LZScene

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//
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// This unit is part of the GLScene Engine https://github.com/glscene
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//
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{
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   Unit to interface with simple star records aimed for background skies.
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	 History :  
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	    05/07/03 - EG - Creation
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}
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unit GLStarRecord;
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interface
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uses
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  GLVectorGeometry;
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type
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   TGLStarRecord = packed record
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      RA : Word;              // x100 builtin factor, degrees
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      DEC : SmallInt;         // x100 builtin factor, degrees
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      BVColorIndex : Byte;    // x100 builtin factor
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      VMagnitude : Byte;      // x10 builtin factor
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   end;
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   PGLStarRecord = ^TGLStarRecord;
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{ Computes position on the unit sphere of a star record (Z=up). }
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function StarRecordPositionZUp(const starRecord : TGLStarRecord) : TAffineVector;
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{ Computes position on the unit sphere of a star record (Y=up). }
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function StarRecordPositionYUp(const starRecord : TGLStarRecord) : TAffineVector;
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{ Computes star color from BV index (RGB) and magnitude (alpha). }
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function StarRecordColor(const starRecord : TGLStarRecord; bias : Single) : TVector;
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// ------------------------------------------------------------------
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// ------------------------------------------------------------------
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// ------------------------------------------------------------------
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implementation
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uses
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  GLVectorTypes;
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// ------------------------------------------------------------------
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// ------------------------------------------------------------------
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// ------------------------------------------------------------------
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// StarRecordPositionYUp
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//
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function StarRecordPositionYUp(const starRecord : TGLStarRecord) : TAffineVector;
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var
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   f : Single;
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begin
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   SinCos(starRecord.DEC*(0.01*PI/180), Result.V[1], f);
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   SinCos(starRecord.RA*(0.01*PI/180), f, Result.V[0], Result.V[2]);
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end;
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// StarRecordPositionZUp
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//
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function StarRecordPositionZUp(const starRecord : TGLStarRecord) : TAffineVector;
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var
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   f : Single;
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begin
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   SinCos(starRecord.DEC*(0.01*PI/180), Result.V[2], f);
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   SinCos(starRecord.RA*(0.01*PI/180), f, Result.V[0], Result.V[1]);
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end;
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// StarRecordColor
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//
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function StarRecordColor(const starRecord : TGLStarRecord; bias : Single) : TVector;
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const
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   // very *rough* approximation
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   cBVm035 : TVector = (X:0.7; Y:0.8; Z:1.0; W:1);
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   cBV015  : TVector = (X:1.0; Y:1.0; Z:1.0; W:1);
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   cBV060  : TVector = (X:1.0; Y:1.0; Z:0.7; W:1);
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   cBV135  : TVector = (X:1.0; Y:0.8; Z:0.7; W:1);
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var
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   bvIndex100 : Integer;
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begin
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   bvIndex100:=starRecord.BVColorIndex-50;
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   // compute RGB color for B&V index
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   if bvIndex100<-035 then
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      Result:=cBVm035
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   else if bvIndex100<015 then
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      VectorLerp(cBVm035, cBV015, (bvIndex100+035)*(1/(015+035)), Result)
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   else if bvIndex100<060 then
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      VectorLerp(cBV015, cBV060, (bvIndex100-015)*(1/(060-015)), Result)
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   else if bvIndex100<135 then
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      VectorLerp(cBV060, cBV135, (bvIndex100-060)*(1/(135-060)), Result)
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   else Result:=cBV135;
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   // compute transparency for VMag
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   // the actual factor is 2.512, and not used here
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   Result.V[3]:=Power(1.2, -(starRecord.VMagnitude*0.1-bias));
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end;
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end.
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