[geos-devel] Re: Computing Radials inside a Polygon
Sanak Goe
geosanak at gmail.com
Sat Aug 29 23:39:29 EDT 2009
Hi Jo,
Thanks for your reply.
Ok, your last image seems to be no problem.
Regards,
Sanak.
2009/8/30 Jo <doublebyte at gmail.com>
> Thinking better, I could see what I was doing wrong.
> I was rotating a point around (0,0), rather than rotating it around my
> origin (the centroid). Therefore, I had to add the difference between
> the point and centroid coordinates to the sin and cosine terms:
>
> double
> rx=centroid->getX()+cos(fangle)*(x-centroid->getX())-sin(fangle)*(y-centroid->getY());
> double
> ry=centroid->getY()+sin(fangle)*(x-centroid->getX())+cos(fangle)*(y-centroid->getY());
>
> And calling consecutively this function, gives me the equally spaced
> radials from a centroid of a polygon to its external ring; in this
> example, using a 45 degrees angle:
>
> http://ladybug.no-ip.org/files/radials_calc.png
>
> cheers,
> Jo
>
>
> 2009/8/29 <geos-devel-request at lists.osgeo.org>:
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> > 1. Re: Computing Radials inside a Polygon (Jo)
> >
> >
> > ----------------------------------------------------------------------
> >
> > Message: 1
> > Date: Sat, 29 Aug 2009 16:16:23 +0100
> > From: Jo <doublebyte at gmail.com>
> > Subject: [geos-devel] Re: Computing Radials inside a Polygon
> > To: geos-devel at lists.osgeo.org
> > Message-ID:
> > <23ab5f0a0908290816j66928c44y58de481b26525657 at mail.gmail.com>
> > Content-Type: text/plain; charset=ISO-8859-1
> >
> > Hi Jo,
> >
> > I think that each rotate angles should be calculated more simply.
> > (I think rotation matrix is valid when centroid is equal to origine(0,
> 0).)
> >
> > The following code is pseudo code. (Sorry I haven't OGR environment...)
> >
> > ===================================
> > // get minCircle radius. (If radius is not calculated yet)
> > double radius=Distance(¢roid, &startPt); // I don't know that
> Distance
> > function is implemented in OGR...
> > // get start angle.
> > double startAngle=atan2(&startPt.getY()-¢roid.getY(),
> > &startPt.getX()-¢roid.getX());
> >
> > for (size_t i=0; i <n; ++i){
> > OGRLineString dLine;
> > dLine.addPoint(¢roid);
> >
> > double rx=centroid.getX() + radius*cos(startAngle+fangle*i);
> > double ry=centroid.getY() + radius*sin(startAngle+fangle*i);
> > :
> > }
> > ===================================
> >
> > Regards,
> >
> > Sanak.
> >
> > Hi Sanak,
> > First of all, thanks for your reply!
> > I think I didn't explain myself clearly though...
> >
> > The idea is to compute radials from the centroid of the polygon, to
> > its external ring, at equal intervals. Something like this:
> >
> > http://ladybug.no-ip.org/files/radials_final.png
> >
> > I had the idea of starting with a vector that starts in the centroid
> > and ends up in the external ring (at any point), and therefore its
> > length equals the radius; to make things simple I choose a point that
> > is in the same x-alignment as the centroid, and so the start angle in
> > your code would be null. My start vector is something like this:
> >
> > http://ladybug.no-ip.org/files/start.png
> >
> > Than my idea, was to rotate this vector around the centroid, at a
> > constant angle, and get the radials.
> > Lets assume for simplicity that the angle is 90 degrees (PI/2 radians)
> > and therefore I want to end up with four radials.
> > As I compute the new locations of the rotated endpoint of the vector,
> > I store this point on a stl container, so that in the end I can loop
> > through all these locations and generate my vectors.
> > Here is a GEOS version of my code:
> >
> > //////////GEOS VERSION /////
> > std::vector<geos::geom::Point*> pts;
> >
> > geos::geom::GeometryFactory factory;
> > geos::geom::Coordinate
> coordC(ogrCentroid.getX(),ogrCentroid.getY());
> >
> > //create centroid from given point
> > geos::geom::Point* centroid = factory.createPoint(coordC);
> >
> > //create start pt as any point on the perimeter (for instance, a
> > point aligned in x with the centroid)
> > geos::geom::Coordinate
> coordS(ogrCentroid.getX()+radius,ogrCentroid.getY());
> > geos::geom::Point* startPt = factory.createPoint(coordS);
> >
> > pts.push_back(startPt);
> >
> > //nr radials=4; angle=90 deg= PI/2
> > double fangle=PI/2.0;
> >
> > for (size_t i=0; i <4; ++i){
> >
> > double x=pts.back()->getX();
> > double y=pts.back()->getY();
> >
> > double rx=x*cos(fangle)-y*sin(fangle);
> > double ry=x*sin(fangle)+y*cos(fangle);
> >
> > geos::geom::Coordinate coords(x+rx,y+ry);
> > geos::geom::Point* aPoint = factory.createPoint(coords);
> >
> > pts.push_back(aPoint);
> > }
> >
> > Let me mention that this code is *not* doing what I want, since:
> > - I am getting the wrong angle between the start point and the first
> > iteration point;
> > - the angles do not appear to be 90 degrees;
> > - I think also the angles are not constant;
> >
> > Here is a screenshot of my output:
> >
> > http://ladybug.no-ip.org/files/output.png
> >
> > So I guess there must be some assumption (or more than one, actually)
> > that is wrong in my code :-)
> > Again, I would be very grateful for any observations or suggestions...
> > thanks in advance for your help!
> >
> > cheers,
> > Jo
> >
> >
> > ------------------------------
> >
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> >
> > End of geos-devel Digest, Vol 82, Issue 25
> > ******************************************
> >
>
>
>
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