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List:       kstars-devel
Subject:    [Kstars-devel] Fwd: KDE/kdeedu/kstars/kstars
From:       Jason Harris <jharris () 30doradus ! org>
Date:       2007-11-04 18:46:37
Message-ID: 200711041146.37616.jharris () 30doradus ! org
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From: Jason Harris <kstars@30doradus.org>
To: kde-commits@kde.org
Subject: KDE/kdeedu/kstars/kstars
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SVN commit 732725 by harris:

Linear interpolation for refraction correction.

We use a lookup table to determine the correction due to atmospheric
refraction for an object's apparent altitude as a function of its true
altitude.  Until now, we adopted the correction value from the nearest
altitude position in the lookup table.  This breaks down near the
horizon because the correction changes rapidly with altitude,
resulting in large discontinuous "jumps" as an object's altitude
carries it between positions in the lookup table.

Now, we are using linear inerpolation to ensure that the refraction
correction is a smooth function of the true altitude.

TODO: doing linear interpolation is more costly than a straight table
lookup.  It might make sense to only interpolate near the horizon,
where the corrections are large and changing rapidly with altitude.
The straight-lookup strategy should be fine above some threshhold
altitude.



 M  +16 -5     skymap.cpp  


--- trunk/KDE/kdeedu/kstars/kstars/skymap.cpp #732724:732725
@@ -1370,14 +1370,25 @@
     if ( alt->Degrees() <= -2.000 ) return dms( alt->Degrees() );
 
     int index = int( ( alt->Degrees() + 2.0 )*2. );  //RefractCorr arrays start at \
                alt=-2.0 degrees.
-    dms result;
+    int index2( index + 1 );
 
-    if ( findApparent ) {
-        result.setD( alt->Degrees() + RefractCorr1[index] );
-    } else {
-        result.setD( alt->Degrees() + RefractCorr2[index] );
+    //compute dx, normalized distance from nearest position in lookup table
+    double x1 = 0.5*float(index) - 1.75;
+    if ( alt->Degrees()<x1 ) index2 = index - 1;
+    if ( index2 < 0 ) index2 = index + 1;
+    if ( index2 > 183 ) index2 = index - 1;
+    double x2 = 0.5*float(index2) - 1.75;
+    double dx = (alt->Degrees() - x1)/(x2 - x1);
+
+    double y1 = RefractCorr1[index];
+    double y2 = RefractCorr1[index2];
+    if ( !findApparent ) {
+        y1 = RefractCorr2[index];
+        y2 = RefractCorr2[index2];
     }
 
+    //linear interpolation to find refracted altitude
+    dms result( alt->Degrees() + y2*dx + y1*(1.0-dx) );
     return result;
 }
 



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