On Wed, 10 Jun 2015 12:23:58 +0200, Thomas Neidhart wrote:
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.
I've given a reasonable (IMHO) practical use-case (in an earlier post).
Another, comparable, situation would be empty collections: it would not
occur to anyone that an exception must be thrown because one would want
to
loop over all elements of an empty list.
And finally, there is an "isEmpty()" method in the "Region" interface.
If an empty region cannot exist, wouldn't it mean that the method
should
always return "false"? Yet Luc's suggestion creates a Region for which
"isEmpty" returns "true"...
Gilles
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