Top 100+ Cgal Interview Questions And Answers
Question 1. What Is Cgal?
CGAL is an acronym for the Computational Geometry Algorithms Library. It is on the identical time the quick name for the CGAL Open Source Project. The purpose of the venture is to offer clean get admission to to green and reliable geometric algorithms to customers in industry and academia.
Question 2. How Do You Pronounce Cgal?
CGAL is stated like "seagull" in English, or "cigale" in French.
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Question 3. Are There Example/demo Programs Somewhere?
Yes. The CGAL distribution includes two directories containing example applications: <CGALROOT>/examples/ and <CGALROOT>/demo/. Here you locate the supply code and make files for those programs. The modern-day CGAL manual gives a pleasing evaluate of packages from wherein you could additionally get the demo source code and Windows executables.
Question 4. Where Do I Find Old Releases Of Cgal?
All launched files in addition to manuals are to be had within the release phase of the CGAL GitHub repository.
Question five. Is Visual C++ 6 Supported?
No, it isn't and there's no wish that you can make the essential fixes your self to get it jogging with this compiler. The trouble lies in the reality that VC6 did now not support the template mechanism properly sufficient. The answer is to either use an old model of CGAL or to improve to newer variations of the Microsoft compiler.
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Question 6. Which Versions Of Qt Are Supported?
All the demos use Qt5. The 2D demos are based totally on the Graphics View framework, the three-D demos on the libQGLViewer.
Question 7. Why Do I Get Errors When Using Compiler Optimization On Mac Os X?
The default compiler on Mac OS X is g++ 4.0 (at the least on Tiger and Leopard). It has a few insects which might be unfortunately encountered by means of a few programs the use of CGAL when optimizing. We recommend that you use a more recent version of g++, which includes g++ four.2, which you may get by way of upgrading to the brand new XCode, or the usage of Fink.
You can then choose it through passing the following flags to CMake:- DCMAKE_CXX_COMPILER=/usr/bin/g++-four.2 -DCMAKE_C_COMPILER=/usr/bin/gcc-four.2
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Question eight. How To Reduce Compilation Time Of Programs Using Cgal?
CGAL is heavily using C++ templates, and this has an impact on compilation pace.
Here are a few guidelines that may help speed up compilation of packages using CGAL:
Remove compiler debugging options like -g if you may. Indeed, generating debug statistics may be very pricey for template instantiations, each in terms of walking time and reminiscence usage by the compiler device chain.
Remove compiler optimization options like -O2 while you do no longer want them, which include in the early steps of the development cycle of your programs.
Regroup translation gadgets: If your application is composed of many small translation units to be compiled and connected, attempt to lessen their numbers via merging them. This conventional fashion for C applications is truely very slow for programs closely using C++ templates. Regrouping them reduces the time spent by way of the compiler to parse the header files, and avoids redundant template instantiations.
Precompiled header files: Some compilers, inclusive of GCC, provide a feature named precompiled headers. This may be mainly beneficial if you may regroup in a single header report maximum of the header documents which you use, collectively with most template instantiations.
Question nine. What Is The Difference Between A Predicate And A Construction And Why Does This Make A Difference For The Robustness Of My Program?
Geometric predicates are used to test residences of geometric objects and return, as an example, a Boolean fee (genuine or fake). An instance of predicate would be trying out whether or not a point lies internal a sphere is a predicate as is testing whether 3 points form a left turn, right turn or are collinear. A geometric construction creates a brand new geometric item, such as the line thru two one-of-a-kind factors or the middle of a circle described through a positive set of points.
Constructions are elaborate with recognize to robustness because the built object has to be stored in a specific illustration. If this illustration does no longer capture the item's homes sufficiently, due, as an example, to confined precision round offs, the correctness of next computations with this object can't be guaranteed. However, presenting precise structures is currently much less green, which is basically the reason why the kernel CGAL::Exact_predicates_inexact_constructions_kernel exists; double is used as the representation type in this example.
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Question 10. I Want To Modify The Coordinates Of A Point, Or The Endpoints Of A Segment, But The Cgal Kernel Does Not Seem To Support That. Why?
