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From Thomas Neidhart <thomas.neidh...@gmail.com>
Subject Re: [Math] Allow empty "ConvexHull2D"
Date Wed, 10 Jun 2015 10:23:58 GMT
On 06/09/2015 11:42 PM, Gilles wrote:
> On Tue, 09 Jun 2015 23:19:35 +0200, Thomas Neidhart wrote:
>> On 06/01/2015 09:36 PM, Thomas Neidhart wrote:
>>> On 06/01/2015 03:52 PM, luc wrote:
>>>> Le 2015-06-01 15:27, Gilles a écrit :
>>>>> Hello.
>>>>>
>>>>> On Mon, 01 Jun 2015 15:03:47 +0200, luc wrote:
>>>>>> Le 2015-06-01 14:38, Gilles a écrit :
>>>>>>> Hi.
>>>>>>
>>>>>> Hi Gilles,
>>>>>>> I have a question regarding
>>>>>>> public Region<Euclidean2D> createRegion() throws
>>>>>>> InsufficientDataException
>>>>>>> in ConvexHull2D.
>>>>>>> It throws the exception when the number of points is < 3.
>>>>>>> One can imagine that rather than aborting it could return an
"empty
>>>>>>> Region"
>>>>>>> (which would seamlessly work with further operations on the Region).
>>>>>>> What do you think?
>>>>>>> Context: in the course of a program, a "valid" region can undergo
>>>>>>> successive
>>>>>>> transformation until it is indeed impossible to compute the hull;
it
>>>>>>> seems
>>>>>>> that it would be interesting to not treat that as a hard-failure
>>>>>>> (warranting
>>>>>>> an exception).
>>>>>>
>>>>>> I'm on the fence on this. The exception is advertised right at the
>>>>>> the top
>>>>>> interface level (ConvexHull in o.a.c.m.geometry.hull package) and
>>>>>> clearly intended
>>>>>> to cover this case. An empty region could be expected from computing
>>>>>> the hull of n >= 3 aligned points, but n < 3 points is something
>>>>>> different to me.
>>>>>
>>>>> This is how I get the "Region" in my program:
>>>>>
>>>>>    final ConvexHullGenerator2D hullGen = new MonotoneChain(false);
>>>>>    final ConvexHull2D hull = hullGen.generate(data);
>>>>>    final Region<Euclidean2D> hullRegion = hull.createRegion();
>>>>
>>>> Looking at AbstractConvexHullGenerator2D and ConvexHull2D, they indeed
>>>> seem inconsistent with respect to the number of points.
>>>>
>>>> In AbstractConvexHullGenerator2D.generate, you can have points.size() <
>>>> 2, there
>>>> is a special handling. In ConvexHull2D you cannot have
>>>> vertices.length < 3.
>>>>
>>>> I think this is a problem.
>>>
>>> My understanding was that creating a 2D region with less than 3 points
>>> does not give a meaningful result, but this assumption may be wrong.
>>>
>>> I did recall doing some tests with less than 3 points and got weird
>>> results, see the test case below:
>>>
>>>   final RegionFactory<Euclidean2D> factory = new
>>> RegionFactory<Euclidean2D>();
>>>
>>>   Vector2D p1 = new Vector2D(0, 0);
>>>   Vector2D p2 = new Vector2D(1, 0);
>>>   Vector2D p3 = new Vector2D(1, 1);
>>>   Vector2D p4 = new Vector2D(0.8, 0.8);
>>>
>>>   final Line[] lineArray = new Line[1];
>>>   lineArray[0] = new Line(p1, p2, 1e-6);
>>>
>>>   Region<Euclidean2D> reg = factory.buildConvex(lineArray);
>>>   System.out.println(reg.getSize());
>>>   System.out.println(reg.checkPoint(p3));
>>>   System.out.println(reg.checkPoint(p4));
>>>
>>> I get different results for 3.3 and 4.0:
>>>
>>> In 3.3, the results are:
>>>
>>>  Infinity
>>>  INSIDE
>>>  INSIDE
>>>
>>> In 4.0:
>>>
>>>  IndexOutOfBoundsException
>>>  INSIDE
>>>  INSIDE
>>>
>>> Doing the same test with 2 line segments (p1 -> p2 and p2 -> p3) leads
>>> to a NullPointerException when calling getSize().
>>>
>>> So, I tried to be conservative when creating the region, and this is
>>> also expressed in the javadoc, although I admit there is no method to
>>> return the minimum number of hull points that need to be present in
>>> order to create a region.
>>>
>>> I am really wondering if a 2D region consisting of less than 3 points
>>> does make sense, and if so, what is the correct way to construct such a
>>> region and what are the expected properties?
>>>
>>> If this is clarified, I am fine to update the ConvexHull2D class to
>>> handle these cases too.
>>
>> There was no further response to my posting, but I am curious how to
>> proceed with this topic.
>>
>> Is it reasonable to return an empty region if the number of hull
>> vertices is below 3?
> 
> In the code excerpt I had given (cf. above), I now use
> 
>   final Region<Euclidean2D> hullRegion = data.size() < 3 ?
>     EMPTY_REGION :
>     hull.createRegion();
> 
> where EMPTY_REGION is defined as suggested by Luc:
> 
>   new PolygonsSet(new BSPTree<Euclidean2D>(false), 0d);
> 
> Hence my question in the other post about a factory method that
> would create such a region.
> 
> It seems logical to me that if no hull exists, the region it covers
> is "empty", and can be used as such through the "Region" interface
> without any problem; hence raising an exception hinders functionality
> and code flow.
> 
>> Or should we add an additional method: isValidRegion() to check if the
>> given convex hull can form a valid region?
> 
> What is a valid/invalid region?
> [Cf. other post.]

according to
http://en.wikipedia.org/wiki/Region_%28mathematical_analysis%29 a region
must be non-empty, but I am not sure if the Region interface is intended
to follow this convention, that's why I am asking.

Thomas

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