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4. PRINCIPALES RESULTADOS
Photo modelling is not a new concept: it is developed and heavily utilised, especially in the computer science field. Generated 3D models in the literature are referred to in various ways, such as “Cybercity”, “Virtual City”, or “Digital City”. Even though the aim is the same - to build a virtual city of the existing city - in the literature different research projects developed different methods and techniques, and consequently yielded different results. Research in the computer field includes: Singh, S. Jain, K and Mandla, R. (2013); Mohan, S and Murali, S. (2012);
Oh et. al. (2001); Reid, A and Zisserman, A. (2000); and, van den Hengel et. al. (2006). The following section will discuss the research threads that are close to the present study, based on the modelling process.
2.4.1 Multi photo modelling:
2.4.1.1 Geomatics research; a thread of research based on the extraction of a 3D model using methods such as remote sensing, geographical information systems, and Photogrammetric43. The Photogrammetric, a fusion of photographs and laser, “is the most effective solution to create 3D city model. It gives a better result and good accuracy”44.
43 Surendra Pal Singh and others, 'Virtual 3d City Modelling: Techniques and Applications', in International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. 8th 3DGeoInfo Conference & WG II/2 Workshop, 27 – 29 November, (Istanbul, 2013).
44 lbid,. p.85.
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However, the accuracy of this 3D model method and its error margin depend heavily on the resolution of the satellite images. The process is also constrained by the cost of the laser equipment, and the availability of satellite images as some countries restrict aerial flight.
2.4.1.2 Structure from motion (SfM): SfM is based on the idea of extracting a 3D model from a projection of 2D photos. This is a fully automated system based on photo sequencing, during which the camera takes photos of the structure from different angles, taking into an account the need to overlap photos in certain instances (i.e.: edges, corners). The corresponding points in the images are identified (fig.1a). The rays intersecting these points are generated, after which the 3D structure is automatically restructured (fig.1b).
Figure 1 The structure from motion modelling process.
Source: https://www.youtube.com/watch?v=i7ierVkXYa8
Photo Tourism is a system that inspired a new line of research; it involves exploring photo collections in 3D. Goesele et al. (2007) is one example of this kind of research. A multi-view stereo algorithm “capable of computing high quality reconstructions of a wide range of
a. Photos of the structure from different sides. b. Corresponding points in in the photos
c. Redefining the position of these points in a 3d space and forming the shape of the structure.
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scenes from large, shared, multi-user photo collections available on the Internet”45. Structures can be rebuilt by creating depth maps that are then combined into a single mesh.
Figure 2 Modelling using the photo tourism approach. Source: Goesele 2007.
Even though this system produces some good results, it cannot be employed in the context of historical research, as photos are limited and taking new photos is impossible as many historic buildings are demolished or changed.
2.4.2 Single-view modelling techniques
3D reconstruction from a single image must necessarily be through an interactive process in which the user provides information about the scene structure. Such information may be in terms of vanishing points or vanishing lines, co-planarity, spatial inter-relationship of features, surface normal, and camera parameters. Some of the traditional approaches based on shape, shading and texture have complicated user interaction in terms of specifying the inputs.46
Drawing perspectives from photography, deconstructing prospective images. A prospective image is one in which all the parallel lines on the plane lead to a vanishing point. Each image has a unique vanishing point that depends on the height of the camera, the type of lens and the angle of the camera, the type of lens and most importantly the location of the camera.
Depending on the content prospective photos could include 1 point perspective, two points prospective, or three points prospective.
45 Michael Goesele and others, ' Multi-view stereo for community photo collections ', in: The international conference on computer vision. (Rio de Janeiro, 2007).
46 S Mohan and S Murali, 'Multi-View Stereo Reconstruction Technique', in: Image Based 3D Modelling and Rendering from Single View Perspective Images, by S Mohan and S Murali (IGI Global, 2013). 604-619.
Example of the photo tourism modelling results. St. Peter cathedral modelling: its corresponding depth maps and its shaded renderings depth map
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Figure 3 Illustration of parallel lines and the convergence to vanishing points. Source:
<http://mathworld.wolfram.com/VanishingPoint.html>
The latest version of SketchUp includes single modelling techniques under the name of PhotoMatch. PhotoMatch is a feature that allows the researcher to first configure the image’s vanishing points by manually configuring the parallel lines, and then trace over a photograph and model its geometry47.
47 A. CriminisiI. Reid A. Zisserman, 'Single View Metrology'. International Journal of Computer, 40 (2000), 123–148.
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Figure 4 configuring the photos vanishing points using SketchUp.
source:Sketchup help centre. https://help.sketchup.com/pl/article/3000115
This is a powerful modelling aid in the architectural field, as it has become an automated process. Researchers such as Sinha et al 2008.48 developed “snap line segments to vanishing directions” in the form of vanishing point constraints, that snip the extra lines when intersected in the plane. The automation of the process results in smooth modelling, however, this automated technique needs more than one photo to fully generate the structure.
Figure 5 configuration of the vanishing in different photos of the structure and snipping the intersections Source: Sinha, S. et al. 2008
Tripoli’s urban core transformed dramatically across the span of the research: new urban developments were constructed and buildings were demolished. The research is limited to the photos that already exist of the space. Thus, for this research a single-view modelling
48 Sudipta N. Sinha and others, 'Interactive 3D Architectural Modelling from Unordered Photo Collections'. ACM Transactions on Graphics, 27 (2008).
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method was used to rebuild Tripoli’s core areas in three different sets of time. Following this, a walk through tour was generated within the site to provide a valuable perspective on the urban changes.