3D Data Acquisition and Processing 91 Hubert Mara – Robert Sablatnig Evaluation of 3D Shapes of Ceramics for the Determination of Manufacturing Techniques Abstract: Motivated by the requirements of today’s archaeologists we are developing a system for the docuPHQWDWLRQ RI GDLO\ ÀQGV RI H[FDYDWLRQV XVLQJ ' DFTXLVLWLRQ 7KH PRVW ZLGHVSUHDG ÀQGV DUH IUDJPHQWV RI ceramics called sherds. We have shown in previous work the acquisition and documentation of these sherds using 3D scanners based on the principle of structured light. The traditional documentation of sherds is EDVHG RQ WKH H[WUDFWLRQ RI WKH SURÀOH OLQH ZKLFK LV D KRUL]RQWDO LQWHUVHFWLRQ WKURXJK WKH RULHQWDWHG VKHUG 2XU V\VWHP DXWRPDWLFDOO\ HVWLPDWHV WKH SURÀOH OLQH E\ XVH RI FRPSXWHUL]HG PHWKRGV LQVSLUHG E\ WUDGLWLRQDO DUFKDHRORJLFDO ZRUN %DVHG RQ WKH PHWKRGV XVHG IRU HVWLPDWLRQ RI WKH SURÀOH OLQH ZH GHPRQVWUDWH D PHWKRG for determination of ancient manufacturing techniques, which is important to determine the technological advancement of an ancient culture. As ceramics were generally manufactured on rotational plates, the proÀOH OLQH LV WKHRUHWLFDOO\ LGHQWLFDO IRU D FRPSOHWH V\PPHWULF YHVVHO 'XH WR WKH PDQXIDFWXULQJ WHFKQLTXH WKH V\PPHWU\ LV EURNHQ DQG WKHUHIRUH ZH FDQ GHWHUPLQH LW E\ HVWLPDWLQJ WKH YDULDQFHV RI WKH VKDSH RI WKH SURÀOH line. For the proposed method we use complete vessels, because sherds of excavations of living places have EHHQ GXPSHG DQG UH XVHG DV ÀOOLQJ PDWHULDO IRU ÁRRUV DQG ZDOOV 7KHUHIRUH VKHUGV YLUWXDOO\ QHYHU UHDVVHPble a complete vessel and therefore no real ground truth is known. As archaeologists are also excavating burial places where individual unbroken ceramics or complete sets of sherds are found, our method can be applied on, but is not limited to, individual vessels. Results for traditionally manufactured new vessels and DQFLHQW YHVVHOV DFTXLUHG GXULQJ RXU ÀHOG WULS WR WKH H[FDYDWLRQV LQ WKH YDOOH\ RI 3DOSD 3HUX DUH JLYHQ DQG the applicability of the method in archaeology is shown. Introduction Documentation of ceramics is a main task in archaeRORJ\ EHFDXVH FHUDPLFV DUH WKH PRVW FRPPRQ ÀQGV used and produced in large numbers by humans for several thousands of years. Archaeologists use analysis of ceramics (LEUTE 1987) on a daily basis to reveal information about the age, trading relations, advancements in technology, art, politics, religion and many other details of ancient cultures. Therefore we are developing an automated system for ceramics documentation to help archaeoloJLVWV GRFXPHQW WKHLU ÀQGV LQ DQ HIÀFLHQW DQG DFFXUDWH way, which can be used for further (computerized) research. The basis of documentation of ceramics is a manually drawn horizontal intersection, which is FDOOHG WKH SURÀOH OLQH LEUTE 7KH SURÀOH OLQH is the longest elongation around – or cross-section WKURXJK ² WKH ZDOO RI D FHUDPLF GHÀQHG E\ LWV URWDtional axis (axis of symmetry). The term rotational axis relates to the fact that rotational wheels (plates) have been used for thousands of years for manufacturing ceramics. This assumption can be made HVSHFLDOO\ EXW QRW RQO\ IRU GDLO\ ÀQGV RQ DUFKDHR- logical excavations. Therefore we have based our work on the rotational axis to orient a ceramic or LWV IUDJPHQW WR HVWLPDWH WKH SURÀOH OLQH DV LV GRQH manually by drawings. This work presents processing of 3D models of ceramics beyond estimating WKH SURÀOH OLQH EHFDXVH H[SHULPHQWV IRU HVWLPDWLRQ RI PXOWLSOH SURÀOH OLQHV DW UDQGRP SRVLWLRQV RI unbroken ceramics have shown notable deviations ! PP OHDGLQJ WR TXDOLW\ FULWHULD IRU FODVVLÀFDtion and determination of manufacturing techniques, which is another important question for archaeologists. This question is of even more interest for the Americas where – unlike the Mediterranean Area – no written sources about vanished civilizations exist. As the ceramics are found in tens of thousands at virtually every excavation, these drawings require a lot of time, skill and manpower of experts. Therefore we are assisting archaeologists in interdisciplinary projects (KAMPEL / SABLATNIG 1999) by using an automated system for acquisition and documentation of ceramics using a 3D scanner based on the principle of structured light (DEPIERO / TRIVEDI LISKA 1999). 