IMAGE BASED 3D CITY MODELING: COMPARATIVE STUDY

: 3D city model is a digital representation of the Earth’s surface and it’s related objects such as building, tree, vegetation, and some manmade feature belonging to urban area. The demand of 3D city modeling is increasing rapidly for various engineering and non-engineering applications. Generally four main image based approaches were used for virtual 3D city models generation. In first approach, researchers were used Sketch based modeling, second method is Procedural grammar based modeling, third approach is Close range photogrammetry based modeling and fourth approach is mainly based on Computer Vision techniques. SketchUp, CityEngine, Photomodeler and Agisoft Photoscan are the main softwares to represent these approaches respectively. These softwares have different approaches & methods suitable for image based 3D city modeling. Literature study shows that till date, there is no complete such type of comparative study available to create complete 3D city model by using images. This paper gives a comparative assessment of these four image based 3D modeling approaches. This comparative study is mainly based on data acquisition methods, data processing techniques and output 3D model products. For this research work, study area is the campus of civil engineering department, Indian Institute of Technology, Roorkee (India). This 3D campus acts as a prototype for city. This study also explains various governing parameters, factors and work experiences. This research work also gives a brief introduction, strengths and weakness of these four image based techniques. Some personal comment is also given as what can do or what can’t do from these softwares. At the last, this study shows; it concluded that, each and every software has some advantages and limitations. Choice of software depends on user requirements of 3D project. For normal visualization project, SketchUp software is a good option. For 3D documentation record, Photomodeler gives good result. For Large city reconstruction; CityEngine is a good product. Agisoft Photoscan software creates much better 3D model with good texture quality and automatic processing. So this image based comparative study is useful for 3D city user community. Thus this study will provide a good roadmap for geomatics user community to create photo-realistic virtual 3D city model by using image based techniques


INTRODUCTION
At present scenario, there are four main image based techniques available to create virtual 3D city model.First, sketch based method, second is procedural based method, third close range photogrammetry based method and fourth technique based on computer vision algorithms.SketchUp, CityEngine, Photomodeler and Agisoft Photoscan are the main softwares to represent these approaches respectively.Sketch based modeling is simple and not fulfill the engineering requirements.Procedural based modeling is also not easy for normal user.Close range photogrammetry based method is also not suitable for large landscape modeling.Previous available research literature shows that each and every method has some advantages & limitations.Previous study shows that, till date, there is a no perfect comparative study available to study the image based 3D city model.Thus this research paper will fulfill the required gap in 3D modeling community to create 3D city model in efficient way.The main aim of this research is to give a comparative assessment for these images based 3D modeling softwares.There are so many techniques and commercial software available now days in geospatial market to create photorealistic 3D models of a city.Image based modeling software gives a good solution for 3D city modeling.Each techniques and softwares have some advantages and limitations too.The main problem is to find out the suitable software and method according to the need of project for 3D modeling.Level of Details (LoD) is also an important key issue for any 3D city modeling project.Same 3D product is not economically feasible for all 3D city modeling project.First, it should analyze, what is the main aim of 3D city model?At what level, it needs the details?How much area of city?What is the composition of city?What is the project cost?After that it should think about that, which software will be more suitable for this work?What methodology should adopt to create 3D city model?At present, there is no such type of study is available anywhere, so in this research paper work, it is trying to give a solution to solve the above problem.Some previous literature, related with this work summarized as given below: A good collection of literature can be found on image based 3D city modeling techniques and applications (Singh et al., 2013c), Image based 3D modeling: a review (Remondino, Fabio, and Sabry El-Hakim., 2006).
Zhou and Zhang (2004) explained about a preliminary review on three-dimensional city model.This review was based on the comparison of three 3D data model of GIS, (3D FDS, V3D, GeoToolKit).The literature study shows that, till date, there is no complete such type of comparative study available for 3D city modeling user community.So this research paper will fulfill gap and useful for image based 3D city modeling community.
The main purpose of this paper is to find out the suitable software for 3D city modeling project.In this research work, SketchUp, CityEngine, Photomodeler and Agisoft Photoscan software, were explained.This comparative study is based on mainly, general overview of software, method of image data acquisition, data processing methods, and result of output 3D model.This study shows that every software has some advantages and limitations.This research paper will play an important role for 3D city modeling scientist community.At the last, the strengths and weakness of these four images based 3D city modeling software are explored.With the help of this paper, they can easily understand that, which software is more suitable for which project.
Choice of software depends on user requirements.

