flightgear/src/Scenery/tileentry.cxx

// tileentry.cxx -- routines to handle a scenery tile
//
// Written by Curtis Olson, started May 1998.
//
// Copyright (C) 1998 - 2001  Curtis L. Olson  - curt@flightgear.org
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; either version 2 of the
// License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
//
// $Id$


#ifdef HAVE_CONFIG_H
#  include <config.h>
#endif

#include <simgear/compiler.h>

#include <simgear/bucket/newbucket.hxx>
#include <simgear/debug/logstream.hxx>
#include <simgear/math/sg_geodesy.hxx>
#include <simgear/math/sg_random.h>
#include <simgear/misc/sgstream.hxx>

#include <Aircraft/aircraft.hxx>
#include <Include/general.hxx>
#include <Main/globals.hxx>
#include <Scenery/scenery.hxx>
#include <Time/light.hxx>
#include <Objects/matlib.hxx>
#include <Objects/newmat.hxx>
#include <Objects/obj.hxx>

#include "tileentry.hxx"
#include "tilemgr.hxx"


// Constructor
FGTileEntry::FGTileEntry ( const SGBucket& b )
    : ncount( 0 ),
      center( Point3D( 0.0 ) ),
      tile_bucket( b ),
      terra_transform( new ssgTransform ),
      terra_range( new ssgRangeSelector ),
      loaded(false),
      pending_models(0)
{
    nodes.clear();

    // update the contents
    // if ( vec3_ptrs.size() || vec2_ptrs.size() || index_ptrs.size() ) {
    //     SG_LOG( SG_TERRAIN, SG_ALERT, 
    //             "Attempting to overwrite existing or"
    //             << " not properly freed leaf data." );
    //     exit(-1);
    // }
}


// Destructor
FGTileEntry::~FGTileEntry () {
    // cout << "nodes = " << nodes.size() << endl;;
    // delete[] nodes;
}


#if 0
// This is the current method cut and pasted from 
//  FGTileEntry::load( const SGPath& base, bool is_base )
void
FGTileEntry::WorldCoordinate( sgCoord *obj_pos, Point3D center,
                              double lat, double lon, double elev, double hdg)
{
    // setup transforms
    Point3D geod( lon * SGD_DEGREES_TO_RADIANS,
                  lat * SGD_DEGREES_TO_RADIANS,
                  elev );
	
    Point3D world_pos = sgGeodToCart( geod );
    Point3D offset = world_pos - center;
	
    sgMat4 POS;
    sgMakeTransMat4( POS, offset.x(), offset.y(), offset.z() );

    sgVec3 obj_rt, obj_up;
    sgSetVec3( obj_rt, 0.0, 1.0, 0.0); // Y axis
    sgSetVec3( obj_up, 0.0, 0.0, 1.0); // Z axis

    sgMat4 ROT_lon, ROT_lat, ROT_hdg;
    sgMakeRotMat4( ROT_lon, lon, obj_up );
    sgMakeRotMat4( ROT_lat, 90 - lat, obj_rt );
    sgMakeRotMat4( ROT_hdg, hdg, obj_up );

    sgMat4 TUX;
    sgCopyMat4( TUX, ROT_hdg );
    sgPostMultMat4( TUX, ROT_lat );
    sgPostMultMat4( TUX, ROT_lon );
    sgPostMultMat4( TUX, POS );

    sgSetCoord( obj_pos, TUX );
}
#endif


// Norman's 'fast hack' for above
static void WorldCoordinate( sgCoord *obj_pos, Point3D center, double lat,
                             double lon, double elev, double hdg )
{
    double lon_rad = lon * SGD_DEGREES_TO_RADIANS;
    double lat_rad = lat * SGD_DEGREES_TO_RADIANS;
    double hdg_rad = hdg * SGD_DEGREES_TO_RADIANS;

    // setup transforms
    Point3D geod( lon_rad, lat_rad, elev );
	
