Conference Paper
Contents
1. gt gt 5 se Ste Fig 1 Two printed city block from the Toul scale model To define the following protocol we have tested different configurations in our laboratory These tests were made on 3D printed city blocks of the Toul scale model painted by hand and relying on a textured background Fig 1 The optimal configuration we found for taking pictures that are enough detailed and the most exploitable is a good single lens reflex camera equipped with a powerful flash device and its reflector In the case of our study we used a Canon EOS 6D offering high quality pictures of 16 megapixels equipped with a 24 70 lens We had a laptop on the shooting place to download and verify pictures Also we had the possibility to start a test with 123DCatch granting us a preview for exploitable results or not This contribution has been peer reviewed The peer review was conducted on the basis of the abstract 278 International Archives of the Photogrammetry Remote Sensing and Spatial Information Sciences Volume XL 5 W2 2013 XXIV International CIPA Symposium 2 6 September 2013 Strasbourg France Have been tested aperture sensibility shutter speed settings and different lightning conditions with some help from a professional photographer This protocol was only applied and verified on the Autodesk 123DCatch software It has been provided by our own tests but also by following the user manual the user experiences and previous re2. 5 3 Protocol for the 3D scan acquisition As the photogrammetry 3D scanning needs some preparation The device only scans what is in a radius of 30 45 cm from his sensors Therefore without an expensive robot arm to do this the operator can only use his own arms and count on the access he has to the scale model As much as possible it 1s recommended to study every access possibilities to the model as if it is extremely rare to have a free access everywhere The treated area by the scan is defined by its own acquisition quality settings In the case of thin detailed scale models the precision has to be high one point each semi millimetre This setting limits the acquisition volume to a cube of 15cm side The full acquisition time for this volume with an accurate precision is about one hour To scan the full model you have to sweep over the model from many different points of view The software displays in real time every acquired point which allows us to control the missing areas If there are areas that are inaccessible to the scanner sensors it will leave a hole in the final mesh which can be difficult to fill In the case of very dark painted surfaces it is possible to change the light sensibility setting of the scanner to avoid other holes The scanner cannot acquire areas painted that are near to the absolute black in our case the openings of the buildings which absorbs every light With laser scanning we have been able to ac
3. many of the operations are automated and never ask to interfere directly on the mesh That s why we decided to use Geomagic Studio 12 7 2 Protocol for the segmentation In order to segment the meshes we will describe the use of Geomagic Studio 12 step by step The general way to operate the mesh is first correction of the mesh in order to avoid any calculation error then optimization of the mesh in order to simplify the computing in the following steps and finally mesh segmentation Each of these steps asks for many parameters that we have to define We have to find the optimal default values to have the best conditions to automate the process Let us detail the three steps Many functions help to correct the mesh The first one is automatically proposed by the program called the Mesh Doctor This operation reduces little spikes holes replaces the normal of false oriented triangles and also improves the mesh quality in order to avoid any error on next steps Others manual functions let the user complete the correction step as hole filling mesh trimming etc In the fortified areas this process is a tedious task because of the vegetation cover which has to be removed This operation can only be done manually given the proximity of the vegetation with the smallest details of the fortification works Even with these precautions the loss of information is inevitable and prejudicial for the segmentation step Thus it re
4. 2004 Comparison and evaluation of laser scanning and photogrammetry and their combined use for digital recording of cultural heritage Kersten T P Keller F Saenger J and Schiewe J 2012 Automated generation of an historic 4D city model of Hamburg and its visualization with the GE Engine Skarlatos D and Kuiparissi S 2012 Comparison of laser scanning photogrammetry and SFM MVS pipeline applied in structures and artificial surfaces JSPRS Annals of the Photogrammetry Remote Sensing and Spatial Information Sciences Volume I 3 2012 XXII ISPRS Congress 25 August 01 September 2012 Melbourne Australia This contribution has been peer reviewed The peer review was conducted on the basis of the abstract 282
