Textured model of the Temple of Augustus and Livia (Vienne, Isère)

Context

The Temple of Augustus and Livia is found in the town of Vienne in the département of Isère. It stood on the west side of a Forum decorated with majestic porticoes, commemorative monuments and imperial statues. Converted into a Christian church at the beginning of the middle ages, it hosted the Jacobin Club during the Revolution before becoming a court house, a museum and finally a library at the beginning of the 19th century. These successive and varied uses were the drivers of its remarkable preservation. In 1850, Prosper Mérimée, protector of French heritage in his capacity as Inspector General of Historic Monuments, commissioned its restoration to its original condition by attempting to reverse the changes undergone over two millennia.

The problem

The objective of this project was to produce a 3D textured model of the object which would meet the needs of the architects, archaeologists and conservationists, for whom the conservation of heritage objects is a significant challenge and who would all be major users of the results.

Setting of the project

• Roman temple built around the 1st or 2nd century BC.

Objectives

• Architectural representation of the monument
• “As -built” and “as-is” modelling
• Assess the accuracy of the resulting 3D textured model

Requirements

• Textured 3D model of the temple as it is today
• Subject of research

Constraints

• Complex architecture
• Monument includes numerous damaged areas

Our response

We decided to use laser scanning to meet the requirement. This revolutionary new tool enabled us to provide a high density survey in record time.
We carried out research into possible improvements in the 3D textured model:

• Architectural representation of the object: all modelling requires the object to be broken down into simple components. In matters of architectural heritage, this process must respect the conventions of architecture. Thus each component must relate to an expression in the vocabulary of architecture. Moreover, the modelling process for each component must also respect the logic of construction; for example, relevant shapes (that is, objects with a consistent profile) are modelled by extrusion of their profiles.

• “As is” model of the object (existing situation at the time of survey) : 
  • The “as-built” model is an idealised representation of the object as it was immediately after construction. In archaeology, this model is used to mask the effects of weathering of the stone and the minor defects resulting from the passage of time.
  • The “as is” model is the most faithful possible representation of the object as it exists today. It emphasises the areas ruined over time and thus enables direct measurement on the model in order to plan renovations.

The transition between “as-built” and “as-is” models is achieved by means of Boolean operations. The as-built model is “hollowed out” to the as-is state, thus representing the damage since the original construction.

Thanks to this stage of the process, we provide a model which is faithful to reality and is useful especially in order to visualise the damaged areas where renovation is needed. In addition, this level of detail is sometimes essential for texturing the model using photographs taken on site. Oriented photographs are projected onto the model such that the result is perfectly faithful to the present-day reality.

Study of accuracy

This project on the Temple of Augustus and Livia also provided the opportunity to assess the accuracy of the resulting 3D model. Two methods were adopted:

• the first uses the inspection tool (Compare/Inspect) of 3DReshaper. This works by comparing the 3D model to the point cloud; the resulting discrepancies are shown by means of coded colours;
• the other enables the model to be checked in terms of textures. Check points are measured on the object using field survey methods. These measurements must be distributed uniformly over the model and must be sufficiently numerous for the results to be representative of the model as a whole. The same check points are measured on the model. The coordinates are compared and standard deviations are computed. The overall accuracy of the model proved to be of the order of 2 to 3 cm.

The study of the conventions of architecture and the breakdown of the object into simple shapes enables a correct representation of the object from an architectural point of view. It provides the opportunity to associate each component with a corresponding method of modelling and to emphasise the similarities between different sub-objects. From these observations flows a refined survey procedure which results in time savings at the modelling stage. The transition between the “as-built” and “as-is” models by means of Boolean operations confines the meshing process purely to the degraded areas, thus keeping file sizes to a minimum. Use of this method guarantees an accuracy of 2 to 3 cm over the whole model, which can be checked throughout the modelling process by means of 3DReshaper’s Compare/Inspect tool.

Deliverables

• Faithful representation of the structure “as is”, for analysis and diagnostic purposes: 3D model in AutoCAD DWG 3D format and 3D textured model (true RGB) for 3ds Max
• 3D animation of the model in the form of a film together with VRML file, for presentation and communication purposes.