    Point3D world_pos = sgGeodToCart( geod );
    Point3D offset = world_pos - center;

    sgMat4 mat;

    SGfloat sin_lat = (SGfloat)sin( lat_rad );
    SGfloat cos_lat = (SGfloat)cos( lat_rad );
    SGfloat cos_lon = (SGfloat)cos( lon_rad );
    SGfloat sin_lon = (SGfloat)sin( lon_rad );
    SGfloat sin_hdg = (SGfloat)sin( hdg_rad ) ;
    SGfloat cos_hdg = (SGfloat)cos( hdg_rad ) ;

    mat[0][0] =  cos_hdg * (SGfloat)sin_lat * (SGfloat)cos_lon - sin_hdg * (SGfloat)sin_lon;
    mat[0][1] =  cos_hdg * (SGfloat)sin_lat * (SGfloat)sin_lon + sin_hdg * (SGfloat)cos_lon;
    mat[0][2] =	-cos_hdg * (SGfloat)cos_lat;
    mat[0][3] =	 SG_ZERO;

    mat[1][0] = -sin_hdg * (SGfloat)sin_lat * (SGfloat)cos_lon - cos_hdg * (SGfloat)sin_lon;
    mat[1][1] = -sin_hdg * (SGfloat)sin_lat * (SGfloat)sin_lon + cos_hdg * (SGfloat)cos_lon;
    mat[1][2] =	 sin_hdg * (SGfloat)cos_lat;
    mat[1][3] =	 SG_ZERO;

    mat[2][0] = (SGfloat)cos_lat * (SGfloat)cos_lon;
    mat[2][1] = (SGfloat)cos_lat * (SGfloat)sin_lon;
    mat[2][2] =	(SGfloat)sin_lat;
    mat[2][3] =  SG_ZERO;

    mat[3][0] = offset.x();
    mat[3][1] = offset.y();
    mat[3][2] = offset.z();
    mat[3][3] = SG_ONE ;

    sgSetCoord( obj_pos, mat );
}


// recurse an ssg tree and call removeKid() on every node from the
// bottom up.  Leaves the original branch in existance, but empty so
// it can be removed by the calling routine.
static void my_remove_branch( ssgBranch * branch ) {
    for ( ssgEntity *k = branch->getKid( 0 );
	  k != NULL; 
	  k = branch->getNextKid() )
    {
	if ( k -> isAKindOf ( ssgTypeBranch() ) ) {
	    my_remove_branch( (ssgBranch *)k );
	    branch -> removeKid ( k );
	} else if ( k -> isAKindOf ( ssgTypeLeaf() ) ) {
	    branch -> removeKid ( k ) ;
	}
    }
}


#ifdef WISH_PLIB_WAS_THREADED // but it isn't

// Schedule tile to be freed/removed
void FGTileEntry::sched_removal() {
    global_tile_mgr.ready_to_delete( this );
}

#endif


// Clean up the memory used by this tile and delete the arrays used by
// ssg as well as the whole ssg branch
void FGTileEntry::free_tile() {
    int i;
    SG_LOG( SG_TERRAIN, SG_INFO,
	    "FREEING TILE = (" << tile_bucket << ")" );

    SG_LOG( SG_TERRAIN, SG_DEBUG,
	    "  deleting " << nodes.size() << " nodes" );
    nodes.clear();

    // delete the ssg structures
    SG_LOG( SG_TERRAIN, SG_DEBUG,
	    "  deleting (leaf data) vertex, normal, and "
	    << " texture coordinate arrays" );

    for ( i = 0; i < (int)vec3_ptrs.size(); ++i ) {
	delete [] vec3_ptrs[i];
    }
    vec3_ptrs.clear();

    for ( i = 0; i < (int)vec2_ptrs.size(); ++i ) {
	delete [] vec2_ptrs[i];
    }
    vec2_ptrs.clear();

    for ( i = 0; i < (int)index_ptrs.size(); ++i ) {
	delete index_ptrs[i];
    }
    index_ptrs.clear();