5. manipulation is allowed 123DCatch keeps running in the background you can also close the program asking Autodesk to send an e mail when the mesh has finished calculation Whatever the size of the photographed area the number of polygons in the mesh will remain the same The more you photograph closely the more the mesh is accurate Once the pictures are sent the mesh is computed and downloaded This step can take several minutes between 10 to 30 minutes We have for the first time a good preview of the calculated object If the result is satisfying you can continue working on it otherwise you have to restart with others settings At this stage the program allows four different interventions Define common points between unused pictures and the used ones to add them correctly to the calculation Add new pictures to clarify complex areas making possible round trips between photo shoots and 123DCatch Give a precise scale to the model for a future exportation hence the importance of taking pictures with a ruler And above all recalculate a more precise model on a selected area of the mesh This is highly recommended because the first computing is generally inaccurate After these steps the model is ready to be exported 123Dcatch has the advantage of offering many export file formats obj compatible with many programs will be used to export to Geomagic fox compatible with Autodesk Maya to eventually modify the 3D las wh
6. to the museums to see the physical scale models In this paper we focus on the very first steps in the 3D digitising modelling pipeline that are the raw data acquisition and processing 3D data acquisition from scale models is a challenging task for many reasons the level of detail of the scale models the high accuracy needed for modelling the limited access to scale models and the low luminous conditions in which they are exposed We have been experimenting different acquisition methods and we defined a full operation protocol in order to automate the process We will first introduce the town scale models their specificities and their complexities in comparison to the full scale models section 2 Then related works are presented section 3 in terms of town scale model modelling and comparisons between photogrammetrical and 3D laser scanning methods Later we will determinate the most appropriate software and method for high quality meshes acquisition and define a protocol for each of these methods by explaining our experiments sections 4 and 5 One of these methods will be chosen section 6 It will be followed by our experiments on segmentation of these meshes section 7 And finally we will conclude with a presentation of our results section 8 2 SCALE MODELS The town scale model tradition began in the 17 century In France a collection of 144 scale models has been initiated by Vauban named Plans Reliefs T
7. International Archives of the Photogrammetry Remote Sensing and Spatial Information Sciences Volume XL 5 W2 2013 XXIV International CIPA Symposium 2 6 September 2013 Strasbourg France ACQUIRE HIGH QUALITY MESHES OF SCALE MODELS FOR AN AUTOMATIC MODELLING PROCESS F Giraud K Jacquot C Chevrier G Halin MAP CRAI National School of Architecture of Nancy 2 rue Bastien Lepage 54000 Nancy France giraud jacquot chevrier halin crai arch1 fr KEY WORDS 3D Modelling Scale Models Photogrammetry Laser Scan Segmentation ABSTRACT Urban scale models depicting whole towns such as the hundred scale model collection known as plans reliefs are a valuable source of information of cities and their surroundings These physical representations of French strongholds from the 17 through the 19 century suffer from many problems that are among other things wear and tear or the lack of visibility and accessibility A virtual collection would allow remote accessibility for visitors as well as history researchers Moreover it may also be linked to other digital collections and therefore promote the collection to make people come to the museums to see the physical scale models We also work on other physical town scale models like Epinal for which the scale is a bit higher In a first part we define a protocol for acquiring 3D meshes of town scale models from both photogrammetric and scanning methods Then we compare the results of both met
8. an data has only been used to verify buildings height and pictures to create textures Many graphic designers have modelled the Plans Reliefs on SketchUp manually with a loss of accuracy Lastly a PhD Jacquot et al 2013a from our laboratory is being made on the plan relief fortifications The goal is to automate the adjustments of a parametric library of fortifications works thanks to reverse engineering process In a previous paper Chevrier et al 2010 we explained how we managed to automate the modelling of the buildings from roof segments However these roof segments had been manually positioned with PhotoModeler task that took a lot of time 3 2 Comparison between photogrammetry and 3D laser scanning As this paper is about 3D mesh acquisition from real models of cultural heritage we had to care about the different technics of acquisition Most of found studies are on real size objects According to P Grussenmeyer Grussenmeyer et al 2010 there are three methods Laser Scanning Photogrammetry and Tacheometry These methods have been tested on a full scale model a castle We can exclude tacheometry because of the small scale we are working on Each of these has advantages and disadvantages both capture heavy data with high precision photogrammetry is easily textured and better detects edges In the end they seem complementary Some researches insist on the fact that there is no method to be applied in every situation Th
