3D Reconstruction and Auralisation
of the “Painted Dolmen” of Antelas
Virtual Reality is a very active topic of research. A large number of applications of this type of technology can be found in areas as varied as the automotive industry, computer games, industrial training and prototyping, aeronautics, medicine, archaeology, architecture and tourism [1-3]. The European Network of Excellence-INTUITION on this topic joins together more than 58 partners.
Most of the effort in the design and development of VR systems has normally been directed at providing a visually realistic experience to the user. However, whilst vision is undoubtedly our predominant sense, the feeling of immersion in a Virtual Environment can be significantly improved by taking our other senses into account as well. Among them, hearing clearly stands out as the most important for the enhancement of VR experiences.
The focus of this work is precisely on the combination of visual and audio immersion; in other words, the reconstruction of a real-world environment through the development of a 3D model which allows the user to see and hear as if she was really there. This requires not only recording the environment’s actual visual and acoustic properties and integrating them into the model but also tracking the user’s movements and updating the audiovisual scene accordingly in real-time.
The Anta Pintada (painted dolmen) of Antelas was deemed an excellent case-study for this work. Among the numerous remains from the Neolithic period which can be found in the Vouga valley region, this chamber tomb stands out for its extraordinary archaeological value, mainly due to the unique colour drawings found in its interior . Extremely fragile (a considerable part was irremediably lost through exposure to light in early archaeological campaigns), they require strict conservation measures, including severe restrictions to visitor access. This problem – reconciling heritage conservation with the need to provide public access – is by no means exclusive of this particular site. In some cases, the solution has involved building replicas [5, 6]. A less radical, more affordable alternative is offered by the development of VR models. These can also be invaluable in the promotion (especially through the Internet) and museological presentation of a site. The heritage conservation authorities responsible for the Anta Pintada are very keen on investing in these areas.
Additional motivation for choosing the Anta Pintada to test a VR model integrating audio is provided by the emergence of Acoustic Archaeology : there is growing scientific interest in studying the acoustics of ancient man-made structures. Intriguing acoustic properties have been found in many of them; there is a suggestion that those properties might have been deliberately engineered. The suggestion is particularly strong for Neolithic passage-graves (i.e. composed of a corridor and a burial chamber) such as this one .
The 3D Laser Range Scanner prototype used in this work is based on a 2D scanner (SICK LMS 200 laser unit) fixed on a tilt unit to allow rotation. The pan and tilt information are synchronised to produce a spherical representation of points . In May 2006, the prototype was used to acquire 3D information from the “Anta Pintada de Antelas” a Neolithic chamber tomb located in Oliveira de Frades, listed as Portuguese national monument (see Figure 1).
In-situ data acquisition in May 2006
Two views of the complete Anta model obtained by aligning a set of 9 point clouds.
The first step to calculate sound reflections in the 3D model is to work out the position of the virtual sources associated with each triangle. The second step is to check their “visibility”, i.e. whether the line between virtual source and listener intersects the corresponding triangle . The following figure shows the location of the visible virtual sources (represented by grey spheres) corresponding to first-order reflections in two different models. Source and listener are represented respectively by a sphere and a head.
Figure 9: Position of the virtual sources: two examples
The acoustic stimuli at a listener’s eardrums are influenced by the complex interaction between the sound waves and the listener’s torso and head. This interaction is strongly dependent upon the direction of arrival of the sound wave. For each angular position of the sound source relative to the centre of the head (usually specified by two angles: azimuth and elevation), it can be described by a pair of HRTF (Head-Related Transfer Functions ) – one for each ear. Usually, a discrete set of HRTF is defined for regularly distributed values of azimuth and elevation. The HRTF capture the main cues on source localisation, provided by the differences in sound intensity and arrival time between ears, known as Interaural Time Difference (ITD) and Interaural Intensity Difference (IID).
Demonstrations: User in VR Setup, example with (left) a synthetic model and (right) the Anta model
authors wish to thank the City Council of Oliveira
de Frades, for granting them access to the Anta Pintada,
and especially Filipe Soares
(City Council /
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