92 Layers of Perception – CAA 2007 First we describe the acquisition process of ceramics, followed by the description of the symmetry analysis including results of synthetic and real ceramics. Finally a summary and an outlook is given. Acquisition The challenging tasks for developing a documentation system for archaeology are to build a system which is accurate, portable, inexpensive, easy-touse and robust for all kinds of climate, which can range from desert to jungle to arctic. This means technologies like computer tomography and other laboratory equipment are often unsuitable for the daily work of archaeologists - especially not for ceramics. As photography has already proven its reliability for archaeology, we chose to use a camera and a light-source for 3D-acquisition. For recent work we use 3D scanners from the Konica-Minolta Vivid product range (MARA MARA / HECHT IN PRESS), because of their resolution (< 0.1 mm), which meets the requirements given by archaeologists for their documentation. Fig. 1(a) shows one of the vessels acquired and Fig. 1(b) shows a manual drawing RI D SURÀOH OLQH Fig. 1(c, d) shows the setup of our 3D scanner from recent experiments at the excavations in the valley of Palpa, Peru (REINDEL / ISLA 2001). Fig. 1(c) shows the triangulation principle (MARA 2003) using a laser (bottom) and a camera (top) with a known distance and orientation. Additionally the WXUQWDEOH ² DOVR VKRZQ LQ WKLV ÀJXUH ² LV XVHG WR JHW a complete 3D model of the ceramic. The number of 3D scans depends on the complexity of the ceramic and typically ranges from two scans for sherds up to eight scans for vessels. The 3D scans are registered using the method proposed by TOSOVIC (2002) to reassemble a complete 3D model. After the registration, noise from dust and other objects like holding devices (e.g. clamps or plasticine) are removed from the 3D model. Then the orientation is estimated based on the assumption that ceramics are rotationally symmetric objects (MARA 2003), because they were generally manufactured on rotational plates. The principle of our orientation PHWKRG LV ÀWWLQJ RI FLUFOH WHPSODWHV GANDER / GOLUB / STREBEL 1994). In comparison to other computerized, but manual methods (MELERO ET AL. 2004) our orientation method can be used fully- and semiautomatically (LETTNER ET AL. 2006). Furthermore our system is capable to store the 3D model and further archaeological information (e.g. description, photographs, etc.) in a database. For solving the puzzling problems of other – typically industrially manufactured – rotational objects, other methods (POTTMANN / RANDRUP WILLIS ORRIOLS 2004) can be applied. Once an orientated 3D model is obtained, a vertical cross-section is estimated using the point of maximum height of the 3D model. 