    // delete the terrain branch (this should already have been
    // disconnected from the scene graph)
    ssgDeRefDelete( terra_transform );

    if ( lights_transform ) {
	// delete the terrain lighting branch (this should already have been
    // disconnected from the scene graph)
	ssgDeRefDelete( lights_transform );
    }
}


// Update the ssg transform node for this tile so it can be
// properly drawn relative to our (0,0,0) point
void FGTileEntry::prep_ssg_node( const Point3D& p, float vis) {
    if ( !loaded ) return;

    SetOffset( p );

// #define USE_UP_AND_COMING_PLIB_FEATURE
#ifdef USE_UP_AND_COMING_PLIB_FEATURE
    terra_range->setRange( 0, SG_ZERO );
    terra_range->setRange( 1, vis + bounding_radius );
    lights_range->setRange( 0, SG_ZERO );
    lights_range->setRange( 1, vis * 1.5 + bounding_radius );
#else
    float ranges[2];
    ranges[0] = SG_ZERO;
    ranges[1] = vis + bounding_radius;
    terra_range->setRanges( ranges, 2 );
    if ( lights_range ) {
	ranges[1] = vis * 1.5 + bounding_radius;
	lights_range->setRanges( ranges, 2 );
    }
#endif
    sgVec3 sgTrans;
    sgSetVec3( sgTrans, offset.x(), offset.y(), offset.z() );
    terra_transform->setTransform( sgTrans );

    if ( lights_transform ) {
	// we need to lift the lights above the terrain to avoid
	// z-buffer fighting.  We do this based on our altitude and
	// the distance this tile is away from scenery center.

	sgVec3 up;
	sgCopyVec3( up, globals->get_current_view()->get_world_up() );

	double agl;
	if ( current_aircraft.fdm_state ) {
	    agl = current_aircraft.fdm_state->get_Altitude() * SG_FEET_TO_METER
		- scenery.cur_elev;
	} else {
	    agl = 0.0;
	}

	// sgTrans just happens to be the
	// vector from scenery center to the center of this tile which
	// is what we want to calculate the distance of
	sgVec3 to;
	sgCopyVec3( to, sgTrans );
	double dist = sgLengthVec3( to );

	if ( general.get_glDepthBits() > 16 ) {
	    sgScaleVec3( up, 10.0 + agl / 100.0 + dist / 10000 );
	} else {
	    sgScaleVec3( up, 10.0 + agl / 20.0 + dist / 5000 );
	}
	sgAddVec3( sgTrans, up );
	lights_transform->setTransform( sgTrans );

	// select which set of lights based on sun angle
	float sun_angle = cur_light_params.sun_angle * SGD_RADIANS_TO_DEGREES;
	if ( sun_angle > 95 ) {
	    lights_brightness->select(0x04);
	} else if ( sun_angle > 92 ) {
	    lights_brightness->select(0x02);
	} else if ( sun_angle > 89 ) {
	    lights_brightness->select(0x01);
	} else {
	    lights_brightness->select(0x00);
	}
    }
}


ssgLeaf* FGTileEntry::gen_lights( ssgVertexArray *lights, int inc, float bright ) {
    // generate a repeatable random seed
    float *p1 = lights->get( 0 );
    unsigned int *seed = (unsigned int *)p1;
    sg_srandom( *seed );

    int size = lights->getNum() / inc;

    // Allocate ssg structure
    ssgVertexArray   *vl = new ssgVertexArray( size + 1 );
    ssgNormalArray   *nl = NULL;
    ssgTexCoordArray *tl = NULL;
    ssgColourArray   *cl = new ssgColourArray( size + 1 );

    sgVec4 color;
    for ( int i = 0; i < lights->getNum(); ++i ) {
	// this loop is slightly less efficient than it otherwise
	// could be, but we want a red light to always be red, and a
	// yellow light to always be yellow, etc. so we are trying to
	// preserve the random sequence.
	float zombie = sg_random();
	if ( i % inc == 0 ) {
	    vl->add( lights->get(i) );