9. deling of a town scale model Limassol Cyprus pp 99 107 LNCS 6436 Dylla K Mueller P Ulmer A Haegler S and Frischer B 2009 Rome Reborn 2 0 A framework for virtual city reconstruction using procedural modeling techniques Geneva 2013 Scale model of geneva project pram http youtu be MwLDoZnv70A accessed june 2013 Grussenmeyer P and Al Khalil O 2000 A comparison of photogrammetry software packages for the documentation of buildings The Mediterranean Surveyor in the New Millennium Seminar 18 21 September 2000 Malta Grussenmeyer P Landes T Voegtle T and Ringle K 2008 Comparison methods of terrestrial laser scanning photogrammetry and tacheometry data for recording of cultural heritage buildings The International Archives of the Photogrammetry Remote Sensing and Spatial Information Sciences Vol XXXVII Part B5 Beijing China Jacquot K Chevrier C and Halin G 2013 Validation of a parametric approach for 3D fortification modeling Application to scale models 5th International Workshop 3D ARCH 2013 3D Virtual Reconstruction and Visualization of Complex Architectures 25 26 feb 2013 Trento Italia Jacquot K Chevrier C and Halin G 2013 Reverse Engineering of Scale Models Using Dataflow Programming Application to the fortification of plans reliefs Digital Heritage 28 oct 01 nov 2013 Marseille France submitted Kadobayashi R Kochi N Otani H and Furukawa R
10. ed pictures for the photogrammetry we can improve geometry The available results in the fortified areas of the scale models are along the same line Even if the photogrammetric mesh is more irregular than the laser results the impact on the segmentation process remains invisible 6 2 Difficulties of acquisition Because of the size of scale models we had to find the most convenient method to acquire its geometry As a reminder Epinal scale model one of the smallest is 3 3 x 1 8 m The only city table of Toul is already 2 3 x 2 13m So some areas can be difficult to access Fig 7 Moreover given the little control over the parameters of 123Dcatch it is impossible to forbid its algorithms to create awkward geometries instead of trees and areas it could not capture properly Therefore photogrammetry is easier thanks to the zoom and focal we can modify according to the distance we have to the subject The Single Lens Reflex 1s also a lighter device than the Laser Scan The Laser Scan has the disadvantage of needing a specific distance between it and the subject which can often be difficult to access This is resulting by holes and missing parts on the mesh Fig 6 This disadvantage could be bypassed by the use of a robotic arm or a bridge over the model of a consequent price The additional cost of these operations includes a truck the equipment the LaserScan we used costs about 55 000 and the Single Lens Reflex 3000 which
11. ey made some tests to select the most appropriate method and sometimes to combine them Kadobayashi et al 2004 Boehler et al 2004 Skarlatos et al 2012 4 3D ACQUISITION VIA PHOTOGRAMMETRY In this part we present experiments that have been made on a 3D printed replica of a city block of the Toul plan relief In section 4 1 we compare several photogrammetrical software Then in section 4 2 we defined a protocol to take pictures according to the recommended advices of the user manual software Finally in section 4 3 we define a protocol for the 3D mesh creation in order to obtain a high quality mesh 4 1 Comparison of software We have been looking for a tool that would automate to the maximum the meshing process Previously we used Photomodeler Scanner software that used photogrammetric methods in manual ways Now that the technologies have evolved in this field many software programs exist We have tested several photogrammetrical software that use different algorithms and different protocols for taking pictures to select the most appropriate to our need To do it wisely we followed the main instructions for each software program to have a proper set of pictures rather than providing an identical set for all software Elements of comparison have already been made on real buildings Grussenmeyer et al 2008 We considered these researches to make our experiments but the tested software often ask for manual inte
