Re: [Flightgear-devel] Possible cause of JSBSim airport bug

[<Wol...@t-...>]

Ok, I just had a look and maybe advanced a little bit.

The problem is that the hitcount is zero and therefore these lines are
taken:

	SG_LOG( SG_TERRAIN, SG_INFO, "no terrain intersection" );
	*terrain_elev =3D 0.0;


So the question is, why is the hitcount zero?
Look at:

bool FGTileMgr::current_elev_ssg( sgdVec3 abs_view_pos, double
*terrain_elev ) {
    sgdVec3 view_pos;
    sgdVec3 sc;
    sgdSetVec3( sc, scenery.center.x(), scenery.center.y(),
scenery.center.z());
    sgdSubVec3( view_pos, abs_view_pos, sc );

    sgdVec3 orig, dir;
    sgdCopyVec3(orig, view_pos );
    sgdCopyVec3(dir, abs_view_pos );

    hit_list.Intersect( terrain, orig, dir );

The bug has to be in these lines or before.
abs_view_pos is in the order of magnitude -2millions, -4millions,
3millions.

scenery.center.x(), scenery.center.y(), scenery.center.z() is
*exactly* the same, so that view_pos becomes exactly (0.0, 0.0, 0.0).
I think this is the problem:
I would guess that hit_list.Intersect looks from the "orig" point in
the direction dir. (and not backwards). That would mean, that orig has
to be above the terrain, but obviously it is *in* the terrain.

Bye bye,
Wolfram.