04-084 3/14/06 9:02 PM Page 365 Principles and Evaluation of Autostereoscopic Photogrammetric Measurement Jie Shan, Chiung-Shiuan Fu, Bin Li, James Bethel, Jeffrey Kretsch, and Edward Mikhail Abstract display alternating pairs of remote sensing images. Usery Stereoscopic perception is a basic requirement for photogram- (2003) discusses the critical issues such as color, resolution, metric 3D measurement and accurate geospatial data collec- and refresh rate for autostereoscopic display of geospatial tion. Ordinary stereoscopic techniques require operators data. Despite these visualization-oriented studies, properties wearing glasses or using eyepieces for interpretation and and performance of autostereoscopic measurement are measurement. However, the recent emerging autostereoscopic unknown to the photogrammetric community. In this paper, technology makes it possible to eliminate this requirement. we focus on the metric properties of the autostereoscopic This paper studies the principles and implementation of display and evaluate its performance for photogrammetric autostereoscopic photogrammetric measurement and evalu- measurement. The paper starts with a brief introduction ates its performance. We first describe the principles and to the principles of the autostereoscopic technology. As properties of the parallax barrier-based autostereoscopic an important metric property, we quantitatively show the display used in this study. As an important metric property, autostereoscopic geometry, including viewing zones and we quantitatively present the autostereoscopic geometry, the boundary of operator’s movement for autostereoscopic including viewing zones and the boundary of a viewer’s perception. To carry out autostereoscopic measurement and movement for autostereoscopic measurement. A toolkit evaluate its performance, a photogrammetric toolkit, AUTO3D, AUTO3D is developed that has common photogrammetric is developed based on a DTI (Dimension Technologies, Inc., functions. The implementation principles are described by 2004) autostereoscopic monitor. Principles and design con- addressing the differences compared to the ordinary stereo- siderations in the AUTO3D development are discussed. Finally, scopic technology. To evaluate the performance of the auto- we compare the autostereoscopic measurement results with stereoscopic measurement, images at a resolution of 25 ␮m the ones obtained from a conventional stereo photogrammet- and 50 ␮m are measured by a group of seven (7) operators, ric workstation. Seven (7) operators are involved in the tests who are asked to digitize 18 well-defined roof points and by measuring a number of carefully selected feature points. 18 ground points. These results are evaluated by comparing The results and discussion of the experiments are presented the same measurements obtained from a popular stereoscopic in this paper. photogrammetric workstation. It is shown that the precision of autostereoscopic measurement is about 16 percent to 25 percent lower than the conventional stereo workstation. Autostereoscopic Principle The term “autostereoscope” is used to refer that a viewer can perceive 3D without viewing aids, such as goggles or Introduction eyepieces. The most popular autostereoscopic technology Accurate and realistic 3D data collection requires stereoscopic is based on the parallax barrier principle (Kaplan, 1952; perception and measurement. Throughout the history of Okoshi, 1976; Sexton, 1992). As shown in Figure 1, a spe- photogrammetry for over 150 years, this requirement has been cially designed image, called parallax stereogram, is placed met by merely using eyepieces or glasses attached to the behind a barrier made of opaque material with fine transpar- equipment or operators. However, recent development in auto- ent vertical slits. The parallax stereogram is composed of stereoscopic technology provides alternatives to this conven- (vertically) interleaved stripes from the left and right images tion. In contrast to the traditional stereo technology, autostereo of a stereo pair. Each transparent slit acts as a window to is goggle-free or aid-free (Okoshi 1976; Eichenlaub and Martens, the corresponding image stripes. The image and the barrier 1990; Pastoor and Wöpking, 1997). Its potential utilization in are arranged in such a way that the left eye and right eye of photogrammetric practice has been brought to the attention of the viewer at certain location will respectively see the left photogrammetrists and photogrammetry vendors (Petrie, 2001). image and right image, so that the stereoscopic effect can be However, so far, the primary application fields of this obtained. technology are collective visualization and the entertainment In practice, this principle can be implemented