12. hey were made at a scale of 1 600 and their average size often approaches 50 square meters Most of the old scale models are damaged by the time and are difficult to move In order to protect them and to grant a larger access to these pieces of history there is a real interest to create 3D semantic models of the scale models that would be available for viewing by the public on the Internet In parallel to these olds scale models and collections a new wave of freshly realised scale models is actually coming They sometimes have different scales as Epinal s one 1 300 scale In order to acquire their 3D meshes there are many differences between a scale model and a full scale object The acquisition of a scale model needs much more precision than the full scale object Indeed they have the same details of architectural complexity only represented much smaller 10 centimetres long in real life are represented by only a half millimetre on the scale model When the streets are 5 to 10 metres wide they only measure to 2 centimetres The 3D acquisition of town scale models is much more difficult and needs to be precise The scale model reconstruction is a real difficult work From this point of view this operation needs some preparation and a protocol must be defined in order to realise the acquisition in optimal conditions 3 RELATED WORKS 3 1 Town scale model acquisition and modelling Town scale model acquisition and remodelling has a
13. hods The photogrammetric protocol has been elaborated by choosing the most accurate software 123DCatch which asks for about 60 pictures and defining the settings needed to obtain exploitable photographs In the same way we defined the devices and settings needed for the laser scan acquisition method In a second part we segment the 3D meshes in planes by using Geomagic which has been chosen between several programs for its accurate resulting geometry 1 INTRODUCTION Urban scale models depicting whole towns such as the hundred scale models collection known as plans reliefs are a valuable source of information of cities and their surroundings These physical representations of French strongholds from the 17 through the 19 century suffer from many problems that are among other things wear and tear or the lack of visibility and accessibility Nevertheless they are objects that come back with a new visibility to the public with the apparition of the computer tools and the new interest for city history This can be seen by the new wave of copies of scale models and the new exhibitions in which they are highlighted To grant an access to a larger public some researchers work on the possibilities to recreate them faithfully A virtual collection would allow remote accessibility for visitors as well as history researchers Moreover it may also be linked to other digital collections and therefore promote the collection to make people come
14. ich is a highly used point cloud file and dwg compatible with AutoCAD Following this protocol the acquired mesh quality is high and exploitable for next steps of treatment Here are 123DCatch screenshots of these results on the Toul printed replica scale model Fig 3 and the real Epinal scale model Fig 4 As presented meshes are well textured and are accurate enough even in the centre courtyards We always have small not captured details as they could not be seen from any point of view as areas under the roof overhangs small alleys lt 2mm and the smaller and deeper courtyards lcm in 2cm high courtyards 19 mm Fig 4 123DCatch results for the Epinal scale model This contribution has been peer reviewed The peer review was conducted on the basis of the abstract 279 International Archives of the Photogrammetry Remote Sensing and Spatial Information Sciences Volume XL 5 W2 2013 XXIV International CIPA Symposium 2 6 September 2013 Strasbourg France 5 3D SCANNING ACQUISITION 5 1 Technical constraints Technical constraints due to the scale models and plans reliefs are almost the same for scanning and photo shooting Then the access to the scale model often difficult is even more important for the 3D scan because of the necessity to sweep the model from many directions Moreover the 3D scan needs to be wired to electric power and to a laptop It becomes much more difficult to use than the Single Len
15. in for geometric and semantic segmentations and the early results are encouraging Jacquot et al 2013b 8 CONCLUSION This study about the use of photogrammetry and laser scanning on historical city scale model has revealed some important knowledge about the protocols for acquisition and segmentation From the different sizes of the scale models compositions and epochs we could define a unique protocol to have the same quality of results Tests have been focused on the city and fortifications but they have to be extended to towers vegetation historical monuments etc The next step of our researches is in progress the automation of the creation of the building parametric models from the 3D segmented meshes The texture of each building will be computed from the 3D textured mesh The benefit of that kind of semantic and digitalized city model would be great it will be accessible to the public it will represent an information source useful for historians patrimonial architects and town planners it will promote the scale model collections to everyone ACKNOWLEDGEMENTS Thanks are due to the photographer Gilles Andr of the SRI Service Regional de Inventaire de Lorraine for his advices REFERENCES Boehler W and Marbs A 2004 3D Scanning and photogrammetry for heritage recording a comparison Geoinformatics 2004 University of G vle Sweden 7 9 June 2004 Chevrier C Jacquot K and Perrin J 2010 3D Mo
