Abstract
The Hindu and Buddhist monuments of Angkor in Cambodia are the legacy of highly developed Khmer empires. Well-known for their structural and surface complexity they constitute a great challenge to any attempt towards precise and detailed 3D measurements and modeling. This paper reports about a pilot project using modern techniques of analytical and digital photogrammetry to derive a photorealistic 3D model of one of the very complex towers of the famous Bayon temple of the ancient city of Angkor Thom. The project is based on the use of small format analogue tourist-type photography. Described processing steps include phototriangulation on an analytical plotter, automated image matching for surface model generation and the use of novel point cloud editing and view-dependent texture mapping techniques.
1. Introduction
The Khmer kingdom of Angkor in Cambodia is one of the most remarkable empires in human history. On 75 square miles of fertile plains, north of the modern town of Siem Reap, a succession of 42 kings built during the Angkor Period 802-1432 A.D. about 72 major monuments, temples, palaces, canals with dikes, moats and reservoirs. This extended complex ranks among the most spectacular sites currently listed in the UNESCO World Heritage List. Many of the treasures are now lying in ruins or have been partially restored in the past by various international expert teams. One of the many monuments has been subject to particular attention throughout the years (Preston, 2000, White, 1982): The Temple of Angkor Wat.
Today, after a decade of forced neglect during the rule of the Khmer Rouge, the precious Angkor monuments receive much help again from various countries. Among those is the JSA project (Japanese government team for Safeguarding Angkor), which aims at the conservation and restoration of the Northern Library of the Bayon temple, the Royal Plaza (Prasat Suor Prat and its Terraces) of Angkor Thom and the Northern Library of Angkor Wat (JSA News, 1999). While on sabbatical leave at the Asian Institute of Technology (AIT) in Bangkok in spring 1999 the third author was invited to join the Japanese team of geographers and surveyors from the Keio University, Tokyo as photogrammetrist for a balloon photogrammetry mission over the Bayon temple. The main purpose of this mission was to generate a 3D computer model of the very complex structure of Bayon from small format balloon images. The results of this work will be published elsewhere. During this one week mission the author took a number of amateur photographs of one of the many Bodhisattva-faced towers. This paper reports about the processing of these images and shows the resulting texture mapped 3D model of the tower.
2. The Temple of Bayon
Bayon was the last great temple built in Angkor and remains one of the most enigmatic. Jayavarman VII became king of the Khmer in 1181. After the Cham (ancient Vietnamese) sacked Angkor in 1177 he conquered their homeland Champa and began a frenzy of buildings in Angkor, among which are Preah Khan, Ta Prohm, and Banteai Kdei, all adjacent to the city Angkor Thom, which he also created. Angkor Thom was then the Khmer capital, a four-sided enclosure surrounded by a wall 8 miles long and a vast 100 m broad moat.
Since the old Hindu gods failed to support the Khmers in 1177 he dedicated his largest temple, the Bayon, to Buddha. Today Bayon is considered an interesting mixture of both Hindu and Buddhist elements of style. Bayon is situated right in the center of Angkor Thom. It is an extremely complex structure, which is best revealed by aerial photographs (Figure 1). It is said to represent the Mount Meru of an old Hindu legend. It consists of three levels of platforms and two galleries with spectacular and well-preserved basreliefs, showing the live of the ordinary people and stories from the Hindu mythology.
Figure 1: Aerial image of Bayon, taken from a Hellium balloon
Figure 2: The smiling faces of Bayon (one of 13 non-metric images used for the photogrammetric
processing
A special feature of Bayon are its 54 towers with four large faces on each, pointing in all four geographical directions (such adding up to 216 faces in total, not counting the many more on other objects). These faces on the towers and gateways of Angkor Thom represent Lokesvara, a Bodhisattva from the Mahayana Buddhism, a holy one who stayed at earth to do good work and help people. The origine and purpose of these faces are still under scientific dispute. Their symbolism is not yet untangled and they continue to amaze visitors from all over the world (Figure 2).
3. Data Acquisition
The main goal of the Bayon field campaign was the taking of small format camera balloon images over Bayon for the 3D reconstruction of the complete and utterly complex temple. Balloon images where also taken over the Northern Library of Angkor Wat and over the several Prasat Suor Prat buildings. During the mission a sequence of small format terrestrial images was taken with a Minolta Dynax 500si camera (c = 35 mm) of one of the Bayon towers on the third platform in the north-eastern corner of the temple. The 13 images covering the full horizon were meant as a test of photogrammetric procedures rather than a serious project aiming at a complete recording of the object. Figure 3 shows the arrangement of images, as they were used for bundle triangulation around the tower. Since a 360 degree azimuth coverage was necessary and the light conditions were fairly extreme the production of good, evenly illuminated pictures was practically impossible without artificial lighting, which was not available at the site. Therefore, the images suffer under strong variations of the illuminated and shadow areas. Also, the shadow and light parts will vary from image to image, depending on the time of the day the images were taken. This will cause problems with texture mapping from multiple images, if an even distribution of light is aimed at allover the 3D model. Thus a great deal of our work went into a modification of the standard procedure of texture mapping. We developed a specific technique of view-dependent texture mapping, which is object face oriented and picks for each object face a combination of the best possible corresponding image patches for texture mapping.
Figure 3: Images used for bundle triangulation
