File indexing completed on 2025-01-12 09:34:20

 /*
     SPDX-FileCopyrightText: 2015 Jasem Mutlaq <mutlaqja@ikarustech.com>
 
     SPDX-License-Identifier: GPL-2.0-or-later
 */
 
 #include "artificialhorizoncomponent.h"
 
 #include "greatcircle.h"
 #include "kstarsdata.h"
 #include "linelist.h"
 #include "Options.h"
 #include "skymap.h"
 #include "skymapcomposite.h"
 #include "skypainter.h"
 #include "projections/projector.h"
 
 #define UNDEFINED_ALTITUDE -90
 
 ArtificialHorizonEntity::~ArtificialHorizonEntity()
 {
     clearList();
 }
 
 QString ArtificialHorizonEntity::region() const
 {
     return m_Region;
 }
 
 void ArtificialHorizonEntity::setRegion(const QString &Region)
 {
     m_Region = Region;
 }
 
 bool ArtificialHorizonEntity::enabled() const
 {
     return m_Enabled;
 }
 
 void ArtificialHorizonEntity::setEnabled(bool Enabled)
 {
     m_Enabled = Enabled;
 }
 
 bool ArtificialHorizonEntity::ceiling() const
 {
     return m_Ceiling;
 }
 
 void ArtificialHorizonEntity::setCeiling(bool value)
 {
     m_Ceiling = value;
 }
 
 void ArtificialHorizonEntity::setList(const std::shared_ptr<LineList> &list)
 {
     m_List = list;
 }
 
 std::shared_ptr<LineList> ArtificialHorizonEntity::list() const
 {
     return m_List;
 }
 
 void ArtificialHorizonEntity::clearList()
 {
     m_List.reset();
 }
 
 namespace
 {
 
 // Returns true if angle is "in between" range1 and range2, two other angles,
 // where in-between means "the short way".
 bool inBetween(const dms &angle, const dms &range1, const dms &range2)
 {
     const double rangeDelta = fabs(range1.deltaAngle(range2).Degrees());
     const double delta1 = fabs(range1.deltaAngle(angle).Degrees());
     const double delta2 = fabs(range2.deltaAngle(angle).Degrees());
     // The angle is between range1 and range2 if its two distances to each are both
     // less than the range distance.
     return delta1 <= rangeDelta && delta2 <= rangeDelta;
 }
 }  // namespace
 
 double ArtificialHorizonEntity::altitudeConstraint(double azimuthDegrees, bool *constraintExists) const
 {
     *constraintExists = false;
     if (m_List == nullptr)
         return UNDEFINED_ALTITUDE;
 
     SkyList *points = m_List->points();
     if (points == nullptr)
         return UNDEFINED_ALTITUDE;
 
     double constraint = !m_Ceiling ? UNDEFINED_ALTITUDE : 90.0;
     dms desiredAzimuth(azimuthDegrees);
     dms lastAz;
     double lastAlt = 0;
     bool firstOne = true;
     for (auto &p : *points)
     {
         const dms az = p->az();
         const double alt = p->alt().Degrees();
         if (qIsNaN(az.Degrees()) || qIsNaN(alt)) continue;
         if (!firstOne && inBetween(desiredAzimuth, lastAz, az))
         {
             *constraintExists = true;
             // If the input angle is in the interval between the last two points,
             // interpolate the altitude constraint, and use that value.
             // If there are other line segments which also contain the point,
             // we use the max constraint. Convert to GreatCircle?
             const double totalDelta = fabs(lastAz.deltaAngle(az).Degrees());
             if (totalDelta <= 0)
             {
                 if (!m_Ceiling)
                     constraint = std::max(constraint, alt);
                 else
                     constraint = std::min(constraint, alt);
             }
             else
             {
                 const double deltaToLast = fabs(lastAz.deltaAngle(desiredAzimuth).Degrees());
                 const double weight = deltaToLast / totalDelta;
                 const double newConstraint = (1.0 - weight) * lastAlt + weight * alt;
                 if (!m_Ceiling)
                     constraint = std::max(constraint, newConstraint);
                 else
                     constraint = std::min(constraint, newConstraint);
             }
         }
         firstOne = false;
         lastAz = az;
         lastAlt = alt;
     }
     return constraint;
 }
 