16. is much cheaper and the loss of time from moving and installation Fig 7 Detail of a small and deep courtyar on the left the rea scale model in the middle 123DCatch finds an average round geometry on the right the scan leaves holes instead of proposing a fake geometry 6 3 Exploitation possibilities and extras The exploitation possibilities offered by each method are similar Thanks to the accuracy of the laser scan segmentation part can be a bit easier than the photogrammetry because of the slight noise on 123DCatch results Scan reorganization is also easier thanks to the geo referencing of each captured mesh but it is also possible with photogrammetry in Geomagic Studio 12 In contrast photogrammetry offers well textured meshes exploitable for the next steps 6 4 Chosen method After this comparison we chose to give priority to the photogrammetry because of its ease of use its rapidity in situ its price and the needed capacity of texturing the models 7 SEGMENTATION AND PLANES RECOGNITION IN THE 3D MESHES After acquiring meshes from the whole scale model we have many separated point clouds These point clouds are heavy about 1 million points per city block and represents a very complex geometry composed of many polygons which are too heavy to be viewable online So we have to treat this point cloud with respect to building shapes and roofs in order to have a lighter model The aim of plane recogn
17. ition is to facilitate an automatic parametric modelling that will be explained in a another paper The texturing of the parametric model is automatically computed from the textured mesh In section 7 1 we compare software applications for segmentation Then in section 7 2 we define a protocol for this step And finally in section 7 3 we will see the final results of segmentation 7 1 Comparison of software In the domain of segmentation of 3D meshes many software applications exists We decided to test three software programs to evaluate which one is the most adapted to our needs Many of these are commercial and expensive because of their rarity and complexity Tested software includes Geomagic Studio 12 Rapidform XOR3 EDF CloudCompare and Mesh2Surface 4 Rhino plug in EDF CloudCompare is free it applies RANSAC algorithms with medium quality results but it offers very low export possibilities Mesh2Surface 4 Rhino is a Rhino plug in that costs around 600 It has been tested on its demo version and did not offer exploitable results Rapidform and Geomagic are recognized among the best point cloud processing software Their price is high 15 000 and their functionalities are very close to each other However Geomagic Studio 12 is an easy to use software that offers great exploitable results It has many parameters that allow the refinement and the enhancement of the meshes to prepare them to the phase of segmentation Also
18. lated works on photogrammetry in the Digital Heritage domain The Single Lens Reflex is set on manual mode assembled with the flash device and its reflector The flash device allows us to take pictures even in the low luminous condition on which the scale models are often exposed Its shutter speed is set to 200 milliseconds It is a very short shutter speed which makes the ambient light shadows disappear when synchronized with the flash device The aperture is set up to 11 to have a good depth of field Its ISO Sensibility is set on 800 allowing us to take quite a lot of pictures without consuming too fastly the flash device battery with a low quality loss compared to an ISO 100 setting The Auto focus is on and the focal length is optimized according to the distance from the subject The focal length has to remain the same for every picture on one unique subject The shooting conditions in which we work are not always great Also we tried to take pictures after verifying our own stability and then to verify each picture We have to choose parts of the subject of a reasonable size The referent here is a city block and its near environment as it can have in itself a real complexity This represents an average surface of three square decimetres Also the distance to the subject has to be the shortest In the first shoots the subject must be entirely seen in every picture and take the most of the area Then we can shoot scale model detail
19. lly correctly segmented depending of the amount of residual noise but smallest parts of fortification are nearly out of range even with a high sensibility and a minimum area about of a tenth of a millimetre Almost every face connection on scale models are edges so we set the connexion type on sharp Then comes the phase of surfaces and connections fitting And finally the result computing that offers a real segmentation separated in trimmed primary surfaces By following these steps and settings we have a segmented model approaching the real one and allowing us further automatic treatments for the creation of the parametric models of the buildings Fig 8 Fig 8 Results of segmentations from Geomagic for Toul replica scale model and for the real Epinal scale model 7 3 Results Thanks to the plane recognition the model becomes simpler only composed of real plane polygons following their real edges There are still some improvements to do such as missing corners that needs even more precision and the presence of chimneys that can be easily found they are formed by spikes In the fortified part of scale models the level of detail is higher than in the city area as the plans reliefs were used above all as a military tool Geomagic treatments and especially segmentation step are time consuming processes whose outcomes are far from being satisfactory An alternative method is being tested right now It uses Grasshopper plug