7KLV FURVV VHFWLRQ LV WKH VR FDOOHG SURÀOH OLQH ZKLFK concludes the traditional archaeological documentation. Fig. 4 shows a result for an automatically HVWLPDWHG SURÀOH OLQH Symmetry Analysis $V VXFK D UDWKHU VLPSOH WZR GLPHQVLRQDO SURÀOH line – as shown in Fig. 4 ² GRHV QRW UHÁHFW DQ\ LQformation about the manufacturing quality leading to the manufacturing technique, we decided to enhance our system by giving the archaeologists a tool to gather further information about the acquired 3D model. Even though the Nasca ceramics may not have been produced on rotational plates – as asVXPHG E\ RXU PHWKRG ² WKH HVWLPDWLRQ RI WKH SURÀOH line is possible. But like manual orientation, varia- Fig. 1. (a) Photograph of vessel 2801-V3 and its twin found near Palpa, Peru and (b) a manual drawing (CARMICHAEL 1986) of a vessel having the same shape. (c) Konica-Minolta Vi-9i 3D-scanner projecting a laser plane (bottom arrow) onto a sherd, while the camera (top) acquires the projection of the laser. Having a well-known distance and orientation of the ODVHU SODQH DQG WKH ÀHOG RI YLHZ RI WKH FDPHUD WKH GLVWDQFH UDQJH FDQ EH HVWLPDWHG G 'HWDLO RI WKLV VHWXS VKRZLQJ D sherd mounted with plasticine on the turntable, which is used for controlled acquisition of all sides of an object. 3D Data Acquisition and Processing )LJ $XWRPDWLFDOO\ HVWLPDWHG SURÀOH OLQHV DQG IURQWYLHZ RI WKH 1DVFD VKHUG WLRQV RI WKH RULHQWDWLRQ RI WKH SURÀOH OLQH WDNH DSproximately twice the time than for ceramics manufactured using rotational plates. Therefore we had to investigate the question of manufacturing technique and quality of the symmetry of Nasca vessels to determine these variations. Furthermore there is an ongoing discussion between archaeologists about the existence of rotational plates for the manufacture of ceramics in South America. The general opinion is that in this region the wheel was not invented, therefore ceramics were produced without a rotational plate (CARMICHAEL 1986 RQ WKH RWKHU KDQG WKHUH is evidence that rotational plates were used (WIECZOREK / TELLENBACH 2002). In general technological advancement is determined by archaeologists from ceramics which have been produced either on slow or fast turning rotational plates. As we use structured light as 3D acquisition method, we cannot make assumptions about the internal structure of a ceramic as others have (WIECZOREK / TELLENBACH 2002), but we can estimate the surface with a high resolution (0.1 mm). Therefore we can analyze the symmetry and estimate features like deviation of real surfaces in respect to a perfectly symmetrical surface. Such features can help archaeologists to decide about the technological advancements of ancient cultures. )LJ 93 As archaeologists also excavate burial places, where unbroken ceramics or complete sets of sherds are found, we are presenting a method to determine the manufacturing process of ceramics, which reveals information about the technological advancement of an ancient culture. Furthermore, this method can be applied, but is not limited to, unbroken or reconstructed vessels. To begin our investigation and answer questions about the manufacturing process of ceramics, we chose to use two modern pots which were manufactured in a traditional way. Therefore this data can be interpreted as mixture between synthetic and real data, because we used real objects. However, unlike real archaeological fragments, we know how they were produced. In addition, we decided to use the method for ÀQGLQJ WKH RULHQWDWLRQ RI D VKHUG MARA / KAMPEL :H EHJDQ ZLWK WKH SURÀOH OLQH ZKLFK FDQ be estimated in a similar way to the process used with sherds. The difference is that for complete vessels the bottom plane can be used for orientation, because it is the counterpart to the rotational plate, ZKLFK GHÀQHV WKH RUWKRJRQDO D[LV RI URWDWLRQ :H HVWLPDWHG PXOWLSOH SURÀOH OLQHV ZKLFK FDQ EH overlaid by transforming them into the same coordinate system, where the y-axis equals the rotational D[LV 7KHUHIRUH WKH GLVWDQFH EHWZHHQ SURÀOH OLQHV FDQ D F /RQJHVW SURÀOH OLQHV DQG E G PXOWLSOH SURÀOH OLQHV RI PRGHUQ FHUDPLFV PDQXIDFWXUHG LQ D WUDGLWLRQDO ZD\ which are supposed to be identical. 