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. 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 meta-data like tolerance, which are often problem-specific. > > 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 >> >> >> --------------------------------------------------------------------- >> To unsubscribe, e-mail: user-unsubscr...@commons.apache.org >> For additional commands, e-mail: user-h...@commons.apache.org > > --------------------------------------------------------------------- > To unsubscribe, e-mail: user-unsubscr...@commons.apache.org > For additional commands, e-mail: user-h...@commons.apache.org > --------------------------------------------------------------------- To unsubscribe, e-mail: user-unsubscr...@commons.apache.org For additional commands, e-mail: user-h...@commons.apache.org