20. lready been the subject of many studies Most of these experiments use the scan data only as a support to manually model the object The This contribution has been peer reviewed The peer review was conducted on the basis of the abstract 277 International Archives of the Photogrammetry Remote Sensing and Spatial Information Sciences Volume XL 5 W2 2013 XXIV International CIPA Symposium 2 6 September 2013 Strasbourg France Project Relief Auguste Magnin Geneva 2013 is the project of reconstruction of the Geneva Plan Relief It has a high fidelity to the original using a 3D scan but has mainly been made manually Rome Reborn Dylla et al 2010 is an interesting project of restitution of the Plastico Di Roma Antica that uses procedural modelling on the basis of scan data on certain areas a program recreates plausible models but not exact ones in order to have the historic atmosphere of the city It uses architectural rules from treatises translated to modelling language Going further a study on the automated generation of a Historical city Model of Hamburg has been done including five different epochs Kersten et al 2012 It has been conceived by scanning acquisition on a scale model thanks to a robotic arm and automation of reconstruction from maps Finally for the Plans Reliefs exhibition in Paris in 2012 Google digitized many scale models in a short time using pictures and laser scanning Unfortunately the laser sc
21. quire high quality 0 2mm in our case non textured meshes similar in quality to the photogrammetry Each different acquisition is geo referenced due to the dot grid making easier the assemblage between them Also captured models are already at scale which is a benefit for further operations We present the results of each scan on the Toul printed replica scale model Fig 5 and then on the real Epinal scale model Fig 6 The meshes are of good quality but there are always some holes corresponding to parts that the scanner could not capture But no textures are available Earlier tests have been made with a textured scan but results were not convincing Textures are very rough and they are not captured on the whole scale model so they are not exploitable For the Epinal acquisition we focused on a part of the model that was easily accessible on the border HOED gt Uae AO ts x t x KAVRAN AC SSS 2 See OO r 2SDOr COR ee x DER OTR AOADA OO ae ge SSO ae OMAR OO te fi Fig 5 Scan results for Toul replica scale model with resulting holes j from non captured black surfaces Fig 6 Scan results for Epinal scale model the back sides are missing because it is not accessible by the hand 6 COMPARISON BETWEEN PHOTOGRAMMETRY AND 3D SCAN In order to know which method is the most accurate in the case of city scale model we compared each method results We compared the me
22. rvention Also others photogrammetric programs appeared recently Tested softwares are Autodesk 123DCatch Agisoft Photoscan EOS System Photomodeler Other softwares as Pistou a research program in MAP Gamsau or Topcon s Imagemaster Pro have been considered but we excluded them for their apparent complexity Here are the results of our comparison each one tested on the 3D printed scale model of Toul 123DCatch Nb Pict 60 Photomodeler 2 to 10 Photoscan gt 100 Calc time 30min 30hrs Manual Quality of the results Price Good Good 2500 Medium Free 3500 Table 1 Comparison of the different software As seen on table 1 123DCatch asks for about 60 pictures with a maximum of 80 Photoscan needs pictures as much as possible the user manual mentioning 100 to 500 Photomodeler is mainly usable manually but an automation exists which demands for couples of pictures to work with stereogrammetry After experiments the most appropriate software to acquire high quality 3D meshes from scale models using photogrammetry appeared to be Autodesk 123DCatch It has the quality of being free fast and offers easily exploitable results On the other side it leaves a very limited control to the user for its parameterization and does not display any preview before it is completely calculated So you have to wait for the results before knowing if the model is usable or not 4 2 Protocol for taking pictures
