CGAL users discussion list

["Kenneth Tham" <>]

Hi,
   Thanks for your help. Yes, the polygon has 1 common point so it is 
probably not simple, which may be the cause of the failure. I have added a 
check using the method is_simple() to check if the joined polygon is simple 
after joining. It seems to work better with the code now. What is the 
difference between simple and strictly simple polygons? Thank you.
Regards,
Kenneth


-----Original Message-----
From: 

 
[mailto:]
Sent: Tuesday, February 19, 2008 4:41 AM
To: 

Subject: RE: [cgal-discuss] CGAL::join Function



Hi Kenneth,

There are a few faults in your example which cause the errors.

I'll refer to the following small part of Q from your example:

Q.push_back (Point_2(103.63966979999999,1.2391405));
Q.push_back (Point_2(103.64162709999999,1.2391810999999999));
Q.push_back (Point_2(103.6397209,1.2391553));
Q.push_back (Point_2(103.63966979999999,1.2391405));
Q.push_back (Point_2(103.6395785,1.2391258000000001));

Q.push_back (Point_2(103.6395201,1.2391295));

Q.push_back (Point_2(103.6394873,1.2391405));


1. Polygon Q maybe simple but is definitely not strictly simple
(points 1 and 4 are the same).The predicate
is_counterclockwise_oriented() has a precondition that allows ony
strictly simple polygons to be tested.

2. 1. Polygon Q is clockwise oriented. The Polygons package reqires
the polygon boundaries to be oriented in a counter clockwise manner
which may be the reason join() fails.

I suggest you try to split your polygon Q into a series of smaller
strictly simple  counterclockwise oriented polygons and try to perform
the join operation.

If this doesn't work, try and minimize Q to the smallest polygon that
fails (hopefully less ten 10 vertices), and I'll have a another look...

Sincerely,

Guy Zucker


-------------

Quoting Kenneth Tham 
<>:

> Hi,
>     Attached is my code that gives the error. The error occurs when   
>  the join function is called. I have commented out the code that    
> checks for orientation of the Polygon Q as that also gives an error.  
>   The interesting thing is that if the first three points are not    
> added in Polygon Q, the resulting polygon is anti-clockwise    
> oriented. But when the 3 points are added, it cannot pass the    
> checking. Thanks for your help.
>
> Regards,
> Kenneth
>
>
>       -----Original Message-----
>       From: 
> 
>  on behalf of Ophir Setter
>       Sent: Thu 2/14/2008 4:53 PM
>       To: 
> 
>       Cc:
>       Subject: Re: [cgal-discuss] CGAL::join Function
>
>
>       What number type? - You shoiuld use a kernel that supports exact    
> construction and exact predicates.
>
>       I think the best way is if you send code and data that creates the 
> bug.
>
>
>       On Thu, Feb 14, 2008 at 10:45 AM, Kenneth Tham    
> <>
>  wrote:
>
>
>               Hi,
>                   I am using a Kernel class inheriting from 
> CGAL::Cartesian<Number_type>.
>               Regards,
>               Kenneth
>
>
>                       -----Original Message-----
>                       From: 
> 
>  
> [mailto:]On
>     
> Behalf Of Ophir Setter
>                       Sent: Thursday, February 14, 2008 3:41 PM
>                       To: 
> 
>                       Subject: Re: [cgal-discuss] CGAL::join Function
>
>
>                       Can send the code that generates the code?
>                       What kernel are you using?
>                       Can you send the data that causes this assertion?
>
>
>
>                       On Thu, Feb 14, 2008 at 8:47 AM, Kenneth Tham    
> <>
>  wrote:
>
>
>                               Hi,
>                                 Thanks for your help. I am now trying to 
> debug the algorithm    
> for joining the polygons. There is one case where it needs to merge   
>  about 300 polygons. After about 200 times, the program gives the    
> error as shown in the attached file. I'm not really sure what it    
> means. Can anybody help? Thanks.
>                               Regards,
>                               Kenneth
>
>
>
>
>                               -----Original Message-----
>                               From: Efi Fogel 
> [mailto:]
>                               Sent: Tuesday, February 12, 2008 10:23 PM
>                               To: 
> 
>                               Subject: Re: [cgal-discuss] CGAL::join 
> Function
>
>
>                               Kenneth Tham wrote:
>
>                               >Hi,
>                               >    I am using the CGAL::join function for 
> joining    
> polygon_with_holes_2 objects. In my program, I use the join function  
>   for about 10000 times consequtively(trying to join a polygon to a   
>  set of polygons). My program takes quite a long time to process  
> this   part ( several minutes ). Is it normal for this function to  
> take   this amount of time? Thanks.
>                               >Regards,
>                               >Kenneth
>                               >
>                               >
>                               >
>                               First of all you should call the join 
> function just once with a range of
>                               input polygons (or polygons with holes). If 
> you intend to perform other
>                               operations in a sequence, you should call the 
> join member function of
>                               Polygon_set_2 instead of the global join 
> function. Finally, I assume you
>                               are using a exact-prediacte 
> exact-construction kernel (for a good
>                               reason). This means that all operations are 
> caried out in an exact
>                               maner. You can try to expedite the 
> computation by reducing the bit
>                               length of the input coordinates.
>
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