STUDY AREA
Study area is civil engineering department, Indian Institute of Technology, Roorkee, (IIT-R), Uttarakhand, India.The civil engineering department surrounded by department of earth science, department of architecture, department of water resources development and management (WRD&M), and this civil engineering department (CED) has the following main sections and buildings: Geomatics section building, Geotechnical section building (having CAD Lab and Research scholar wing), Lecture hall building (having Newton rooms and Rayleigh rooms), Wind engineering section, O.P. Jain auditorium building, Transportation The detail about this section is given in Annexure 1:

COMPARATIVE STUDY BASED ON DATA ACQUISITION:
In SketchUp and CityEngine software, photographs should be taken from front shoot of building.Photographs can be taken for each important feature of building façade.So in this type of photography, more detail will obtain.Editing of photograph is essential and can be done by any photo-editing software like Photoshop software and camera calibration is not required.For Photomodeler software, only convergent photography is useful.Convergent photographs are the photographs in which pairs of photo taken with camera so directed that their optical axes converge.Photomodeler works on basic principles of close range photogrammetry.For any close range photogrammetric project, photos should be taken according to; "3X3 rules", given by Waldhausl and Ogleby, 1994, and further explained by Shashi and Jain, (2007b).These rules are the best suitable for any close range photogrammetric image acquisition with amateur cameras.(Shashi and Jain, 2007b, Singh et al., 2012, Singh et al., 2013d).
Photomodeler generates the 3D positions of points by projecting straight lines from the camera position, through the point on the film or CCD and out into space.When two such rays are used, the intersection of the rays gives the point position.(Photomodeler) Camera calibration is an essential step for Photomodeler project (Singh et al., 2013d).Camera calibration is the process of determining characteristics of a camera so it can be used as a measurement device.(W.Zhang et al., 2010).(Singh et al., 2012) We should follow the following main points during photography to obtained the best result from Photomodeler software,.[Photomodeler] 1.One should try to get the angles between the shots as close to right angles (90 degrees) as possible.2. One should try to take at least three photographs but more photographs will be useful because the returning of a person on project site will increase the work load and cost of project.3.One should try to get all important points on at least three photographs.4. One should try to get as much overlap between adjacent photographs as possible.5.One should try to get photographs from both above and below the object, if possible.6.One should take a special care to ensure good exposure and good focus.7. One should also take accurate measurement of the distance between two clearly visible points in the scene that will be later useful to scale a close range photogrammetry project.Photomodeler gives good result for curves, edge, and cylinders, so special care is needed for photography of these features.
For any close range photography project, initial planning will also play an important role for successful result.First, one should try to decide, how many photographs will take and from where to take them.After that visit on project site and take the photographs.Special care should be taken to ensure better exposure and good focus.
3D model is also possible by using single photograph in Photomodeler software.For single photograph project, the main requirements are: 1. he photographs should be clear and with good focus.2. For good result, the known objects (shapes, control points, constrained edges & lines) should be very clearly visible and should be spread out over the photograph.3.For single photograph project, if someone is trying to recover all the camera parameters in an inverse camera procedure, the photograph should be in three point perspective (i.e. have three vanishing points).For this, one should take a picture of building from its corner instead of face-on.One should follow the following main points during photography for Agisoft Photoscan 3D reconstruction project.

1.
One should choose high resolution digital camera (More than 5 MP).

2.
One should choose wide angle lenses because these lenses are more suitable for reconstructing spatial relations between objects than telephoto ones.