23. s The nearest distance 30cm the littlest focal length 24 and the furthest distance 1m the largest focal length 70 We take two orbits of approximately 20 photos around the object The first orbit at 30 from the ground the second at 60 Fig 2 To complete this collection we can add some photos orbiting around hidden areas It will work better if there is a slight presence of volumes or a textured background all around the object instead of a solid colored one We often arrive to a total of sixty pictures which is generally enough to work with Furthermore in order to have a spatial reference and scale for a future 3D mesh acquisition it is advisable to place a small rule near the model This will define a scale for it and make easier the following phases After photographing the model following our protocol we pass to the 3D acquisition phase with 123DCatch 15 20 photos 15 20 photos Fig 2 Simple scheme for taking photos 4 3 Protocol for the 3D acquisition with 123DCatch 123DCatch is very simple to use At the opening of the software you only have to select the pictures you want to send To simplify the computation it can be useful to reorder the pictures by orbits 123DCatch has a limit of upload of 80 pictures at a maximum resolution of 6 Megapixels So we may have to resize the pictures before sending them Then we can send these pictures to the Autodesk cloud for calculation During this phase no user
24. s Reflex in almost every case The use of our handyscan Creaform EXAscan is easier on little scale models Also the handyscan needs special reference points which consists on a grid marked with many points every 5 centimetres 5 2 Tests To define the following protocol we made many experiments in laboratory always on the same 3D printed scale model of Toul and also in real conditions on the Epinal scale model Experiments on the printed Toul scale model have been useful to define every settings of the scan luminosity sensibility precision and the scanning time These settings are related to each other we had to make a compromise between precision scan time and size of the bounding box These settings have been used on another experiment on the real Epinal scale model where we tried to define how to scan in real conditions of work All these tests have been influenced by previous experiments from our laboratory In the first experiments Chevrier et al 2010 previous settings have been already defined and tested in real conditions These experiments conducted on Toul plan relief have highlighted the difficulties to scan thin or damaged elements As special reference points we suspended a wired doted grid over the scale model thanks to a portable scaffolding This grid remains hardly installable over a massive scale model We are not authorized to place pastilles on the model because it is forbidden by Cultural Heritage
25. sh quality section 6 1 the acquisition difficulty section 6 2 the exploitability and the eventual extras that they offer section 6 3 6 1 Mesh quality The mesh quality is important for us as we need a high quality to exploit the model in order to segment it Considering the amount of polygons as a part of a mesh quality the results between photogrammetry and laser scan are similar Fig 3 4 5 6 In comparison each scanned cubic areas 15x15cm has 1 4 million polygons whereas every 123DCatch mesh of the same size has up to 1 6 million polygons depending on the number of pictures and their resolution The mesh quality of the photogrammetry depends on the area we want to acquire and on the resolution of taken pictures whereas the quality of the scan depends on the amount of defined sized cubic areas The mesh quality is not just about the polygons number Others characteristics come into play the accuracy of the acquired This contribution has been peer reviewed The peer review was conducted on the basis of the abstract 280 International Archives of the Photogrammetry Remote Sensing and Spatial Information Sciences Volume XL 5 W2 2013 XXIV International CIPA Symposium 2 6 September 2013 Strasbourg France geometry and the precision on details On these criterions laser scan has less noise on solid coloured parts but photogrammetry is still enough for segmentation and both cannot access in thin holes Taking detail
26. sults in many program crashes delaying the segmentation process Then we prepare the mesh for the segmentation by cleaning low curvature areas reducing noise and sharpening These options are facultative but they help to the segmentation especially on imprecise meshes Finally the parametric surfacing segmentation phase finishes our process This step passes by many actions First the region detection asks the user to enter sensibility parameters Different parameters are asked at this time as the separator and curvatory sensibility and the minimum area to be computed This contribution has been peer reviewed The peer review was conducted on the basis of the abstract 281 International Archives of the Photogrammetry Remote Sensing and Spatial Information Sciences Volume XL 5 W2 2013 XXIV International CIPA Symposium 2 6 September 2013 Strasbourg France Obviously it often depends of the mesh quality and the nature of the model In the case of city blocks sensibility must be high and the minimum area is about a half square centimetre Next we define the type of geometrical shape we want to recognize Planes for building reconstruction Sphere for spherical trees Freeform for ground These regions are separated by their contours which the user is able to modify and correct In the fortified areas manual interventions are always necessary as the fortified entities vary greatly in terms of size The walls are usua
Download Pdf Manuals
Related Search