	    // factor = sg_random() ^ 2, range = 0 .. 1 concentrated towards 0
	    float factor = sg_random();
	    factor *= factor;

	    if ( zombie > 0.5 ) {
		// 50% chance of yellowish
		sgSetVec4( color, 0.9, 0.9, 0.3, bright - factor * 0.2 );
	    } else if ( zombie > 0.15 ) {
		// 35% chance of whitish
		sgSetVec4( color, 0.9, 0.9, 0.8, bright - factor * 0.2 );
	    } else if ( zombie > 0.05 ) {
		// 10% chance of orangish
		sgSetVec4( color, 0.9, 0.6, 0.2, bright - factor * 0.2 );
	    } else {
		// 5% chance of redish
		sgSetVec4( color, 0.9, 0.2, 0.2, bright - factor * 0.2 );
	    }
	    cl->add( color );
	}
    }

    // create ssg leaf
    ssgLeaf *leaf = 
	new ssgVtxTable ( GL_POINTS, vl, nl, tl, cl );

    // assign state
    FGNewMat *newmat = material_lib.find( "LIGHTS" );
    leaf->setState( newmat->get_state() );

    return leaf;
}


ssgBranch*
FGTileEntry::obj_load( const std::string& path,
		       ssgVertexArray* lights, bool is_base )
{
    ssgBranch* result = 0;

    // try loading binary format
    result = fgBinObjLoad( path, this, lights, is_base );
    if ( result == NULL ) {
	// next try the older ascii format
	result = fgAsciiObjLoad( path, this, lights, is_base );
	if ( result == NULL ) {
	    // default to an ocean tile
	    result = fgGenTile( path, this );
	}
    }

    return result;
}


void
FGTileEntry::load( const SGPath& base, bool is_base )
{
    cout << "load() base = " << base.str() << endl;

    // Generate names for later use
    string index_str = tile_bucket.gen_index_str();

    SGPath tile_path = base;
    tile_path.append( tile_bucket.gen_base_path() );

    SGPath basename = tile_path;
    basename.append( index_str );
    // string path = basename.str();

    SG_LOG( SG_TERRAIN, SG_INFO, "Loading tile " << basename.str() );


    // obj_load() will generate ground lighting for us ...
    ssgVertexArray *light_pts = new ssgVertexArray( 100 );
    ssgBranch* new_tile = new ssgBranch;

    // Check for master .stg (scene terra gear) file
    SGPath stg_name = basename;
    stg_name.concat( ".stg" );

    sg_gzifstream in( stg_name.str() );

    if ( in.is_open() ) {
	string token, name;

	while ( ! in.eof() ) {
	    in >> token;

	    if ( token == "OBJECT_BASE" ) {
		in >> name >> ::skipws;
		SG_LOG( SG_TERRAIN, SG_INFO, "token = " << token
			<< " name = " << name );

		SGPath custom_path = tile_path;
		custom_path.append( name );

		ssgBranch *custom_obj
		    = obj_load( custom_path.str(), light_pts, true );

		if ( custom_obj != NULL ) {
		    new_tile -> addKid( custom_obj );
		}
	    } else if ( token == "OBJECT" ) {
		in >> name >> ::skipws;
		SG_LOG( SG_TERRAIN, SG_DEBUG, "token = " << token
			<< " name = " << name );

		SGPath custom_path = tile_path;
		custom_path.append( name );
		ssgBranch *custom_obj
		    = obj_load( custom_path.str(), NULL, false );
		if ( custom_obj != NULL ) {
		    new_tile -> addKid( custom_obj );
		}
	    } else if ( token == "OBJECT_STATIC" ) {
		// load object info
		double lon, lat, elev, hdg;
		in >> name >> lon >> lat >> elev >> hdg >> ::skipws;
		SG_LOG( SG_TERRAIN, SG_INFO, "token = " << token
			<< " name = " << name 
			<< " pos = " << lon << ", " << lat
			<< " elevation = " << elev
			<< " heading = " << hdg );

