tide.cxx

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// tide.cxx -- interface for tidal movement
//
// Written by Erik Hofman, Octover 2020
//
// Copyright (C) 2020  Erik Hofman <erik@ehofman.com>
//
// 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., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
//
 
#include <simgear/constants.h>
#include <simgear/timing/sg_time.hxx>
#include <simgear/structure/SGExpression.hxx>
#include <Main/globals.hxx>
 
#include "tide.hxx"
#include "light.hxx"
#include "bodysolver.hxx"
 
void FGTide::reinit() {
    _prev_moon_lon = -9999.0;
}
 
void FGTide::bind()
{
    SGPropertyNode *props = globals->get_props();
 
    viewLon = props->getNode("sim/current-view/viewer-lon-deg", true);
    viewLat = props->getNode("sim/current-view/viewer-lat-deg", true);
 
    _tideAnimation = props->getNode("/environment/sea/surface/delta-T-tide", true);
 
    _tideLevelNorm = props->getNode("/sim/time/tide-level-norm", true);
    _tideLevelNorm->setDoubleValue(_tide_level);
}
 
void FGTide::unbind()
{
    viewLon.reset();
    viewLat.reset();
 
    _tideLevelNorm.reset();
    _tideAnimation.reset();
}
 
#include <Main/fg_props.hxx>
void FGTide::update(double dt)
{
    auto l = globals->get_subsystem<FGLight>();
 
    // Don't know where the 60 degrees offset comes from but it matches
    // the tides perfectly at EHAL. Something to figure out.
    // Eureka: It was the latitude (53.45 degrees north).
    // It turns out that the moon is draging the tide with an almost
    // perfect 45 degrees 'bow-wave' along the equator. Tests at SMBQ
    // (0 degrees latitude) confirmed this finding.
    double viewer_lon = (viewLon->getDoubleValue()
                         + fabs( viewLat->getDoubleValue() )
                        ) * SGD_DEGREES_TO_RADIANS;
    double moon_lon = l->get_moon_lon() - viewer_lon;
    if (fabs(_prev_moon_lon - moon_lon) > (SGD_PI/360.0))
    {
        _prev_moon_lon = moon_lon;
 
        double sun_lon = l->get_sun_lon() - viewer_lon;
        _tide_level = cos(2.0*moon_lon);
        _tide_level += 0.15*cos(2.0*sun_lon);
 
        if (_tide_level < -1.0) _tide_level = -1.0;
        else if (_tide_level > 1.0) _tide_level = 1.0;
 
        _tideLevelNorm->setDoubleValue(_tide_level);
        _tideAnimation->setDoubleValue(0.5 - 0.5*_tide_level);
    }
}
 
// Register the subsystem.
SGSubsystemMgr::Registrant<FGTide> registrantFGTide;