[osg-users] Easy way to pick nearest edge?

Robert Osfield robert.osfield at gmail.com
Tue Sep 2 01:06:32 PDT 2008

Hi Simon,

The LineSegmentIntersector records the indices of the vertices of the
primitive that has been hit.  The
LineSegmentIntersector::Intersection::indexList and ratioList provide
the indices and barycentric coords.


On Tue, Sep 2, 2008 at 8:54 AM, Simon Notheis <notheis at ira.uka.de> wrote:
> Hi everyone,
> I want to use mouse events to pick an object of the 3D scene. Instead of
> working with the intersection point, I want to find the nearest vertex
> or edge of the polygon that was hit.
> Until now I used code similar to the following one:
>  // Get the hitlist by using osg::Viewer::computeIntersection(...)
>  const osgUtil::LineSegmentIntersector::Intersection&
>    intersection = *(hitlist.begin())
>  osg::Geometry* geometry = intersection.drawable->asGeometry()
> Then i used intersection.indexList and the geometry's VertexArray to
> iterate through all vertices to find the closest vertex (vx) wrt the
> intersection point.
> Then I calculated the angles between vx and the both neighbouring
> vertices in the indexList to determine which one belongs to the closest
> edge. Finally, the nodePath gave me the local=>world transformation.
> All of that only worked because we had some bad code to load/convert our
> own scene format into OSG. This code generated seperate triangles or
> quads, so actually I always hit a polygon with 3 or 4 vertices, what
> made it easy to determinie the neighbours of vx.
> Now that we're using some optimization steps (e.g. triangle strips), I
> can't use the above code (e.g. the indexList is empty). I tried to use
> the geometry's IndexArray instead, but the problem is that without
> knowing the exact primitive type, it is hard to determine which vertices
> are the neighbouring ones to find the edge. E.g. in case of an triangle
> strip I would have to check the last 2 and next 2 vertices in the
> index/vertex list. Also the code won't work if the drawable can't be
> cast to a Geometry.
> So my question is, if there's a more easy or flexible way to determine
> the closest vertex and edge of the intersected polygon/primitve?
> Looking forward to your ideas or solutions, if something similar was
> already discussed here.....
>  Simon
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