect. (3) Decimation: For each 3D model, randomly select 20% polygons to be deleted. Figure 11 (c) shows a typical example of the effect. Experimental result of the robustness evaluation is shown in Figure 12. Clearly, our approach is robust against similarity transformation, noise and decimation.
## System DemoPlease visit our 3D model retrieval system## Download(backup)3DRetrieval v1.8 Executable3DRetrieval v1.8 Source code and Executable Back |