On 06/01/2015 03:52 PM, luc wrote:
> Le 20150601 15:27, Gilles a écrit :
>> Hello.
>>
>> On Mon, 01 Jun 2015 15:03:47 +0200, luc wrote:
>>> Le 20150601 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 hardfailure
>>>> (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, 1e6);
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.
Thomas
>
>>
>> So, I note that "generate" did not raise an exception whereas the
>> computation
>> request could be deemed invalid. Then "createRegion" raises an
>> exception...
>> Something looks wrong here: if the "hull" instance is valid (from a
>> programming
>> perspective), then "hullRegion" should be valid too (i.e. "empty").
>>
>> Assuming that you don't want to change the existing code, how can I
>> create an
>> empty region? Is there a singleton instance to represent the concept?
>
> There are no singleton for empty space or full space as regions are
> always associated
> with other metadata like tolerance, which are often problemspecific.
>
> You can build one as follows:
>
> Region empty = new PolygonsSet(new BSPTree<Euclidean2D>(false),
> tolerance);
>
> Tolerance is mainly a distance in the space (here the 2D Euclidean
> plane), and
> points this close to a line will be consider to belong to the line.
>
> best regards,
> Luc
>
>>
>> Thanks,
>> Gilles
>>
>>>
>>> best regards
>>> Luc
>>
>>
>> 
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