Discreet Markers for User Localization
Yusuke Nakazato, Masayuki Kanbara, and Naokazu Yokoya
Nara Institute of Science and Technology
8916–5 Takayama, Ikoma, Nara, 630–0192, Japan,{yuusu-n, kanbara, yokoya}@is.naist.jp
Abstract
This paper describes a localization method for wearable
computer users. To realize applications of wearable computers like a navigation system, the position of a user is required for location-based services. Many localization methods in indoor environments have been proposed. One of themethods estimates user’s position using IR beacons or visual markers. However, these methods have problems ofpower supply and/or undesirable visual effects. In order toavoid the problems, we propose a new localization method
which is based on using an IR camera and discreet markers
consisting of translucent retro-reflectors. In the proposed
method, to extract the regions of the markers from the captured images stably, the camera captures the reflection of IR
LEDs which are flashed on and off synchronously.

1. Introduction
In applications of wearable computers like a navigation
system, the position of a user is required for location-based
services. Especially in indoor environments, since a GPS
can not be used, many localization methods have been proposed. One of the methods estimates the user’s position using IR beacons[4]. Another method estimates the user’s position by recognizing the visual markers which are pasted up
on the ceilings or walls[3, 1]. However, both of these methods have problems of power supply and/or undesirable visual effects. In order not to impair the scenery, we propose(a) Visual markers.

a new localization method using discreet markers that consist of translucent retro-reflectors. In the proposed method,translucent retro-reflective markers, which are discreet anddo not need power supply, are captured with an IR camera.

2. Localization System
2.1. Discreet Markers
The markers are set up on the ceilings or walls in indoor environments as infrastructures. The discreet markers
consist of translucent retro-reflectors. When visual markers
are set up on the ceilings, the markers impair the sceneryas shown in Fig. 1(a). Fig. 1(b) shows the scene where discreet markers are set up. Since the markers are translucent,
it is difficult for a user to observe. However, when the image is captured with a flash, the markers can be clearly observed as shown in Fig. 1(c). Because the retro-reflector reflects a light toward a light source, the reflection of themcan be captured clearly by the camera which is located near
the flashing light.

2.2. Localization System Overview
Fig. 2 illustrates the overview of the proposed localization system. Markers consisting of translucent retroreflectors are set up on the ceilings or walls in indoor environments as infrastructures. The user equips an IR camera upward for capturing images. The camera captures thereflection of the IR LEDs that are attached to it. The re-(b) Discreet markers.

Figure 1. Markers of positioning infrastructures.

(c) Discreet markers with a flash.

Translucent
Retro-reflective
Markers

Infrared Camera
Infrared LED

Wearable PC
InfraredUSB

Infrared
CameraVideo Capture
Unit

Infrared
LEDs

47mm

72mm

Control
CircuitFigure 3. Infrared
cameraand IR LEDs.
Figure 2. System overview.
flection of retro-reflector can be captured clearly by the IR
camera. However, the images also capture lights other thanthe IR LEDs; for examples, fluorescent and sun light. To
avoid such a problem, IR LEDs are flashed on and off synchronously under control of a wearable computer. The difference between the captured images when IR LEDs are
switch on and off is calculated. Finally, to estimate theposition of the user, the markers are recognized from thesubtraction image. A position ID is associated with eachmarker. In addition to identifying the received marker, it ispossible to estimate the relative position and orientation ofcamera with respect to the marker coordinate system fromthe four vertices of a square marker of known size using astandard computer vision technique[2].

3. Experiments
In this experiment, a small camera shown in Fig. 3 isused as an IR camera. View angle of the camera is 92.6◦ ,and IR LEDs are attached around the camera. We made acircuit which controls the LEDs using RS-232C serial communication with a PC. The used PC is a mobile computer“InterLink MP-XP7310 (Pentium M 1GHz).” The distance
between the camera and the markers is about 1.8m, and theimage size is 320 × 240 pixels. The retro-reflective marker
is a square whose edge length is 160mm.
Fig. 4 shows results of marker recognition. In these images, the position IDs associated with the markers and conesthat show the direction of the user are drawn. We confirm
that the ID of marker can be correctly recognized. The system can calculate the relative position and direction of thecamera with respect to the marker coordinate system. The
estimated orientation is indicated by a cone heading north.
The processing rate is about 15 frames per second, because
two images when the IR LEDs are switch-on and switch-off
are required for recognition.
Fig. 5 shows estimated camera position when the cameratranslates. The markers were set up on the ceiling, and thecamera moved in parallel to the ceiling (y-axis). The distance between the camera and the ceiling was 144cm. We
can confirm that the error of estimated camera position isat most about 20cm. We conclude that the accuracy of estimated camera position is comparable with the case of using

Figure 4. Results of marker recognition.

ｘ(true valu)ｙ(true valu)ｚ(true valu)

xyz

150
100

position[cm]RS-232C50
0-50 1112131

4151

61

71

81-100-150-200

framesFigure 5. Estimated camera position.
visual markers.

4. Summary
This paper has proposed a new localization method
based on using IR camera and discreet markers which consist of translucent retro-reflectors. In the method, user’s position is identified by recognizing markers which are illuminated from IR LEDs. In order to eliminate the influence ofinfrared other than the reflection from retro-reflective markers, the IR LEDs are flashed on and off continuously. In theexperiments, we have confirmed that the accuracy of estimated camera position is comparable with the case ofusing visual markers. In future work, we should investigate a method of marker setting.
Acknowledgements This research is partially supported by Core
Research for Evolutional Science and Technology (CREST) Program “Advanced Media Technology for Everyday Living” ofJapan Science and Technology Agency (JST).

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