[Kstars-devel] [kstars] kstars/skyobjects: + Separate the test for significant gravitational lensing around the

[Akarsh Simha]

Git commit 6853ce95835de7d7cfbe74137593d72c77e2fbd8 by Akarsh Simha.
Committed on 17/07/2011 at 10:00.
Pushed by asimha into branch 'master'.

+ Separate the test for significant gravitational lensing around the
  sun from the actual lensing calculation. Now there are two methods
  -- checkBendLight() and bendlight(), the former doing the job of
  checking whether the skypoint is close enough to the sun to be
  lensed by its gravity.

+ [Major] Recompute coordinates of a SkyPoint only when about a minute
  has passed since the last recomputation. This means that precession,
  nutation, aberration, are calculated only when there are
  sufficiently large changes in time. This is expected to increase the
  speed of recomputing the skymap, although I have not tested the
  improvement.

  This is also a potentially dangerous change. I've not tested to
  check that precession is recomputed when the step is sufficiently
  large.

CCMAIL: kstars-devel at kde.org

M  +9    -0    kstars/skyobjects/skypoint.h
M  +48   -33   kstars/skyobjects/skypoint.cpp

http://commits.kde.org/kstars/6853ce95835de7d7cfbe74137593d72c77e2fbd8

diff --git a/kstars/skyobjects/skypoint.cpp b/kstars/skyobjects/skypoint.cpp
index 9930928..8a5eff1 100644
--- a/kstars/skyobjects/skypoint.cpp
+++ b/kstars/skyobjects/skypoint.cpp
@@ -31,6 +31,8 @@
 #include "Options.h"
 #include "skycomponents/skymapcomposite.h"
 
+KSSun *SkyPoint::m_Sun = 0;
+
 SkyPoint::SkyPoint() {
     // Default constructor. Set nonsense values
     RA0.setD(-1); // RA >= 0 always :-)
@@ -257,36 +259,33 @@ SkyPoint SkyPoint::moveAway( const SkyPoint &from, double dist ){
     return SkyPoint( ra() + dtheta, lat1 );
 }
 
+bool SkyPoint::checkBendLight() {
+    // First see if we are close enough to the sun to bother about the
+    // gravitational lensing effect. We correct for the effect at
+    // least till b = 10 solar radii, where the effect is only about
+    // 0.06".  Assuming min. sun-earth distance is 200 solar radii.
+    static const dms maxAngle( 1.75 * ( 30.0 / 200.0) / dms::DegToRad );
+
+    if( !m_Sun )
+        m_Sun = (KSSun*) KStarsData::Instance()->skyComposite()->findByName( "Sun" );
+
+    // TODO: This can be optimized further. We only need a ballpark estimate of the distance to the sun to start with.
+    return ( fabs( angularDistanceTo( static_cast<const SkyPoint *>(m_Sun) ).Degrees() ) <= maxAngle.Degrees() ); // NOTE: dynamic_cast is slow and not important here.
+}
+
 bool SkyPoint::bendlight() {
 
     // NOTE: This should be applied before aberration
+    // NOTE: One must call checkBendLight() before unnecessarily calling this.
+    // We correct for GR effects
+    Q_ASSERT( m_Sun );
+    double corr_sec = 1.75 * m_Sun->physicalSize() / ( m_Sun->rearth() * AU_KM * angularDistanceTo( static_cast<const SkyPoint *>(m_Sun) ).sin() );
+    Q_ASSERT( corr_sec > 0 );
 
