Real-Time Polygon Reconstruction for Digital archives of CulturalProperties
Megumi Okumoto1 , Yuri Iwakata2 , Asuka Komeda2 , Tomoko Yonezawa3 , Masami Takata4 , and Kazuki Joe4
1

Graduate School of Humanities and Sciences, Nara Women’s University, Nara, Nara, Japan2
Department of Science, Nara women’s University, Nara, Nara, Japan
3
Faculty of Informatics, Kansai University, Takatsuki, Osaka, Japan
4
Academic Group of Information and Computer Sciences, Nara women’s University, Nara, Nara, Japan

1

Introduction

To display special purpose 3D objects on Virtual Reality (VR) systems, when they can be analyzed intuitively
and easily, it is possible for those 3D objects to be applied
to cultural property research. It is sometimes diﬃcult forusers who are not familiar with VR to control objects displayed in VR according to their intention. Therefore, tocontrol VR objects for such users, operations of VR systems should be intuitive and very easy. Motions are oneof intuitive human’s operations. Kinect[1] is a device for
capturing human’s motions. In this paper, we develop aVR system using Kinect as a VR interface device.
To faithfully describe the detail of a cultural property,
its digital archive measured by a 3D scanner becomes extremely complex and huge, and the number of meshes ofthe polygon model is too many to be eﬃciently displayedin VR systems. As the number of the meshes increases,
the operability of VR systems deteriorates. To use digital archives of cultural properties for research, which aredisplayed in VR, it is required that the number of meshes
is changeable on demand from users. Consequently, inthis paper, we propose a real-time polygon reconstruction method. To apply this method to a VR system withKinect, the VR system is intuitive and easy for any users.
2A Virtual Reality System using Kinect

In this paper, we develop a VR system using Kinect asa VR interface device. Figure 1 shows a conceptual imageof our developing VR system with Kinect. The flow of theVR system operations using Kinect is shown as follows:
Step A-1 Display objects on VR.
Step A-2 Capture user’s motion by Kinect.
Step A-3 Update the objects as control requests from thecaptured motions.
Step A-4 Return to Step A-1.
Figure 2 (a), (b) and (c) show the three patterns. Table 1
shows the corresponding resolution for captured each motion. In this way, these motions are applied to the target
object so that three kinds of resolution for the object areselectable.
3

Real-Time Polygon Reconstruction

In this paper, we propose a real-time polygon reconstruction method for digital archives of cultural properties.

Fig. 1. Conceptual image of a VR system using Kinect.

(a) Upstanding(b) Bowing (c) Deep bowing

Fig. 2. Motions to be captured.
Table 1. Captured motions, object resolutions and polygon resolutions.

resolutionscaptured motions
upstanding
bowingdeep bowing

objectnormalscale-up
more scale-up

polygonnormaldetailmore detail

Polygon reconstruction needs to be executed in real-time
so that user feels comfortable to operate the VR objects.
In [3], the meaning of real-time for interactive systems isdefined as less execution time than 1.0 second that is thetime user can wait without so much stress.
The flow of real-time polygon reconstruction is shownas following steps B.
Step B-1 By using QSlim, create a history of polygon
reductions which are to be used for the polygon reconstruction.
Step B-2 Display a 3D object with the current polygon
model on VR.
Step B-3 Capture user’s motion by Kinect.
Step B-4 According to the captured user’s motion,change the polygon model to another model from thepolygon reduction history.
Step B-5 Return to Step B-2.

Table 2. Execution time for polygon reduction [sec.].

Reducing-1
Reducing-2

Setup
135.2
67.3

Init
29.5
14.9

Run
31.3
23.7

Output
11.0
9.6

Total
206.9
115.5

(a) Upstanding(b) Bowing (c) Deep bowing

Fig. 4. The scenery of the second experiment.

Table 3. Required Time for Polygon Reduction [sec.].

Setup(a) 1, 653, 199(b) 330, 637(c) 33, 064

181.703change1→20.813

2→30.781

3→20.782

2→1
0.765

Fig. 3. Polygon models for the experiment.
4.3
Changing polygon models can be quickly done with thepolygon reduction history, where QSlim is applied to eachpolygon reduction. QSlim is an application for polygon
reduction based on QEM[2]. The change of polygon models in the proposed system works at the same time withscaling, which is described in section 2.
Table 1 shows the relationship between the inputteduser’s motion and the number of polygons.

4Experiments

In this section, we explain the results of two experimentsfor validating the proposed real-time polygon reconstruction method.

4.1

Polygon data for the experiment

For the experiments, we use a part of 3D coordinate
measuring data of the five-story small pagoda (nationaltreasure), which is enshrined in Kairyuouji temple, Nara,
Japan. The data was measured by Nakagawa’s laboratoryof Waseda University, Ikeuchi’s laboratory of the University of Tokyo and our research group in 2011.

4.2

Computation time for polygon reduction

As the first experiment, the calculation time of polygon
reduction is measured. In polygon reduction, the numberof meshes for a polygon model is reduced in two steps.
In the first step, the number of meshes is reduced from1,6553,199 meshes to 330,637 meshes. In the second step,
the number of meshes is reduced from 330,637 meshes to33,064 meshes. We call these steps Reduce-1 and Reduce2, respectively.
Figure 3 shows the three polygon models. When thenumber of meshes for a polygon model is small, the polygon model is coarse. Table 2 shows calculation time for
Reduce-1 and Reduce-2, where Setup, Init, Run and Output represent file loading, command line parsing, preprocessing, polygon reducing and history file writing times,respectively.
As the result of the first experiment, since polygon reduction takes long time, it is unsuitable to execute polygon
reduction during changing polygon models. Therefore, inthe proposed system, polygons should be restructured witha history of polygon reduction.

Time for the proposed polygon reconstruction method

In the second experiment, the elapsed times for file readand polygon model change in the developed polygon reconstruction method is measured. Figure 4 shows the sceneryof the second experiment. It is observed that the resolution of each polygon model is changed according to themotions. Additionally, examinee can intuitively operate
the developed system by using Kinect. Therefore, we conclude that it is easy for user to operate this system without
any pre-training for the operation.
Table 3 shows various times in the second experiment,
where Setup and Change indicate file loading and polygon
model change for given examinee’s motions, respectively.
As Tab.3 shows, any changes of meshes require less than
1.0 second. Since the definition of real-time in this paperis 1.0 second, the response time for the developed system
is suﬃcient.
5Conclusions

In this paper, we proposed a polygon reconstruction
method for digital archives of cultural properties. To use
digital archives of cultural properties in VR for research
purpose, it is required that mesh resolution is changeableon the demand of users. Additionally, polygon reconstruction needs to be run in real-time so that users feel comfortable for their demands. In this paper, we proposeda real-time polygon reconstruction method using historyof preliminary polygon reductions. To validate the proposed system, the polygon reconstruction method is evaluated. As the result of the experiment, we confirmed thatthe polygon reconstruction is performed within 1.0 second, which is considered as ”real-time” by the definitionof typical user-interface.
Our future work includes the implementation of viewdependent refinement. By implementing view-dependent
refinement, polygon models can be displayed more rapidly.
References[1] Xbox.com, http://www.xbox.com/en-US/Kinect[2] Michael Garland, Paul S. Heckbert, Surface simplication
using quadric error metrics, SIGGRAPH ’97 Proceedings,
New York, NY, USA, pp.209-216, 1997[3] Jakob Nielsen, Kara Pernice, Eyetracking Web Usability,
New Riders Press, 2009