Pierre Corvaisier, Digital Geography marketing manager at THALES Communications, and Xavier Lhomme, development engineer, tell us how their company used Open CASCADE to build an application for 3D photogrammetry based on a particularly innovative CAD approach.
Open CASCADE: The Digital Geography department of THALES Communications devotes 90% of its activity to the development of systems for military use that are based on the mass production of cartographic data with the photogrammetric method. How did you happen to come up with the "RealCity" project, which is aimed at civilians, in particular urban planners?
Pierre Corvaisier: Our research work focuses above all on imaging in a military context, but our tools also give top performance for civilian applications. In general, we try to transfer the technological advances stemming from our R&D projects to civilian industry.
The feedback loop between military and civilian projects is continuous and extremely enriching. Often, working on civilian applications allows us to carry out validations and developments that would not be possible or allowed in a military system. On the other hand, military demands place the bar of performance very high.
Our application, SPDG 3D, is a system for the production of geographical data in 3D. Used for the "RealCity" project, it was built on platforms first developed for the reconnaissance of military targets within limits approaching real time. We later adapted and developed these systems to achieve the mass-production of topographic maps for the general public. Next, Xavier Lhomme had the idea of combining the techniques used in CAD with those used in photogrammetry to create urban environments from aerial images.
OCC: SPDG 3D is a 3D photogrammetric system based on Open CASCADE. Exactly what is a 3D photogrammetric system?
Xavier Lhomme: To simplify, photogrammetry is a measuring technique that allows modeling a 3D space using images. This technique has existed for over 150 years.
Today, traditional digital photogrammetric systems use stereoscopic processors that superimpose two images from analogic or digital captors installed in high-resolutions satellites or airplanes. There are also systems capable of processing land-based images for architectural photogrammetry with an underlying geometry that is slightly different.
Similar to BD TopoŽ, the reference database containing the 3D physical description of French territory, aerial photogrammetry systems allow the reconstitution of planimetric data in 2D5. It is very rare to find a system like SPDG 3D that can reconstruct the data as a 3D volume. SPDG 3D enables the user to build a complete urban environment. Since this reconstruction is the faithful transcription of real building geometry, it becomes possible, for example, to study the integration of architectural projects within a real environment without a physical mock-up. Telecommunications companies can also use these 3D data to calculate the placement of relay antennas, or urban planners to visualize very precisely their impact on coverage quality.
OCC: What advantages did a CAD approach bring to this software?
Thanks to the advanced CAD modeling functionality of Open CASCADE, SPDG 3D enlarges the possibilities of 3D reconstruction. Thus, users can model an interior courtyard by subtracting two elementary volumes through a Boolean operation. They can also attach elements (face, edge, vertex) on reconstructed objects by means of projections, a typical CAD operation. SPDG 3D is, in effect, a complete CAD modeler equipped with 3D visualization in real time that completes stereoscopic processing in the photogrammetric system.
Open CASCADE provides the link between the world of photogrammetry and that of CAD.
OCC: What advantages do you see in working with an open-source development platform?
PC: Not only can be sure of the long-term viability of our developments thanks to transparent access to the source code, we can also obtain top performance with a minimal investment.
Our customers target the long term and we are obliged to answer their demands for stability. Typically, a project stretches over 10 to 15 years. Being able to ensure the maintenance of software developments and continued improvement of the system during this period represents both a challenge and a major advantage. Open CASCADE facilitates the long-term viability of the system.
XL: We reduce development costs by using Open CASCADE in conjunction with other open-source tools, like the Open Inventor visualizer, built on SGI's OpenGL format and providing supplementary graphical functionality. We also use the PostgreSQL relational database system. We are able to choose with more flexibility, more control over the evolution of our applications. And, not least, this reduction in costs allows us to be more competitive on bids.
In addition, the spirit of free exchange and the trend towards the use of standards that are inherent in the Open Source movement go hand in hand with the research and development needs of our company.
OCC: What is the future of SPDG 3D?
XL: This system was developed within the framework of a study carried out for the Ministry of Industry. It is a demonstrator and we are still working on some of the functionality, so that this prototype is in constant evolution. However, we have recently emphasized the ergonomic aspects in order to industrialize the application as soon as possible. Two other military studies have allowed us to add complementary functionality to the prototype.
PC: What we have learned from these projects will help to enrich our other research programs, thus closing the development cycle that goes from the military domain to civilian applications, then returns once again to the starting point.
A number of civilian and military industrialization projects are currently under study. We are also working on a prototype that will allow taking advantage of the reconstructed 3D data and can be used by local communities or other public services.