		// object loading is deferred to main render thread,
		// but lets figure out the paths right now.
		SGPath custom_path = tile_path;
		custom_path.append( name );

		sgCoord obj_pos;
		WorldCoordinate( &obj_pos, center, lat, lon, elev, hdg );
		
		ssgTransform *obj_trans = new ssgTransform;
		obj_trans->setTransform( &obj_pos );

		// wire as much of the scene graph together as we can
		new_tile->addKid( obj_trans );

		// bump up the pending models count
		pending_models++;

		// push an entry onto the model load queue
		FGDeferredModel *dm
		    = new FGDeferredModel( custom_path.str(), tile_path.str(),
					   this, obj_trans );
		FGTileMgr::model_ready( dm );
	    } else if ( token == "OBJECT_TAXI_SIGN" ) {
		// load object info
		double lon, lat, elev, hdg;
		in >> name >> lon >> lat >> elev >> hdg >> ::skipws;
		SG_LOG( SG_TERRAIN, SG_INFO, "token = " << token
			<< " name = " << name 
			<< " pos = " << lon << ", " << lat
			<< " elevation = " << elev
			<< " heading = " << hdg );

		// load the object itself
		SGPath custom_path = tile_path;
		custom_path.append( name );

		sgCoord obj_pos;
		WorldCoordinate( &obj_pos, center, lat, lon, elev, hdg );

		ssgTransform *obj_trans = new ssgTransform;
		obj_trans->setTransform( &obj_pos );

		ssgBranch *custom_obj
		    = gen_taxi_sign( custom_path.str(), name );

		// wire the pieces together
		if ( custom_obj != NULL ) {
		    obj_trans -> addKid( custom_obj );
		}
		new_tile->addKid( obj_trans );
	    } else if ( token == "OBJECT_RUNWAY_SIGN" ) {
		// load object info
		double lon, lat, elev, hdg;
		in >> name >> lon >> lat >> elev >> hdg >> ::skipws;
		SG_LOG( SG_TERRAIN, SG_INFO, "token = " << token
			<< " name = " << name 
			<< " pos = " << lon << ", " << lat
			<< " elevation = " << elev
			<< " heading = " << hdg );

		// load the object itself
		SGPath custom_path = tile_path;
		custom_path.append( name );

		sgCoord obj_pos;
		WorldCoordinate( &obj_pos, center, lat, lon, elev, hdg );

		ssgTransform *obj_trans = new ssgTransform;
		obj_trans->setTransform( &obj_pos );

		ssgBranch *custom_obj
		    = gen_runway_sign( custom_path.str(), name );

		// wire the pieces together
		if ( custom_obj != NULL ) {
		    obj_trans -> addKid( custom_obj );
		}
		new_tile->addKid( obj_trans );
	    } else {
		SG_LOG( SG_TERRAIN, SG_ALERT,
			"Unknown token " << token << " in "
			<< stg_name.str() );
		in >> ::skipws;
	    }
	}
    } else {
        // no .stg file so this must be old scenery

        // fgObjLoad will generate ground lighting for us ...
        ssgVertexArray *light_pts = new ssgVertexArray( 100 );

        new_tile = obj_load( basename.str(), light_pts, is_base );

        // load custom objects
        SG_LOG( SG_TERRAIN, SG_DEBUG, "Checking for custom objects ..." );

        SGPath index_path = tile_path;
        index_path.append( index_str );
        index_path.concat( ".ind" );

        SG_LOG( SG_TERRAIN, SG_DEBUG, "Looking in " << index_path.str() );

        sg_gzifstream in( index_path.str() );

        if ( in.is_open() ) {
            string token, name;

            while ( ! in.eof() ) {
                in >> token;

                if ( token == "OBJECT" ) {
                    in >> name >> ::skipws;
                    SG_LOG( SG_TERRAIN, SG_DEBUG, "token = " << token
                            << " name = " << name );