94 Layers of Perception – CAA 2007 be estimated. Fig. 3 VKRZV WKH ORQJHVW SURÀOH OLQH DQG PXOWLSOH SURÀOH OLQHV FRPELQHG ZLWK WKH VLGH view, as archaeologists show such vessels in their GRFXPHQWDWLRQ ,Q WKH FDVH RI WKH PXOWLSOH SURÀOH lines, we have estimated that the distance between WKH SURÀOH OLQHV GLIIHUV DQG WKHUHIRUH WKHVH SRWV DQG WKHLU SURÀOH OLQHV DUH XQLTXH 7KH PD[LPXP GLVWDQFH EHWZHHQ WZR SURÀOH OLQHV RI WKH ÀUVW SRW ZDV 9.8 mm, whereas for the second pot it was 21.2 mm. ,Q WKH PXOWLSOH SURÀOH OLQHV VKRZQ LQ Fig. 3(b, d), WKH GLVWDQFH EHWZHHQ SURÀOH OLQHV PHDVXUHG SDUDOOHO WR WKH [ D[LV LV QRW HTXDO ,I WKH SURÀOH OLQHV ZHUH (fragments). The estimation of the axis is shown in Fig. 4(b, d). The numeric results for the axis are that they have a minimum distance of 4 mm toZDUGV HDFK RWKHU DQG WR WKH D[LV GHÀQHG E\ WKH bottom plane. Furthermore the angles between the axes differ between 5° and 7°. Using the rotational axis of the lower and upper fragment, we repeated the estimation of the proÀOH OLQHV ZKLFK DUH VKRZQ LQ Fig. 5. The maximum GLVWDQFH EHWZHHQ WKH SURÀOH OLQH DUH PP IRU WKH upper and 2 mm for the lower part. Therefore the ÀUVW FRQFOXVLRQ LV WKDW WKH XSSHU DQG ORZHU SDUWV Fig. 4. (a, c) Top-view and (b, d) side-view of the horizontal cross-sections – the level of darkness corresponds to the KHLJKW 7KH D[LV RI URWDWLRQ IRU WKH ORZHU DQG XSSHU SDUW LV VKRZQ DV D EODFN OLQH GHÀQHG E\ WKH FHQWUHV RI WKH FRQFHQWULF circles (shown as dots). parallel, this would mean that the pots have an elliptic (horizontal) cross-section. As it appears, the asymmetry is more complex. Therefore, we chose to analyze the pots slice-by-slice along the rotational axis, presumed as orthogonal to the bottom plane. Fig. 4 (a, c) shows horizontal intersections, which have been applied with a distance of 10 mm along the rotational axis. The distance of 10 mm corresponds to the manufacturing process, which has left traces in the form of rills as seen along the right hand sides of Fig. 3(b, d). These rills are spaced 10 mm apart, which corresponds to the width of the ÀQJHU RU WRRO XVHG WR µJURZµ WKH SRW DORQJ WKH D[LV of the rotational plate. The intersections at 160 mm and 170 mm in height have been discarded, as they intersect the ”shoulder” of the pot with a very low angle (< 5°), resulting in an intersection having a non-representative, random curvature. Dividing ceramics into sections by characteristic points (like the ”shoulder”) is carried out by archaeRORJLVWV IRU FODVVLÀFDWLRQ 7KHUHIRUH ZH FKRVH WR analyze the object segmented into a lower and an upper part. This means we have two fragments where axis estimation can be applied as for sherds do have a different axis of rotation, which means that these parts have been produced separately and combined without the use of the rotational plate. We can conclude that, based on the different GHYLDWLRQ RI WKH PXOWLSOH SURÀOH OLQHV VKRZQ LQ Fig. 5, the upper part is of lesser quality than the lower part. This leads to the conclusion that these parts have been made by potters with different experience and/or on a slower rotational plate. Conversely, the deviation in the upper part of up to 7 mm compared to less than 2 mm of the lower part shows that a faster turning rotational plate has been used and that more experience was required for manufacturing the upper part. From the differing angle between the axis of rotation based on the bottom plane compared to the axis of rotation of the upper and lower fragment, we can conclude that either the bottom has been post-worked or the pot was contorted before beLQJ ÀUHG LQ WKH RYHQ Even when correcting the axis for the parts