 ArtificialHorizonComponent::ArtificialHorizonComponent(SkyComposite *parent)
     : NoPrecessIndex(parent, i18n("Artificial Horizon"))
 {
     load();
 }
 
 ArtificialHorizonComponent::~ArtificialHorizonComponent()
 {
 }
 
 ArtificialHorizon::~ArtificialHorizon()
 {
     qDeleteAll(m_HorizonList);
     m_HorizonList.clear();
 }
 
 void ArtificialHorizon::load(const QList<ArtificialHorizonEntity *> &list)
 {
     m_HorizonList = list;
     resetPrecomputeConstraints();
     checkForCeilings();
 }
 
 bool ArtificialHorizonComponent::load()
 {
     QList<ArtificialHorizonEntity *> list;
     KStarsData::Instance()->userdb()->GetAllHorizons(list);
     horizon.load(list);
 
     foreach (ArtificialHorizonEntity *horizon, *horizon.horizonList())
         appendLine(horizon->list());
 
     return true;
 }
 
 void ArtificialHorizonComponent::save()
 {
     KStarsData::Instance()->userdb()->DeleteAllHorizons();
 
     foreach (ArtificialHorizonEntity *horizon, *horizon.horizonList())
         KStarsData::Instance()->userdb()->AddHorizon(horizon);
 }
 
 bool ArtificialHorizonComponent::selected()
 {
     return Options::showGround();
 }
 
 void ArtificialHorizonComponent::preDraw(SkyPainter *skyp)
 {
     QColor color(KStarsData::Instance()->colorScheme()->colorNamed("ArtificialHorizonColor"));
     color.setAlpha(40);
     skyp->setBrush(QBrush(color));
     skyp->setPen(Qt::NoPen);
 }
 
 namespace
 {
 
 // Returns an equivalent degrees in the range 0 <= 0 < 360
 double normalizeDegrees(double degrees)
 {
     while (degrees < 0)
         degrees += 360;
     while (degrees >= 360.0)
         degrees -= 360.0;
     return degrees;
 }
 
 // Draws a "round polygon", sampling a circle every 45 degrees, with the given radius,
 // centered on the SkyPoint.
 void drawHorizonPoint(const SkyPoint &pt, double radius, SkyPainter *painter)
 
 {
     LineList region;
     double az = pt.az().Degrees(), alt = pt.alt().Degrees();
 
     for (double angle = 0; angle < 360; angle += 45)
     {
         double radians = angle * 2 * M_PI / 360.0;
         double az1 = az + radius * cos(radians);
         double alt1 = alt + radius * sin(radians);
         std::shared_ptr<SkyPoint> sp(new SkyPoint());
         sp->setAz(az1);
         sp->setAlt(alt1);
         sp->HorizontalToEquatorial(KStarsData::Instance()->lst(), KStarsData::Instance()->geo()->lat());
         region.append(sp);
     }
     // Repeat the first point.
     double az1 = az + radius * cos(0);
     double alt1 = alt + radius * sin(0);
     std::shared_ptr<SkyPoint> sp(new SkyPoint());
     sp->setAz(az1);
     sp->setAlt(alt1);
     sp->HorizontalToEquatorial(KStarsData::Instance()->lst(), KStarsData::Instance()->geo()->lat());
     region.append(sp);
 
     painter->drawSkyPolygon(&region, false);
 }
 
 // Draws a series of points whose coordinates are given by the LineList.
 void drawHorizonPoints(LineList *lineList, SkyPainter *painter)
 {
     const SkyList &points = *(lineList->points());
     for (int i = 0; i < points.size(); ++i)
     {
         const SkyPoint &pt = *points[i];
         if (qIsNaN(pt.az().Degrees()) || qIsNaN(pt.alt().Degrees()))
             continue;
         drawHorizonPoint(pt, .5, painter);
     }
 }
 
 // Draws a points that is larger than the one drawn by drawHorizonPoint().
 // The point's coordinates are the ith (index) point in the LineList.
 void drawSelectedPoint(LineList *lineList, int index, SkyPainter *painter)
 {
     if (index >= 0 && index < lineList->points()->size())
     {
         const SkyList &points = *(lineList->points());
         const SkyPoint &pt = *points[index];
         if (qIsNaN(pt.az().Degrees()) || qIsNaN(pt.alt().Degrees()))
             return;
         drawHorizonPoint(pt, 1.0, painter);
     }
 }
 