                    SGPath custom_path = tile_path;
                    custom_path.append( name );
                    ssgBranch *custom_obj
                        = obj_load( custom_path.str(), NULL, false );
                    if ( (new_tile != NULL) && (custom_obj != NULL) ) {
                        new_tile -> addKid( custom_obj );
                    }
                } else if ( token == "OBJECT_STATIC" ) {
                    // load object info
                    double lon, lat, elev, hdg;
                    in >> name >> lon >> lat >> elev >> hdg >> ::skipws;
                    SG_LOG( SG_TERRAIN, SG_INFO, "token = " << token
                            << " name = " << name 
                            << " pos = " << lon << ", " << lat
                            << " elevation = " << elev
                            << " heading = " << hdg );

                    // object loading is deferred to main render thread,
                    // but lets figure out the paths right now.
                    SGPath custom_path = tile_path;
                    custom_path.append( name );

                    sgCoord obj_pos;
                    WorldCoordinate( &obj_pos, center, lat, lon, elev, hdg );
		
                    ssgTransform *obj_trans = new ssgTransform;
                    obj_trans->setTransform( &obj_pos );

                    // wire as much of the scene graph together as we can
                    new_tile->addKid( obj_trans );

                    // bump up the pending models count
                    pending_models++;

                    // push an entry onto the model load queue
                    FGDeferredModel *dm
                        = new FGDeferredModel( custom_path.str(),
                                               tile_path.str(),
                                               this, obj_trans );
                    FGTileMgr::model_ready( dm );
                } else if ( token == "OBJECT_TAXI_SIGN" ) {
                    // load object info
                    double lon, lat, elev, hdg;
                    in >> name >> lon >> lat >> elev >> hdg >> ::skipws;
                    SG_LOG( SG_TERRAIN, SG_INFO, "token = " << token
                            << " name = " << name 
                            << " pos = " << lon << ", " << lat
                            << " elevation = " << elev
                            << " heading = " << hdg );

                    // load the object itself
                    SGPath custom_path = tile_path;
                    custom_path.append( name );

                    sgCoord obj_pos;
                    WorldCoordinate( &obj_pos, center, lat, lon, elev, hdg );

                    ssgTransform *obj_trans = new ssgTransform;
                    obj_trans->setTransform( &obj_pos );

                    ssgBranch *custom_obj
                        = gen_taxi_sign( custom_path.str(), name );

                    // wire the pieces together
                    if ( (new_tile != NULL) && (custom_obj != NULL) ) {
                        obj_trans -> addKid( custom_obj );
                    }
                    new_tile->addKid( obj_trans );
                } else if ( token == "OBJECT_RUNWAY_SIGN" ) {
                    // load object info
                    double lon, lat, elev, hdg;
                    in >> name >> lon >> lat >> elev >> hdg >> ::skipws;
                    SG_LOG( SG_TERRAIN, SG_INFO, "token = " << token
                            << " name = " << name 
                            << " pos = " << lon << ", " << lat
                            << " elevation = " << elev
                            << " heading = " << hdg );

                    // load the object itself
                    SGPath custom_path = tile_path;
                    custom_path.append( name );

                    sgCoord obj_pos;
                    WorldCoordinate( &obj_pos, center, lat, lon, elev, hdg );

                    ssgTransform *obj_trans = new ssgTransform;
                    obj_trans->setTransform( &obj_pos );

                    ssgBranch *custom_obj
                        = gen_runway_sign( custom_path.str(), name );

                    // wire the pieces together
                    if ( (new_tile != NULL) && (custom_obj != NULL) ) {
                        obj_trans -> addKid( custom_obj );
                    }
                    new_tile->addKid( obj_trans );
                } else {
                    SG_LOG( SG_TERRAIN, SG_ALERT,
                            "Unknown token " << token << " in "
                            << index_path.str() );
                    in >> ::skipws;
                }
	    }
	}
    }

    if ( new_tile != NULL ) {
        terra_range->addKid( new_tile );
    }

    terra_transform->addKid( terra_range );

