[osg-users] Text Intersecting Earth

Robert Osfield robert.osfield at gmail.com
Wed Oct 17 01:13:40 PDT 2007

Hi Jeffrey,

In the 2.x series of the OSG there is an osgfadetext example that uses
a ClusterCullingCallback to cull the text when its on the far side of
the earth, and disables depth test when its on the front side - this
sounds similar to your needs.


On 10/16/07, Jeffrey M Short <jshort8 at csc.com> wrote:
> Hi,
> First off I'm using OSG 1.0.
> I am creating an application that has globe viewing features.  The user can
> add an location object at a given Latitude, Longitude and Altitude.  Then a
> 3d model and a text label is placed at that location.  Both the model node
> and the text geode are placed under a MatrixTransform with the matrix set
> using the
> osg::EllipsoidModel->computeLocalToWorldTransformFromLatLongHeight.  The
> text uses SCREEN_COORDS as the CharacterSizeMode and uses SCREEN as the
> AxisAlignment.
> The problem is that the text is intersects the earth when viewed from
> certain viewpoints.  I would like to draw the earth first, then draw all
> the text, but only draw any particular text geode if and only if the
> location is not clipped by the Earth.
> My first attempt was as follow:
>      I set the text geode depth test to osg::Depth::ALWAYS.
>      I added a custom osg::Drawable::CullCallback to the text geodes.  In
> the callback's virtual bool cull(osg::NodeVisitor* nv, osg::Drawable*
> drawable, osg::State*) const function I got the eye location and the text
> location in Earth Centered Cartesian Coordinates.  Then if the dot product
> of these 2 position vectors is < 0, return true so the text is culled.
> One problem I encountered here was getting the eye location from
> CullVisitor::getEyePoint().  This function seems to return the EyePoint in
> local coordinates.  I tried using the
> osg::computeLocalToWorld(nv->getNodePath()) matrix to transform the
> EyePoint to Earth Centered coordinates but that didn't work.  I had
> transform osg::Vec4(EyePoint, 1) by the inverse of
> osg::EllipoidModel::computeLocalToWorldTransformFromXYZ to get the job
> done.
> This method works but only when the camera is targeted at the center of the
> earth and it only works for locations with 0 altitude (imagine a location
> on the "other side" of the Earth from where you are viewing.  If the
> location has a large enough altitude you could still see it).
> Any suggestions on a better way to accomplish this?
> Thanks
>      Jeff
> Computer Sciences Corporation
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