 // This creates a set of connected line segments from az1,alt1 to az2,alt2, sampling
 // points on the great circle between az1,alt1 and az2,alt2 every 2 degrees or so.
 // The errors would be obvious for longer lines if we just drew a standard line.
 // If testing is true, HorizontalToEquatorial is not called.
 void appendGreatCirclePoints(double az1, double alt1, double az2, double alt2, LineList *region, bool testing)
 {
     constexpr double sampling = 2.0;  // degrees
     const double maxAngleDiff = std::max(fabs(az1 - az2), fabs(alt1 - alt2));
     const int numSamples = maxAngleDiff / sampling;
 
     // Hy 9/25/22: These 4 lines cause rendering issues in equatorial mode (horizon mode is ok).
     // Not sure why--though I suspect the initial conditions computed in drawPolygons().
     // Without them there are some jagged lines near the horizon, but much better than with them.
     // std::shared_ptr<SkyPoint> sp0(new SkyPoint());
     // sp0->setAz(az1);
     // sp0->setAlt(alt1);
     // region->append(sp0);
 
     if (numSamples > 1)
     {
         GreatCircle gc(az1, alt1, az2, alt2);
         for (int i = 1; i < numSamples; ++i)
         {
             const double fraction = i / static_cast<double>(numSamples);
             double az, alt;
             gc.waypoint(fraction, &az, &alt);
             std::shared_ptr<SkyPoint> sp(new SkyPoint());
             sp->setAz(az);
             sp->setAlt(alt);
             if (!testing)
                 sp->HorizontalToEquatorial(KStarsData::Instance()->lst(), KStarsData::Instance()->geo()->lat());
             region->append(sp);
         }
     }
     std::shared_ptr<SkyPoint> sp(new SkyPoint());
     sp->setAz(az2);
     sp->setAlt(alt2);
     // Is HorizontalToEquatorial necessary in any case?
     if (!testing)
         sp->HorizontalToEquatorial(KStarsData::Instance()->lst(), KStarsData::Instance()->geo()->lat());
     region->append(sp);
 }
 
 }  // namespace
 
 // Draws a polygon, where one of the sides is az1,alt1 --> az2,alt2 (except that's implemented as series
 // of connected line segments along a great circle).
 // It figures out the opposite side depending on the type of the constraint for this entity
 // (horizon line or ceiling) and the other contraints that are enabled.
 bool ArtificialHorizon::computePolygon(int entity, double az1, double alt1, double az2, double alt2,
                                        double sampling, LineList *region)
 {
     const bool ceiling = horizonList()->at(entity)->ceiling();
     const ArtificialHorizonEntity *thisOne = horizonList()->at(entity);
     double alt1b = 0, alt2b = 0;
     bool exists = false;
     LineList left, top, right, bottom;
 
     double lastAz = az1;
     double lastAlt = alt1;
     const double azRange = az2 - az1, altRange = alt2 - alt1;
 
     if (az1 >= az2)
         return false;
 
     if (az1 + sampling > az2)
         sampling = (az2 - az1) - 1e-6;
 
     for (double az = az1 + sampling; az <= az2; az += sampling)
     {
         // Put it at the end, if we're close.
         if (az + sampling > az2)
             az = az2;
 
         double alt = alt1 + altRange * (az - az1) / azRange;
         double alt1b = 0, alt2b = 0;
 