Our kernel concept is illustration-impartial. There isn't any assumption on how a Point_3, as an instance, is represented. In particular, it need not be represented by means of Cartesian coordinates. Thus writing code that is based on being capable of alternate those coordinates might also cause inefficient code. Our simple library algorithms are designed around the predicates that function at the geometric gadgets and do no longer require this mutability.
Non-modifiability additionally makes the underlying handle-rep scheme used (by means of default) to allocate, reproduction, etc., CGAL gadgets extremely less complicated. We understand, however, that users may additionally need this flexibility and for this reason are working on imparting mutable kernel items.
The simplest manner to do this currently is to construct a new factor and assign it to the antique one:
use p = Point_2 (p.X (), p.Y () +1);
as in case you would love to do p.Y () += 1.
Question eleven. I Want To Convert A Number Type, E.G. Ft To Double. How Do I Do That?
All variety types used by CGAL provide a conversion function called CGAL::to double.
For example: double x = CGAL::to double (p.X ()).
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Question 12. My Program Reports Microsoft C++ Exception: Cgal::uncertain_conversion_exception At Memory Location ... Why?
This is an internal exception, which is caught by means of CGAL code. Users are not purported to be worried by this, it's far accurate behaviour, except that it every now and then shows up, as an example whilst debugging. It is safe to ignore these exceptions.
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Question 13. How Can I Do Boolean Operations On Polygons?
There are two approach provided in CGAL for acting Boolean operations on polygons inside the plane. One is supplied through the class template Nef_polyhedron_2. The other is provided thru the Boolean Set-Operations bundle. There are demo applications illustrating Boolean operations using each of these. The manual pages offer extra facts concerning, for example, additional functionality available with those magnificence templates.
Question 14. How Do I Compute The Triangulation Of The Interior Of A Polygon?
For this, one should use one of the confined triangulation lessons: Constrained Triangulation 2, Constrained Delaunay Triangulation 2 or Constrained Triangulation plus 2. The edges of the polygon must be enter as constraints to the triangulation. The built triangulation might be a triangulation of the entire convex hull, but such as as edges the rims of the polygon which might be marked as restrained edges. From there it is straightforward to output the faces inner (resp. Outdoor) the polygon or to mark those faces as such.
Question 15. Is The Outer Ccb (counterclockwise Boundary) Of A Face Of An Arrangement A Simple Polygon?
The outer CCB of a face of an arrangement is NOT always a easy polygon. Consider the case where there are "internal antennas"; imagine a face represented through a easy polygon and a vertex v at the outer CCB of the face. Now connect a factor that starts offevolved and ends at v and lies in the indoors of the face (one edge incident at v suffices).
Mind that if you are the use of an inexact variety kind you may get a non-simple polygon even supposing the outer CCB represents one because of numerical errors.
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Question 16. Boolean Set Operations: How Can I Compute Them?
The package deal Regularized Boolean Set-Operation consists of the implementation of Boolean set-operations on factor units bounded by way of weakly x-monotone curves in 2-dimensional Euclidean space. In specific, it contains the implementation of regularized Boolean set-operations, intersection predicates, and point containment predicates.
Ordinary Boolean set-operations that perform on (linear) polygons, which distinguish between the interior and the boundary of a polygon, are supported by the Planar Neff Polyhedra bundle.
For intersections of the interiors of polygons additionally discuss with the polygon intersection demo program, which is likewise to be had within the demo listing of the CGAL distribution.
Question 17. How Can I Attach Information To A Vertex, A Half Edge, Or A Face?
You need to change the DCEL, in order that its vertex, half of part, and/or face sorts will comprise the records you want. The package allows the coding essential to reap this challenge. Refer to Section Extending the DCEL for similarly details.
You have to deliver your version of the concept Arrangement Dcel, which represents the prolonged DCEL and instantiate CGAL::Arrangement On Surface 2 <Traits, Dcel> properly.
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Question 18. Why Is There No Transform Function For Polyhedron_3?
The polyhedral floor (and other records systems inside the Basic Library) could have brought geometric attributes via the user that would make a normally working transform feature for affine differences hard to provide inside the elegance, and however it is simple for a user to use wellknown everyday algorithms for that purpose.
As an example: to transform the points stored in a polyhedral surface P with a CGAL affine transformation given in A (that is a proper STL functor) one can write:
std::remodel (P.Points_begin (), P.Points_end (), P.Points_begin (), A);
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