CODEWITHSUNDEEP
qtcomputational-geometry
spraff
spraff

Reputation: 33435

QPolygonF/QLineF intersection

QPolygonF has methods to union, intersect, and subtract with other QPolygonFs but I need to perform an intersection test with a QLineF. This appears to be missing from the API.

I suppose I could do something like this:

if (polygon .containsPoint (line .p1 ()) != polygon .containsPoint (line .p2 ())
    return true;

QPointF a = polygon .back ();
foreach (QPointF b, polygon)
{
    if (QLineF :: BoundedIntersection == line .intersect (QPointF (a, b))
       return true;
    a = b;
}

return false;

There are probably some numerical or edge-case surprises lurking in the above, so I'd rather not.

Is there a provided method somewhere in the Qt API that I can't see?

Upvotes: 4

Views: 6717

Answers (2)

hmuelner
hmuelner

Reputation: 8231

The implementation of QPolygonF::intersected is:

QPainterPath subject; subject.addPolygon(*this);
QPainterPath clip; clip.addPolygon(r);

return subject.intersected(clip).toFillPolygon();

i.e. QPolygonF uses QPainterPath methods to do the intersections. So you could do:

  • make a QPainterPath from the lines of the polygon
  • make a QPainterPath from your line
  • intersect them
  • check the result with isEmpty()
  • get the intersection point with boundingRect()

I DID NOT TRY THIS! You have to check this yourself. I am not sure if this is better in regard to numerical stability and performance than your solution, so you should write some tests.

Upvotes: 5

spraff
spraff

Reputation: 33435

Unfortunately, the answer is "no".

Upvotes: 1

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