         if (!ceiling)
         {
             // For standard horizon lines, the polygon is drawn down to the next lower-altitude
             // enabled line, or to the horizon if a lower line doesn't exist.
             const ArtificialHorizonEntity *constraint = getConstraintBelow(lastAz, lastAlt, thisOne);
             if (constraint != nullptr)
             {
                 double altTemp = constraint->altitudeConstraint(lastAz, &exists);
                 if (exists)
                     alt1b = altTemp;
             }
             constraint = getConstraintBelow(az, alt, thisOne);
             if (constraint != nullptr)
             {
                 double altTemp = constraint->altitudeConstraint(az, &exists);
                 if (exists)
                     alt2b = altTemp;
             }
             appendGreatCirclePoints(lastAz, lastAlt,   az, alt, &top, testing);
             appendGreatCirclePoints(lastAz, alt1b,   az, alt2b, &bottom, testing);
         }
         else
         {
             // For ceiling lines, the polygon is drawn up to the next higher-altitude enabled line
             // but only if that line is another cieling, otherwise it not drawn at all (because that
             // horizon line will do the drawing).
             const ArtificialHorizonEntity *constraint = getConstraintAbove(lastAz, lastAlt, thisOne);
             alt1b = 90;
             alt2b = 90;
             if (constraint != nullptr)
             {
                 if (constraint->ceiling()) return false;
                 double altTemp = constraint->altitudeConstraint(lastAz, &exists);
                 if (exists) alt1b = altTemp;
             }
             constraint = getConstraintAbove(az, alt, thisOne);
             if (constraint != nullptr)
             {
                 if (constraint->ceiling()) return false;
                 double altTemp = constraint->altitudeConstraint(az, &exists);
                 if (exists) alt2b = altTemp;
             }
             appendGreatCirclePoints(lastAz, lastAlt,   az, alt, &top, testing);
             // Note that "bottom" for a ceiling is above.
             appendGreatCirclePoints(lastAz, alt1b,   az, alt2b, &bottom, testing);
         }
         lastAz = az;
         lastAlt = alt;
     }
 
     if (!ceiling)
     {
         // For standard horizon lines, the polygon is drawn down to the next lower-altitude
         // enabled line, or to the horizon if a lower line doesn't exist.
         const ArtificialHorizonEntity *constraint = getConstraintBelow(az1, alt1, thisOne);
         if (constraint != nullptr)
         {
             double altTemp = constraint->altitudeConstraint(az1, &exists);
             if (exists)
                 alt1b = altTemp;
         }
         appendGreatCirclePoints(az1, alt1b,   az1, alt1, &left, testing);
 
         const ArtificialHorizonEntity *constraint2 = getConstraintBelow(az2, alt2, thisOne);
         if (constraint2 != nullptr)
         {
             double altTemp = constraint2->altitudeConstraint(az2, &exists);
             if (exists)
                 alt2b = altTemp;
         }
         appendGreatCirclePoints(az2, alt2,   az2, alt2b, &right, testing);
     }
     else
     {
         // For ceiling lines, the polygon is drawn up to the next higher-altitude enabled line
         // but only if that line is another cieling, otherwise it not drawn at all (because that
         // horizon line will do the drawing).
         const ArtificialHorizonEntity *constraint = getConstraintAbove(az1, alt1, thisOne);
         alt1b = 90;
         alt2b = 90;
         if (constraint != nullptr)
         {
             if (!constraint->ceiling()) return false;
             double altTemp = constraint->altitudeConstraint(az1, &exists);
             if (exists) alt1b = altTemp;
         }
         appendGreatCirclePoints(az1, alt1b,   az1, alt1, &left, testing);
 
         const ArtificialHorizonEntity *constraint2 = getConstraintAbove(az2, alt2, thisOne);
         if (constraint2 != nullptr)
         {
             if (!constraint2->ceiling()) return false;
             double altTemp = constraint2->altitudeConstraint(az2, &exists);
             if (exists) alt2b = altTemp;
         }
         appendGreatCirclePoints(az2, alt2,   az2, alt2b, &right, testing);
     }
 
     // Now we have all the sides: left, top, right, bottom, but the order of bottom is reversed.
     // Make a polygon with all the points.
     for (const auto &p : * (left.points()))
         region->append(p);
     for (const auto &p : * (top.points()))
         region->append(p);
     for (const auto &p : * (right.points()))
         region->append(p);
     for (int i = bottom.points()->size() - 1; i >= 0; i--)
         region->append(bottom.points()->at(i));
 
     return true;
 }
 
 // Draws a series of polygons of width in azimuth of "sampling degrees".
 // Drawing a single polygon would have "great-circle issues". This looks a lot better.
 // Assumes az1 and az2 in range 0-360 and az1 < az2.
 // regions is only not nullptr during testing. In this wasy we can test
 // whether the appropriate regions are drawn.
 void ArtificialHorizon::drawSampledPolygons(int entity, double az1, double alt1, double az2, double alt2,
         double sampling, SkyPainter *painter, QList<LineList> *regions)
 {
     if (az1 > az2)
     {
         // Should not generally happen. Possibility e.g. az 0 -> 0.01 in a wrap around.
         // OK to ignore.
         return;
     }
 