    // calculate initial tile offset
    SetOffset( scenery.center );
    sgCoord sgcoord;
    sgSetCoord( &sgcoord,
		offset.x(), offset.y(), offset.z(),
		0.0, 0.0, 0.0 );
    terra_transform->setTransform( &sgcoord );
    // terrain->addKid( terra_transform );

    lights_transform = NULL;
    lights_range = NULL;
    /* uncomment this section for testing ground lights */
    if ( light_pts->getNum() ) {
	SG_LOG( SG_TERRAIN, SG_DEBUG, "generating lights" );
	lights_transform = new ssgTransform;
	lights_range = new ssgRangeSelector;
	lights_brightness = new ssgSelector;
	ssgLeaf *lights;

	lights = gen_lights( light_pts, 4, 0.7 );
	lights_brightness->addKid( lights );

	lights = gen_lights( light_pts, 2, 0.85 );
	lights_brightness->addKid( lights );

	lights = gen_lights( light_pts, 1, 1.0 );
	lights_brightness->addKid( lights );

	lights_range->addKid( lights_brightness );
	lights_transform->addKid( lights_range );
	lights_transform->setTransform( &sgcoord );
        // ground->addKid( lights_transform );
    }
    /* end of ground light section */
}


void
FGTileEntry::add_ssg_nodes( ssgBranch* terrain, ssgBranch* ground )
{
    // bump up the ref count so we can remove this later without
    // having ssg try to free the memory.
    terra_transform->ref();
    terrain->addKid( terra_transform );

    SG_LOG( SG_TERRAIN, SG_DEBUG,
            "connected a tile into scene graph.  terra_transform = "
            << terra_transform );
    SG_LOG( SG_TERRAIN, SG_DEBUG, "num parents now = "
            << terra_transform->getNumParents() );

    if ( lights_transform != 0 ) {
	// bump up the ref count so we can remove this later without
	// having ssg try to free the memory.
	lights_transform->ref();
	ground->addKid( lights_transform );
    }

    loaded = true;
}


void
FGTileEntry::disconnect_ssg_nodes()
{
    SG_LOG( SG_TERRAIN, SG_INFO, "disconnecting ssg nodes" );

    if ( ! loaded ) {
        SG_LOG( SG_TERRAIN, SG_INFO, "removing a not-fully loaded tile!" );
    } else {
        SG_LOG( SG_TERRAIN, SG_INFO, "removing a fully loaded tile!  terra_transform = " << terra_transform );
    }
        
    // find the terrain branch parent
    int pcount = terra_transform->getNumParents();
    if ( pcount > 0 ) {
	// find the first parent (should only be one)
	ssgBranch *parent = terra_transform->getParent( 0 ) ;
	if( parent ) {
	    // disconnect the tile (we previously ref()'d it so it
	    // won't get freed now)
	    parent->removeKid( terra_transform );
	} else {
	    SG_LOG( SG_TERRAIN, SG_ALERT,
		    "parent pointer is NULL!  Dying" );
	    exit(-1);
	}
    } else {
	SG_LOG( SG_TERRAIN, SG_ALERT,
		"Parent count is zero for an ssg tile!  Dying" );
	exit(-1);
    }

    // find the terrain lighting branch
    if ( lights_transform ) {
	pcount = lights_transform->getNumParents();
	if ( pcount > 0 ) {
	    // find the first parent (should only be one)
	    ssgBranch *parent = lights_transform->getParent( 0 ) ;
	    if( parent ) {
		// disconnect the light branch (we previously ref()'d
		// it so it won't get freed now)
		parent->removeKid( lights_transform );
	    } else {
		SG_LOG( SG_TERRAIN, SG_ALERT,
			"parent pointer is NULL!  Dying" );
		exit(-1);
	    }
	} else {
	    SG_LOG( SG_TERRAIN, SG_ALERT,
		    "Parent count is zero for an ssg light tile!  Dying" );
	    exit(-1);
	}
    }
}