of the object, the horizontal intersections are not perfectly circular. The horizontal intersections 3D Data Acquisition and Processing )LJ $[LV RI URWDWLRQ DQG PXOWLSOH SURÀOH OLQHV RI WKH XSSHU SDUW D H ORZHU SDUW F J DQG E G I K WKH ORQJHVW SURÀOH lines of the parts of the objects. are elliptic. Therefore we estimated the direction of the major and minor axis of the ellipses. We estimated that the minor axis has the same direction as the orientation of the handle. This means that the symmetry of the pots was broken, when the handle was attached and the pots were still wet. Fig. 6 show the pots intersected by a SODQH GHÀQHG E\ WKH FHQWUH RI JUDYLW\ RI WKH SRW and the direction of the major axis of the ellipses. The angle between the minor axis and the handle of the pot was 7° and 14° for the second pot. Furthermore Fig. 6 shows an example from the excavations in the valley of Palpa, Peru. :H DGGLWLRQDOO\ FRQFOXGH WKDW WKH HOOLSVHV ÀWted (GANDER ET AL. 1994) to the horizontal crosssections can be used as an additional feature. Therefore the distance between the foci of the ellipse is estimated. Ceramics with a distance con- )LJ 95 verging towards zero (circular cross-sections) are of higher quality. The proposed method has also been tested on 17 real vessels (MARA 2006) dated to the Nasca period (CARMICHAEL 1986), which were found in the valley of Palpa, Peru (REINDEL 2001). Therefore we could separate these vessels into three classes determined by the symmetry. The vessels (60%) of two of these three classes were not produced on rotational plates. Beside this information – answer to the question of the manufacturing technique – about the use of rotational plates in South AmerLFD WKLV FODVVLÀFDWLRQ LV XVHG E\ DUFKDHRORJLVWV of the German Archaeological Institute (DAI/ .$$. %RQQ IRU UHÀQHPHQW RI WKHLU FODVVLÀFDWLRQ schemes. 3ODQHV RI V\PPHWU\ RI WKH D ÀUVW DQG E VHFRQG REMHFW F IURQWYLHZ DQG WRSYLHZ RI WKH KRUL]RQWDO LQWHUVHFWLRQ with plane of symmetry of vessel 2827-V1 found in the valley of Palpa, Peru. 96 Layers of Perception – CAA 2007 Summary and Outlook References Summarizing the presented work, we can conclude that symmetry analysis can and will be used to estimate quality features of Nasca ceramics and related FHUDPLFV IRU FODVVLÀFDWLRQ DQG DUFKDHRPHWU\ LEUTE 1987). Furthermore it can be used to approximate a ground truth and therefore estimate possible variDWLRQV RI WKH RULHQWDWLRQ RI WKH SURÀOH OLQH IRU PDQXDO GUDZLQJV DQG DXWRPDWLFDOO\ HVWLPDWHG SURÀOH OLQHV 0HDQZKLOH IRU WKH DXWRPDWHG SURÀOHV ZH FDQ estimate the expected error of ceramics which might not have been manufactured on rotational plates. Finally, we can conclude that symmetry analysis can reveal detailed information of the manufacturing process, such as quality requirements and production steps of ancient ceramics. Beside the improvement of existing methods for 3D vision, another important work has recently begun to ensure the intellectual integrity, reliability, transparency, documentation, standards, sustainability and accessibility of the information gathered by the increasing use of 3D scanners. Therefore we are adopting The London Charter (BEACHAM/ DENARD / NICCOLUCCI 2006), which will be a future standard for the use of 3D vision within Cultural Heritage. 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WILLIS, 6WRFKDVWLF ' *HRPHWULF 0RGHOV IRU &ODVVLÀFDtion, Deformation, and Estimation (Providence 2004). 97 Hubert Mara Robert Sablatnig Institute for Computer Aided Automation Pattern Recognition and Image Processing Group Vienna University of Technology Favoritenstrasse 9/183-2 1040 Vienna, Austria mara@prip.tuwien.ac.at KXEHUW PDUD#SLQ XQLÀ LW sab@prip.tuwien.ac.at