     LineList region;
     if (computePolygon(entity, az1, alt1, az2, alt2, sampling, &region))
     {
         if (painter != nullptr)
             painter->drawSkyPolygon(&region, false);
         if (regions != nullptr)
             regions->append(region);
     }
 }
 
 // This draws a series of polygons that fill the area that the horizon entity with index "entity"
 // is responsible for.  If that is a horizon line, it draws it down to the horizon, or to the next
 // lower line. It draws the polygons one pair of points at a time, and deals with complications
 // of when the azimuth angle wraps around 360 degrees.
 void ArtificialHorizon::drawPolygons(int entity, SkyPainter *painter, QList<LineList> *regions)
 {
     const ArtificialHorizonEntity &ah = *(horizonList()->at(entity));
     const SkyList &points = *(ah.list()->points());
 
     // The skylist shouldn't contain NaN values, but, it has in the past,
     // and, to be cautious, this checks for them and removes points with NaNs.
     int start = 0;
     for (; start < points.size(); ++start)
     {
         const SkyPoint &p = *points[start];
         if (!qIsNaN(p.az().Degrees()) && !qIsNaN(p.alt().Degrees()))
             break;
     }
 
     for (int i = start + 1; i < points.size(); ++i)
     {
         const SkyPoint &p2 = *points[i];
         if (qIsNaN(p2.az().Degrees()) || qIsNaN(p2.alt().Degrees()))
             continue;
         const SkyPoint &p1 = *points[start];
         start = i;
 
         const double az1 = normalizeDegrees(p1.az().Degrees());
         const double az2 = normalizeDegrees(p2.az().Degrees());
 
         double minAz, maxAz, minAzAlt, maxAzAlt;
         if (az1 < az2)
         {
             minAz = az1;
             minAzAlt = p1.alt().Degrees();
             maxAz = az2;
             maxAzAlt = p2.alt().Degrees();
         }
         else
         {
             minAz = az2;
             minAzAlt = p2.alt().Degrees();
             maxAz = az1;
             maxAzAlt = p1.alt().Degrees();
         }
         const bool wrapAround = !inBetween(dms((minAz + maxAz) / 2.0), dms(minAz), dms(maxAz));
         constexpr double sampling = 1.0;  // Draw a polygon for every degree in Azimuth
         if (wrapAround)
         {
             // We've detected that the line segment crosses 0 degrees.
             // Draw one polygon on one side of 0 degrees, and another on the other side.
             // Compute the altitude at wrap-around.
             const double fraction = fabs(dms(360.0).deltaAngle(dms(maxAz)).Degrees() /
                                          p1.az().deltaAngle(p2.az()).Degrees());
             const double midAlt = minAzAlt + fraction * (maxAzAlt - minAzAlt);
             // Draw polygons form maxAz upto 0 degrees, then again from 0 to minAz.
             drawSampledPolygons(entity, maxAz, maxAzAlt, 360, midAlt, sampling, painter, regions);
             drawSampledPolygons(entity, 0, midAlt, minAz, minAzAlt, sampling, painter, regions);
         }
         else
         {
             // Draw the polygons without wraparound
             drawSampledPolygons(entity, minAz, minAzAlt, maxAz, maxAzAlt, sampling, painter, regions);
         }
     }
 }
 
 void ArtificialHorizon::drawPolygons(SkyPainter *painter, QList<LineList> *regions)
 {
     for (int i = 0; i < horizonList()->size(); i++)
     {
         if (enabled(i))
             drawPolygons(i, painter, regions);
     }
 }
 
 void ArtificialHorizonComponent::draw(SkyPainter *skyp)
 {
     if (!selected())
         return;
 
     if (livePreview.get())
     {
         if ((livePreview->points() != nullptr) && (livePreview->points()->size() > 0))
         {
             // Draws a series of line segments, overlayed by the vertices.
             // One vertex (the current selection) is emphasized.
             skyp->setPen(QPen(Qt::white, 2));
             skyp->drawSkyPolyline(livePreview.get());
             skyp->setBrush(QBrush(Qt::yellow));
             drawSelectedPoint(livePreview.get(), selectedPreviewPoint, skyp);
             skyp->setBrush(QBrush(Qt::red));
             drawHorizonPoints(livePreview.get(), skyp);
         }
     }
 
     preDraw(skyp);
 
