CGAL users discussion list

["Kenneth Tham" <>]

Hi,
   Just wondering, is there a function in CGAL for us to plot out the points 
in a polygon to view it visually? Thanks.
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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