Digitization of 3D Objects for Virtual Museum

Transcription

1 Digitization of 3D Objects for Virtual Museum Yi-Ping Hung 1, 2 and Chu-Song Chen 2 1 Department of Computer Science and Information Engineering National Taiwan University, Taipei, Taiwan 2 Institute of Information Science Academia Sinica, Taipei, Taiwan hung@csie.ntu.edu.tw, song@iis.sinica.edu.tw Abstract. Current approaches to digitizing 3D objects for virtual museum can be roughly divided into two categories. One is geometry-based, and the other is image-based. The major advantage of the geometry-based digitization is that the reconstructed 3D models can provide better interactivity for the users to manipulate the 3D virtual objects. Unfortunately, given a 3D artifact, it is usually very expensive, if not impossible, to construct a fine enough 3D model that can generate photo-realistic rendering results. Therefore, we adopt the image-based approach when digitizing 3D artifacts for museums. The digitization results are known as object movies, which are in fact interactive movies for displaying 3D objects. The major drawback of the image-based approach is that it does not directly create a 3D geometric model for the 3D artifact, and hence it is not easy to insert the 3D artifact into a 3D environment for virtual exhibition. With our method, we can insert an object movie, in a geometrically consistent way, into a 3D environment that is represented either by a 3D scene model or by a 2D panoramic image. We have applied this technique to develop a stereoscopic kiosk system for virtually exhibiting 3D artifacts of the National Palace Museum. 1. Introduction Current approaches to digitizing 3D objects for virtual museum can be roughly divided into two categories. One is geometry-based, and the other is image-based. The geometry-based approach first constructs geometric models of 3D objects, and then maps textures onto object surfaces. Usually, the textures are acquired by taking photographs of the 3D objects, in order to make the rendering results look more photo-realistic. There are many techniques for building geometric models of 3D objects, which include laser scanning or structured lighting, model building by using corners or lines in photos, shape from silhouette and image correspondences, probing or 3D touch, and manual creation.

2 Using the geometry-based approach to digitizing a 3D artifact for virtual exhibition, one not only needs to take images of the artifact from various directions for creating texture maps, but also needs to build a 3D geometric model of the artifact by using either automatic 3D digitizing devices (such as laser scanners) or photo/manually editing tools (such as PhotoModeler and 3ds Max). Notice that, although many laser scanners and other active-lighting techniques claim that their 3D digitization processes are automatic, a heavy load of interactive manual post-editing and verification (which include outlier elimination, registration, polygon-mesh generation and reduction, and so on) is almost always required for building an acceptable 3D model with photo-realistic rendering effect. Another possibility is to adopt the image-based approach if our main purpose is for interactive visualization of 3D museum artifacts (neither for replica reproduction, nor for 3D animation). The digitization technique we use is widely known as object movies (OM), where each OM consists of a set of images of an object taken from various viewing directions. The images of an OM can be acquired by using an apparatus consisting of a camera and two controllable rotation axes, as shown in Figure 1. Usually, two-axis OM s are acquired in a regularly sampled manner that, for each tilt angle, a fixed number of images are taken along the pan direction, as illustrated in Figure 2. If n images are taken for each tilt angle, and m pan angles are sampled, we have a total of m n images in an OM. Typically, an OM constructed by using 360 images (36 10) can be browsed smooth enough, and this image number is what we use in our current projects. The critical advantage of adopting OM is that it ensures photo-realistic visual effect in a relatively inexpensive way, compared to the geometry-based approach. However, it suffers from the difficulties on integrating OM s of 3D artifacts with a 3D environment for virtual exhibition, because no 3D geometric models have been reconstructed for displaying 3D artifacts. In the next section, we show how OM s can be inserted into a panorama in a geometrically consistent way. 2. Augmenting Panoramas with OM s A popular image-based representation of a 3D environment is panorama [1]. Many software tools have been developed for constructing a panoramic image by stitching multiple overlapping images taken from approximately the same projection center. However, until now, none of the software tools has the capability to insert an OM into a panorama in a geometrically 3D consistent way. The purpose of inserting OM s into a panorama is to make the virtual exhibition more visually appealing and the virtual navigation smoother and seamless. We are the first team to propose a systematic

3 method that can augment a panorama with OM s in a geometrically consistent way [2]. Our method for augmenting panoramas with OM s allows the users to set a 3D coordinate system in a 2D panorama by using the method presented in [3]. The users simply have to draw such a coordinate system in a panorama with his visual perception, and then our method can automatically compute the 3D geometrical transformation between this coordinate system and the panorama coordinate system defined at the panorama projection center. In addition, our method can generate convincing shadows for an OM by using the silhouettes in images of the OM. Figure 3 shows some composite examples of the interactive browsing system constructed by using our method. More details of this technique can be found in [2]. By augmenting a viewer-centered panorama with object-centered OM s, virtual exhibition of museum artifacts can be realized in a photo-realistic way. This approach is, to our knowledge, the most efficient and inexpensive way for constructing an interactive photo-realistic virtual exhibition of real museum artifacts. With this approach, no complex 3D geometric models are required. We have applied this technology to construct some virtual exhibitions for National Palace Museum and National Museum of History. We have also applied this technique to develop a stereoscopic kiosk system for interactive virtual exhibition of 3D artifacts. 3. Conclusions We have briefly introduced our techniques developed for interactive virtual exhibition of 3D museum artifacts, and explained the advantages of our approach. Based on this approach, we have recently carried out a pilot project on interactive virtual display of five famous 3D artifacts of the National Palace Museum. The results will be demonstrated in F@imp References [1] S. E. Chen, QuickTime VR an Image-Based Approach to Virtual Environment Navigation, Proceedings of SIGGRAPH 95, pp , [2] Y. P. Hung, C. S. Chen, Y. P. Tsai, and S. W. Lin, Augmenting Panoramas with Object Movies by Generating Novel Views with Disparity-Based View Morphing Journal of Visualization and Computer Animation -- Special Issue on Hallucinating the Real World from Real Images, Vol. 13, pp , September [3] C. S. Chen and W. T. Hsieh, Composition of 3D Graphic Objects and Panoramas

4 Proceedings of International Conference on Artificial Reality and Tele-existence, ICAT2000, pp , (a) (b) Figure 1. The images of a two-axis object movie can be easily acquired by using an apparatus consisting of a turntable and a rotation arm. (a) An apparatus built by Kaidan. (b) An apparatus built by Texnai. Figure 2. A two-axis object movie consists of a set of object-centered images taken by rotating the camera with respect to the pan and tilt axes.

5 (a) (b) (c) (d) (e) Figure 3. (a) A panoramic image of a real scene in the Nation Museum of History in Taipei. (b) A de-warped view of the panorama. (c)(d) Augmenting the panorama with an OM of a replica of a Qin Terra-cotta Warrior. (e) Augmenting the panorama with an OM of the Cockscomb Pot and an OM of a Tang Tri-color (tri-color pottery of the Tang Dynasty).