3.
One should avoid non-textured and flat object or scenes.

4.
One should avoid shiny and transparent objects.
5. One should avoid unwanted foregrounds and moving objects.6.If objects are shiny then photo should be shoot in cloudy sky condition.7. Overlap in photographs should be more during photography.8. Capture most important scene content from multiple viewpoints (3 or more).9. Agisoft Photoscan is not support manipulated photographs.So one cannot crop or geometrically transform the images.10.More photos are better than not enough.11.Before taking the images, spending some time for shot plan might be very useful.12. Reference coordinate system is also possible for reconstructed 3D model.13.One should also measure reference distance.

ANNEXURE-1. ANNEXURE 2:
The The main aim of this research paper is, to explore and analyze the potential of various image based 3D city modeling software and to find out a simple and cost effective image based photogrammetric method for 3D modeling user community.This study is based on a small area like campus of any educational institute.3D model of any educational institute is acts as a prototype of a large city.With the help of this study, anyone can find the suitable method and software to create image based 3D city model.Choice of software depends on user requirements.This study is also useful for various kinds of engineering and non-engineering 3D applications.
Wang (2011), explained about a comparative study of five 3D modeling systems based on the SfM principles.He has taken only four software; Bundler, PMVS2, Project Photofly (Autodesk), and ARC 3D Web Service in their work.The working principle of these software is same ie.computer vision based technique, mainly Structure from Motion (SfM).

Figure 4 ,
Figure 4, is showing the example of some photographs, suitable for photomodeler project.

Figure 4 .
Figure 4. Showing photographs for multi-images project (for Photomodeler software)

Figure 5 .
Figure 5. Showing photographs for single-image project (for Photomodeler software)

Figure- 7
Figure-7, showing the example of some photographs, suitable for Agisoft Photoscan project.

Figure 7 .
Figure 7.Some photographs suitable for Agisoft Photoscan software International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XL-5, 2014 ISPRS Technical Commission V Symposium, 23 -25 June 2014, Riva del Garda, Italy This contribution has been peer-reviewed.doi:10.5194/isprsarchives-XL-5-537-2014SkechUp software is an easy and useful tool to create 3D city model.CityEngine works on procedural modeling but also has image based approach to create 3D city model.It gives good realistic virtual scene of a large area in less time.Photomodeler software is works on close range photogrammetric principles and has good measurement accuracy.Agisoft Photoscan works of computer vision based algorithms and gives good 3D model products.So the final conclusion of this work is: these softwares are good and most suitable for image based 3D city modeling.According to user requirement, one can use these softwares.If user wants only a simple 3D city model for visualization purpose, then SkechUp gives a good solution.If user wants only good visualization in less time, then CityEngine gives the best solution.Photomodeler is the best solution photogrammetric documentation of a city.It gives the best measurement accuracy.Agisoft Photoscan is also gives good 3D model and most of the work done automatic and easy to use.The study area is Civil Engineering Department (CED), Indian Institute of Technology-Roorkee, India.
Camera calibration method is different in Agisoft Photoscan software from Photomodeler software.Free software Agisoft Lens is available for camera calibration.Agisoft Lens is automatic lens calibration software, which uses LCD screen as a calibration target.It supports estimation of the full camera calibration matrix, including non-linear distortion coefficients.Agisoft Lens estimates the following camera calibration parameters:  f x , f y -focal length  c x , c y -principal point coordinates  K 1 , K 2 , K 3 , P 1 , P 2 -radial distortion coefficients, using Brown's distortion model Figure6, is showing the camera calibration grid for Agisoft Photoscan.Camera calibration is essential for photomodeler 3D project but it is not necessary for Agisoft Photoscan project.Camera calibration is not useful for SketchUp and CityEngine software.

Table 1 ,
is showing the mode of photography and image input format for these softwares.

Table 1 .
Comparison of mode of photography and image input

COMPARATIVE STUDY BASED ON DATA PROCESSING:Table 2 .
Comparison of basic working principles (based on

Summary of Output model format, Export and Import facility
: (Please see table 3)