QPainterPath or QPolygon centroid/center/origin - How to determine ?

[muenalan]

I want to graphically depict the center point (center point of gravity) of a complex/non-symmetrical Polygon. There is no

QPointF QPolygon::center()

Finding the center of the boundingRect was easy, but wrong.

Any code/idea ?

[init2null]

I can't provide you with any mathematical code snippets, but I use the GEOSGetCentroid function from the C++ library GEOS to get the centroid. It has an easy-to-use C wrapper, which the developers recommend. It's quite a nice library and is under the LGPL. Here's an example of using the library for getting the centroid.

GEOSGeometry *getPolygon(bool &error) {
    //free the returned value with GEOSGeom_destroy
    error = false;
    if (points.size() < 3) {
        error = true;
        return 0;
    } else {
        GEOSCoordSequence *seq = GEOSCoordSeq_create(points.size()+1, 2);
        for (int a=0;a<points.size();++a) {
            GEOSCoordSeq_setX(seq, a, points[a].x());
            GEOSCoordSeq_setY(seq, a, points[a].y());
        }
        GEOSCoordSeq_setX(seq, points.size(), points[0].x());
        GEOSCoordSeq_setY(seq, points.size(), points[0].y());
        GEOSGeometry *ring = GEOSGeom_createLinearRing(seq);
        return GEOSGeom_createPolygon(ring, NULL, 0);
    }
}
 
QPointF getCenter(bool &error) {
    GEOSGeometry *poly = getPolygon(error);
    if (error) {
        return QPointF();
    } else {
        GEOSGeometry *cpoint = GEOSGetCentroid(poly);
        const GEOSCoordSequence *cseq = GEOSGeom_getCoordSeq(cpoint);
        double x=0, y=0;
        GEOSCoordSeq_getX(cseq, 0, &x);
        GEOSCoordSeq_getY(cseq, 0, &y);
        GEOSGeom_destroy(poly);
        GEOSGeom_destroy(cpoint);
        return QPointF(x, y);
    }
}

GEOSGeometry *getPolygon(bool &error) {
    //free the returned value with GEOSGeom_destroy
    error = false;
    if (points.size() < 3) {
        error = true;
        return 0;
    } else {
        GEOSCoordSequence *seq = GEOSCoordSeq_create(points.size()+1, 2);
        for (int a=0;a<points.size();++a) {
            GEOSCoordSeq_setX(seq, a, points[a].x());
            GEOSCoordSeq_setY(seq, a, points[a].y());
        }
        GEOSCoordSeq_setX(seq, points.size(), points[0].x());
        GEOSCoordSeq_setY(seq, points.size(), points[0].y());
        GEOSGeometry *ring = GEOSGeom_createLinearRing(seq);
        return GEOSGeom_createPolygon(ring, NULL, 0);
    }
}

QPointF getCenter(bool &error) {
    GEOSGeometry *poly = getPolygon(error);
    if (error) {
        return QPointF();
    } else {
        GEOSGeometry *cpoint = GEOSGetCentroid(poly);
        const GEOSCoordSequence *cseq = GEOSGeom_getCoordSeq(cpoint);
        double x=0, y=0;
        GEOSCoordSeq_getX(cseq, 0, &x);
        GEOSCoordSeq_getY(cseq, 0, &y);
        GEOSGeom_destroy(poly);
        GEOSGeom_destroy(cpoint);
        return QPointF(x, y);
    }
}

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[muenalan]

Thanks, I'll have a look at

GEOSGetCentroid(poly)

Thanks for the helpfull reply,
Mu