-    // First see if we are close enough to the sun to bother about the
-    // effect. We correct for the effect at least till b = 10 solar
-    // radii, where the effect is only about 0.06".  Assuming
-    // min. sun-earth distance is 200 solar radii.
-
-    static KSSun* thesun;
-    const dms maxAngle( 1.75 * ( 30.0 / 200.0) / dms::DegToRad );
-
-    if( !thesun ) // Hope we don't destroy the sun before the program ends!
-        thesun = (KSSun*) KStarsData::Instance()->skyComposite()->findByName( "Sun" );
-    //    kDebug() << "bendlight says maximum correctable angle is " << maxAngle.Degrees();
-
-    // TODO: We can make this code more efficient
-    SkyPoint sp = *thesun;
-    //    kDebug() << "The unaberrated sun is " << sp.angularDistanceTo( sun ).Degrees() << "deg away";
-    if( fabs( angularDistanceTo( &sp ).radians() ) <= maxAngle.radians() ) {
-        // We correct for GR effects
-        double corr_sec = 1.75 * thesun->physicalSize() / ( thesun->rearth() * AU_KM * angularDistanceTo( &sp ).sin() );
-        Q_ASSERT( corr_sec > 0 );
-        
-        sp = moveAway( sp, corr_sec );
-        setRA(  sp.ra() );
-        setDec( sp.dec() );
-        return true;
-    }
-    return false;
+    SkyPoint sp = moveAway( *m_Sun, corr_sec );
+    setRA(  sp.ra() );
+    setDec( sp.dec() );
+    return true;
 }
 
 void SkyPoint::aberrate(const KSNumbers *num) {
@@ -317,14 +316,30 @@ void SkyPoint::aberrate(const KSNumbers *num) {
     Dec.setD( Dec.Degrees() + dDec );
 }
 
+// Note: This method is one of the major rate determining factors in how fast the map pans / zooms in or out
 void SkyPoint::updateCoords( KSNumbers *num, bool /*includePlanets*/, const dms *lat, const dms *LST ) {
     //Correct the catalog coordinates for the time-dependent effects
     //of precession, nutation and aberration
-    precess(num);
-    nutate(num);
-    if(Options::useRelativistic())
-        bendlight();
-    aberrate(num);
+    bool recompute, lens;
+
+    if( Options::useRelativistic() && checkBendLight() ) {
+        recompute = true;
+        lens = true;
+    }
+    else {
+        recompute = (( lastPrecessJD - num->getJD() ) >= 0.0005); // 0.0005 solar days is less than a minute
+        lens = false;
+    }
+
+    if( recompute ) {
+        precess(num);
+        nutate(num);
+        if( lens )
+            bendlight();
+        aberrate(num);
+        lastPrecessJD = num->getJD();
+    }
+
     if ( lat || LST )
         kWarning() << i18n( "lat and LST parameters should only be used in KSPlanetBase objects." ) ;
 }
@@ -410,7 +425,7 @@ void SkyPoint::apparentCoord(long double jd0, long double jdf){
     precessFromAnyEpoch(jd0,jdf);
     KSNumbers num(jdf);
     nutate( &num );
-    if(Options::useRelativistic())
+    if( Options::useRelativistic() && checkBendLight() )
         bendlight();
     aberrate( &num );
 }
diff --git a/kstars/skyobjects/skypoint.h b/kstars/skyobjects/skypoint.h
index 0e9a732..494dd0f 100644
--- a/kstars/skyobjects/skypoint.h
+++ b/kstars/skyobjects/skypoint.h
@@ -27,6 +27,7 @@
 #include "kstarsdatetime.h"
 
 class KSNumbers;
+class KSSun;
 
 /**@class SkyPoint
 	*
@@ -285,6 +286,12 @@ public:
     	*/
     void nutate(const KSNumbers *num);
 
+    /**
+     *@short Check if this sky point is close enough to the sun for
+     * gravitational lensing to be significant
+     */
+    bool checkBendLight();
+
     /**Correct for the effect of "bending" of light around the sun for
      * positions near the sun.
      *
@@ -500,6 +507,8 @@ private:
     dms RA0, Dec0; //catalog coordinates
     dms RA, Dec; //current true sky coordinates
     dms Alt, Az;
+    long double   lastPrecessJD; // JD at which the last coordinate update (see updateCoords) for this SkyPoint was done
+    static KSSun *m_Sun;
 };
 
 #endif