differ- industry with only limited application in geospatial data ently for different autostereoscopic monitors. The key is display. Jones and McLaurin (1993) take advantage of the to generate the parallax barrier. In the popular lenticular capability of video and computer technology to rapidly autostereoscopic technology, a plate or sheet with an array of small cylindrical lenses (lenticular plate) is used as the Jie Shan, Chiung-Shiuan Fu, Bin Li, James Bethel, and Edward Mikhail are with Geomatics Engineering, School of Civil Photogrammetric Engineering & Remote Sensing Engineering, Purdue University, 550 Stadium Mall Drive, West Vol. 72, No. 4, April 2006, pp. 365–372. Lafayette, IN 47907-2051 ([email protected]). 0099-1112/06/7204–0365/$3.00/0 Jeffrey Kretsch is with the National Geospatial-Intelligence © 2006 American Society for Photogrammetry Agency. and Remote Sensing PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING April 2006 365 04-084 3/14/06 9:02 PM Page 366 TABLE 1. SPECIFICATIONS OF THE DTI 2018XL MONITOR Display Size 46 cm (18Љ) Display Type TFT LCD Max. Display 1280 1024 Resolution Computer 640 480 @ 60 Hz*; 800 600 @ 60 Hz*; Resolution 1024 768 @ 60 Hz*; 1284 1024 @ 60 Hz; Supported 720 400 @ 70 Hz* (PC text mode) *Resolutions other than 1280 1024 are scaled to full-screen. User Control 2D/3D; 3D Mode; Stereo Reverse on/off Display Area 35.9 cm (W) ϫ 28.7 cm (H) Pixel size 0.2805 mm (H) ϫ 0.2805 mm (V) Display Colors 16.7 million (24-bit color, 8 bits/color) Viewing Distance 71.1 cm ϩ/Ϫ10.2 cm is one bright line for every two columns of pixels. The lines Figure 1. Principle of parallax barrier autostereoscope are spaced such that a viewer sitting at the average viewing (From http://astronomy.swin.edu.au/ෂpbourke/ distance from the display sees all of the light lines through stereographics/lenticular/). the odd columns of pixels with the left eye and the same set of light lines through the even columns of pixels with the right eye. In this way, the illumination plate actually plays the role of the required parallax barrier. Shown in Figure 2 is the geometry of the DTI monitor, where S is the distance parallax barrier (Okoshi, 1976; Pastoor and Wöpking, 1997). between two light lines or the pitch of the parallax barrier; The lenticular plate is designed and placed in front of D is the distance between the LCD display plane and the the parallax stereogram in such a way that a viewer’s left illumination plate; P is the pixel width of the monitor; e is eye and right eye can only respectively see the left image the eye base and d the viewing distance perpendicular to the and right image to achieve the stereo effect. As for the monitor plane. As a result of this design, the monitor is 2D autostereoscopic monitor used in this study, it is imple- and 3D switchable. Namely, when the light lines are turned mented by a different technique as will be addressed below. off, the monitor can be used as a common 2D monitor, which we regard as an important feature in order to display ordi- DTI Autostereoscopic Monitor nary computer programs that are not specifically designed for autostereoscopic applications. A summary of the main The autostereoscopic monitor 2018XL made by DTI is used specifications of the DTI monitor used in this study is listed in this study. The parallax barrier is formed by the so-called in Table 1 (Dimension Technologies, Inc., 2004). parallax illumination technique (Eichenlaub, 1993). As shown in Figure 2, an illumination plate is placed behind and spaced apart from the Liquid Crystal Display (LCD) Viewing Geometry display plane. It produces a large number of thin, bright The principle of the autostereoscopic display requires that vertical illuminating lines with dark space in between. There a viewer stays in a certain range relative to the monitor to acquire stereo effect. This section studies the viewing geometry through theoretic reduction. As shown in Figure 2, the geometry of similar triangles yields the following re- lationships (Eichenlaub, 1993): P e ϭ (1) D d ϩ D 2P d 2P(d ϩ D) ϭ or S ϭ . (2) S d ϩ D d Combining Equations 1 and 2 produces e S ϭ (3) d 2D where S is the distance between two light lines or the pitch of the parallax barrier; D is the distance between the LCD display plane and the illumination plate; d the viewing distance perpendicular to the monitor plane; and e is the eye base. Equation 3 reveals the basic relationship in the (paral- lax barrier based) autostereoscope. It shows that the ratio of the eye base and the viewing distance needs to be equal to a constant related to the monitor manufacture to obtain the best stereo effect. This constant essentially is half of the ratio between the pitch and the illumination plate’s dis- tance to the monitor. To achieve