影像式三維重建於水下考古之先期研究

A preliminary study on image-based 3D modeling for the application of underwater archaeology

水下考古對於文化資產保存及歷史研究相當重要，然國內目前在數位典藏技術上大都以二維靜態照片或動態視訊進行記錄，欠缺三維視覺化與測量的功能，無法提供陸上研究人員對水下文化資產之三維空間分布有更具象的感受與量化的數據。影像式三維重建技術可提供互動式的三維仿真模型與可量測的空間資訊，有利於水下考古研究。因此本先期研究之目標即在探討如何應用此技術於水下文化資產之三維模型重建，以輔助水下考古相關研究之進行。由於水下和陸地上攝影環境不同，水下攝影之困難度較高，且會受到各種環境因素影響，例如照明、洋流強度、懸浮物與混濁度等問題，都會影響影像和三維模型的品質。為了將相關變數降低以探討水下三維模型重建問題，本先期研究以室內拖航水槽作為測試環境，將水槽底部現有的鐵架與石頭模擬為水下文資標的物進行三維模型重建。主要研究項目為水下相機率定與測量成果之精度分析，並提出水下三維仿真模型重建之程序，最後則為評估其應用在海洋水下考古之可行性。研究成果顯示在水下環境攝影其鏡頭之焦距變化與水之折射指數1.33相同，而所重建之三維模型絕對誤差可在1公分以內，已可滿足水下考古之需求。

Underwater archaeology is an essential part of cultural heritage preservation and historical study. However, current recording methods for digital archiving mostly use 2D static pictures or video. They lack 3D visualization and measurement functionalities and cannot provide a 3D spatial concept and quantization data to a researcher who cannot enter the water. Conversely, image-based 3D reconstruction technology can offer an interactive photo-realistic 3D model and measurable spatial information that is beneficial to underwater archaeological study. Thus, the goal of this preliminary study is to evaluate its capability for underwater cultural heritage 3D model reconstruction. Due to the fact that the underwater environment is different to that on land, the level of difficulty for acquiring photographs is higher when under the water. The quality of image and reconstructed 3D model will therefore be affected by several environmental factors, such as illumination, water flow speed, suspended matter, turbidity, etc. For the purposes of reducing these and related factors, during this investigation of underwater 3D model reconstruction, this preliminary study adopted an indoor towing tank as the test environment. We simulated cultural heritage targets for 3D model reconstruction using rocks and mental-frames at the bottom of towing tank. The major tasks include underwater camera calibration and accuracy analysis. Here we present 3D model reconstruction procedure for underwater environment and discuss its feasibility for underwater archaeology. Experimental results demonstrate that the lens’focal length was changed according to the water's refractive index, i.e. 1.33, and the absolute error for the reconstructed 3D model was less than 1cm, which is sufficient for the requirement of underwater archaeology.