     QList<LineList> regions;
     horizon.drawPolygons(skyp, &regions);
 }
 
 bool ArtificialHorizon::enabled(int i) const
 {
     return m_HorizonList.at(i)->enabled();
 }
 
 ArtificialHorizonEntity *ArtificialHorizon::findRegion(const QString &regionName)
 {
     ArtificialHorizonEntity *regionHorizon = nullptr;
 
     foreach (ArtificialHorizonEntity *horizon, m_HorizonList)
     {
         if (horizon->region() == regionName)
         {
             regionHorizon = horizon;
             break;
         }
     }
 
     return regionHorizon;
 }
 
 void ArtificialHorizon::removeRegion(const QString &regionName, bool lineOnly)
 {
     ArtificialHorizonEntity *regionHorizon = findRegion(regionName);
 
     if (regionHorizon == nullptr)
         return;
 
     if (lineOnly)
         regionHorizon->clearList();
     else
     {
         m_HorizonList.removeOne(regionHorizon);
         delete (regionHorizon);
     }
     resetPrecomputeConstraints();
     checkForCeilings();
 }
 
 void ArtificialHorizonComponent::removeRegion(const QString &regionName, bool lineOnly)
 {
     ArtificialHorizonEntity *regionHorizon = horizon.findRegion(regionName);
     if (regionHorizon != nullptr && regionHorizon->list())
         removeLine(regionHorizon->list());
     horizon.removeRegion(regionName, lineOnly);
 }
 
 void ArtificialHorizon::checkForCeilings()
 {
     noCeilingConstraints = true;
     for (const auto &r : m_HorizonList)
     {
         if (r->ceiling() && r->enabled())
         {
             noCeilingConstraints = false;
             break;
         }
     }
 }
 
 void ArtificialHorizon::addRegion(const QString &regionName, bool enabled, const std::shared_ptr<LineList> &list,
                                   bool ceiling)
 {
     ArtificialHorizonEntity *horizon = new ArtificialHorizonEntity;
 
     horizon->setRegion(regionName);
     horizon->setEnabled(enabled);
     horizon->setCeiling(ceiling);
     horizon->setList(list);
 
     m_HorizonList.append(horizon);
     resetPrecomputeConstraints();
     checkForCeilings();
 }
 
 void ArtificialHorizonComponent::addRegion(const QString &regionName, bool enabled, const std::shared_ptr<LineList> &list,
         bool ceiling)
 {
     horizon.addRegion(regionName, enabled, list, ceiling);
     appendLine(list);
 }
 
 bool ArtificialHorizon::altitudeConstraintsExist() const
 {
     foreach (ArtificialHorizonEntity *horizon, m_HorizonList)
     {
         if (horizon->enabled())
             return true;
     }
     return false;
 }
 
 const ArtificialHorizonEntity *ArtificialHorizon::getConstraintAbove(double azimuthDegrees, double altitudeDegrees,
         const ArtificialHorizonEntity *ignore) const
 {
     double closestAbove = 1e6;
     const ArtificialHorizonEntity *entity = nullptr;
 
     foreach (ArtificialHorizonEntity *horizon, m_HorizonList)
     {
         if (!horizon->enabled()) continue;
         if (horizon == ignore) continue;
         bool constraintExists = false;
         double constraint = horizon->altitudeConstraint(azimuthDegrees, &constraintExists);
         // This horizon doesn't constrain this azimuth.
         if (!constraintExists) continue;
 
         double altitudeDiff = constraint - altitudeDegrees;
         if (altitudeDiff > 0 && constraint < closestAbove)
         {
             closestAbove = constraint;
             entity = horizon;
         }
     }
     return entity;
 }
 
 // Estimate the horizon contraint to .1 degrees.
 // This significantly speeds up computation.
 constexpr int PRECOMPUTED_RESOLUTION = 10;
 
 double ArtificialHorizon::altitudeConstraint(double azimuthDegrees) const
 {
     if (precomputedConstraints.size() != 360 * PRECOMPUTED_RESOLUTION)
         precomputeConstraints();
     return precomputedConstraint(azimuthDegrees);
 }
 
 double ArtificialHorizon::altitudeConstraintInternal(double azimuthDegrees) const
 {
     const ArtificialHorizonEntity *horizonBelow = getConstraintBelow(azimuthDegrees, 90.0, nullptr);
     if (horizonBelow == nullptr)
         return UNDEFINED_ALTITUDE;
     bool ignore = false;
     return horizonBelow->altitudeConstraint(azimuthDegrees, &ignore);
 }
 
 // Quantize the constraints to within .1 degrees (so there are 360*10=3600
 // precomputed values).
 void ArtificialHorizon::precomputeConstraints() const
 {
     precomputedConstraints.clear();
     precomputedConstraints.fill(0, 360 * PRECOMPUTED_RESOLUTION);
     for (int i = 0; i < 360 * PRECOMPUTED_RESOLUTION; ++i)
     {
         const double az = i / static_cast<double>(PRECOMPUTED_RESOLUTION);
         precomputedConstraints[i] = altitudeConstraintInternal(az);
     }
 }
 
 void ArtificialHorizon::resetPrecomputeConstraints() const
 {
     precomputedConstraints.clear();
 }
 
 double ArtificialHorizon::precomputedConstraint(double azimuth) const
 {
     constexpr int maxval = 360 * PRECOMPUTED_RESOLUTION;
     int index = azimuth * PRECOMPUTED_RESOLUTION + 0.5;
     if (index == maxval)
         index = 0;
     if (index < 0 || index >= precomputedConstraints.size())
         return UNDEFINED_ALTITUDE;
     return precomputedConstraints[index];
 }
 
 const ArtificialHorizonEntity *ArtificialHorizon::getConstraintBelow(double azimuthDegrees, double altitudeDegrees,
         const ArtificialHorizonEntity *ignore) const
 {
     double closestBelow = -1e6;
     const ArtificialHorizonEntity *entity = nullptr;
 
     foreach (ArtificialHorizonEntity *horizon, m_HorizonList)
     {
         if (!horizon->enabled()) continue;
         if (horizon == ignore) continue;
         bool constraintExists = false;
         double constraint = horizon->altitudeConstraint(azimuthDegrees, &constraintExists);
         // This horizon doesn't constrain this azimuth.
         if (!constraintExists) continue;
 
         double altitudeDiff = constraint - altitudeDegrees;
         if (altitudeDiff < 0 && constraint > closestBelow)
         {
             closestBelow = constraint;
             entity = horizon;
         }
     }
     return entity;
 }
 
 bool ArtificialHorizon::isAltitudeOK(double azimuthDegrees, double altitudeDegrees, QString *reason) const
 {
     if (noCeilingConstraints)
     {
         const double constraint = altitudeConstraint(azimuthDegrees);
         if (altitudeDegrees >= constraint)
             return true;
         if (reason != nullptr)
             *reason = QString("altitude %1 < horizon %2").arg(altitudeDegrees, 0, 'f', 1).arg(constraint, 0, 'f', 1);
         return false;
     }
     else
         return isVisible(azimuthDegrees, altitudeDegrees, reason);
 }
 
 // An altitude is blocked (not visible) if either:
 // - there are constraints above and the closest above constraint is not a ceiling, or
 // - there are constraints below and the closest below constraint is a ceiling.
 bool ArtificialHorizon::isVisible(double azimuthDegrees, double altitudeDegrees, QString *reason) const
 {
     const ArtificialHorizonEntity *above = getConstraintAbove(azimuthDegrees, altitudeDegrees);
     if (above != nullptr && !above->ceiling())
     {
         if (reason != nullptr)
         {
             bool ignoreMe;
             double constraint = above->altitudeConstraint(azimuthDegrees, &ignoreMe);
             *reason = QString("altitude %1 < horizon %2").arg(altitudeDegrees, 0, 'f', 1).arg(constraint, 0, 'f', 1);
         }
         return false;
     }
     const ArtificialHorizonEntity *below = getConstraintBelow(azimuthDegrees, altitudeDegrees);
     if (below != nullptr && below->ceiling())
     {
         if (reason != nullptr)
         {
             bool ignoreMe;
             double constraint = below->altitudeConstraint(azimuthDegrees, &ignoreMe);
             *reason = QString("altitude %1 > ceiling %2").arg(altitudeDegrees, 0, 'f', 1).arg(constraint, 0, 'f', 1);
         }
         return false;
     }
     return true;
 }