Module JNISpice
Package spice.basic

Class IlluminationAngles


  • public class IlluminationAngles
    extends java.lang.Object
    Class IlluminationAngles supports illumination angle computations.

    Particulars

       Illumination angles 
       =================== 
     
       The term "illumination angles" refers to following set of 
       angles: 
     
     
          phase angle              Angle between the vectors from the 
                                   surface point to the observer and 
                                   from the surface point to the  
                                   illumination source. 
     
          incidence angle          Angle between the surface normal at 
                                   the specified surface point and the 
                                   vector from the surface point to the 
                                   illumination source. 
     
          emission angle           Angle between the surface normal at 
                                   the specified surface point and the 
                                   vector from the surface point to the 
                                   observer. 
      
       The diagram below illustrates the geometric relationships 
       defining these angles. The labels for the incidence, emission, 
       and phase angles are "inc.", "e.", and "phase". 
     
     
     
                                                illumination source 
     
                      surface normal vector 
                                ._                 _. 
                                |\                 /|  illumination  
                                  \    phase      /    source vector 
                                   \   .    .    / 
                                   .            . 
                                     \   ___   / 
                                .     \/     \/ 
                                      _\ inc./ 
                               .    /   \   / 
                               .   |  e. \ / 
                 &lt--------------- *  surface point on 
        viewing            vector            target body 
        location           to viewing 
        (observer)         location 
     
     
       Note that if the target-observer vector, the target normal vector 
       at the surface point, and the target-illumination source vector 
       are coplanar, then phase is the sum of the incidence and emission 
       angles. This rarely occurs; usually 
     
          phase angle  &lt  incidence angle + emission angle 
     
       All of the above angles can be computed using light time 
       corrections, light time and stellar aberration corrections, or no 
       aberration corrections. In order to describe apparent geometry as 
       observed by a remote sensing instrument, both light time and 
       stellar aberration corrections should be used. 
        
       The way aberration corrections are applied by the constructors
       of this class is described below. 
     
          Light time corrections 
          ====================== 
     
             Observer-target surface point vector 
             ------------------------------------ 
     
             Let `et' be the epoch at which an observation or remote 
             sensing measurement is made, and let et-lt ("lt" stands 
             for "light time") be the epoch at which the photons 
             received at `et' were emitted from the surface point `spoint'. 
             Note that the light time between the surface point and 
             observer will generally differ from the light time between 
             the target body's center and the observer. 
     
     
             Target body's orientation 
             ------------------------- 
     
             Using the definitions of `et' and `lt' above, the target body's 
             orientation at et-lt is used. The surface normal is 
             dependent on the target body's orientation, so the body's 
             orientation model must be evaluated for the correct epoch. 
     
     
             Target body -- illumination source vector 
             ----------------------------------------- 
     
             The surface features on the target body near `spoint' will 
             appear in a measurement made at `et' as they were at et-lt. 
             In particular, lighting on the target body is dependent on 
             the apparent location of the illumination source as seen 
             from the target body at et-lt. So, a second light time 
             correction is used to compute the position of the 
             illumination source relative to the surface point. 
     
     
          Stellar aberration corrections 
          ============================== 
     
          Stellar aberration corrections are applied only if 
          light time corrections are applied as well. 
     
             Observer-target surface point body vector 
             ----------------------------------------- 
     
             When stellar aberration correction is performed, the 
             direction vector `srfvec' is adjusted so as to point to the 
             apparent position of `spoint': considering `spoint' to be an 
             ephemeris object, `srfvec' points from the observer's 
             position at `et' to the light time and stellar aberration 
             corrected position of `spoint'. 
     
             Target body-illumination source vector 
             -------------------------------------- 
     
             The target body-illumination source vector is the apparent 
             position of the illumination source, corrected for light 
             time and stellar aberration, as seen from the target body 
             at time et-lt. 
     
     
       Using DSK data 
       ============== 
     
          DSK loading and unloading 
          ------------------------- 
     
          DSK files providing data used by this class are loaded by
          calling KernelDatabase.load(java.lang.String) and can be unloaded by calling 
          KernelDatabase.unload(java.lang.String) or KernelDatabase.clear() 
          See the documentation of KernelDatabase.load(java.lang.String) for limits 
          on numbers of loaded DSK files.
     
          For run-time efficiency, it's desirable to avoid frequent 
          loading and unloading of DSK files. When there is a reason to 
          use multiple versions of data for a given target body---for 
          example, if topographic data at varying resolutions are to be 
          used---the surface list can be used to select DSK data to be 
          used for a given computation. It is not necessary to unload 
          the data that are not to be used. This recommendation presumes 
          that DSKs containing different versions of surface data for a 
          given body have different surface ID codes. 
     
     
          DSK data priority 
          ----------------- 
     
          A DSK coverage overlap occurs when two segments in loaded DSK 
          files cover part or all of the same domain---for example, a 
          given longitude-latitude rectangle---and when the time 
          intervals of the segments overlap as well. 
     
          When DSK data selection is prioritized, in case of a coverage 
          overlap, if the two competing segments are in different DSK 
          files, the segment in the DSK file loaded last takes 
          precedence. If the two segments are in the same file, the 
          segment located closer to the end of the file takes 
          precedence. 
     
          When DSK data selection is unprioritized, data from competing 
          segments are combined. For example, if two competing segments 
          both represent a surface as a set of triangular plates, the 
          union of those sets of plates is considered to represent the 
          surface.  
     
          Currently only unprioritized data selection is supported. 
          Because prioritized data selection may be the default behavior 
          in a later version of the class, the UNPRIORITIZED keyword is 
          required in the `method' argument. 
     
           
          Syntax of the `method' input argument 
          ----------------------------------- 
     
          The keywords and surface list in the `method' argument 
          are called "clauses." The clauses may appear in any 
          order, for example 
     
             "DSK/&ltsurface list&gt/UNPRIORITIZED"
             "DSK/UNPRIORITIZED/&ltsurface list&gt"
             "UNPRIORITIZED/&ltsurface list&gt/DSK"
     
          The simplest form of the `method' argument specifying use of 
          DSK data is one that lacks a surface list, for example: 
     
             "DSK/UNPRIORITIZED" 
     
          For applications in which all loaded DSK data for the target 
          body are for a single surface, and there are no competing 
          segments, the above string suffices. This is expected to be 
          the usual case. 
     
          When, for the specified target body, there are loaded DSK 
          files providing data for multiple surfaces for that body, the 
          surfaces to be used by the constructors of this class
          for a given call must be specified in a surface list, unless 
          data from all of the surfaces are to be used together. 
     
          The surface list consists of the string 
     
             "SURFACES ="
     
          followed by a comma-separated list of one or more surface 
          identifiers. The identifiers may be names or integer codes in 
          string format. For example, suppose we have the surface 
          names and corresponding ID codes shown below: 
     
             Surface Name                              ID code 
             ------------                              ------- 
             "Mars MEGDR 128 PIXEL/DEG"                1 
             "Mars MEGDR 64 PIXEL/DEG"                 2 
             "Mars_MRO_HIRISE"                         3 
     
          If data for all of the above surfaces are loaded, then 
          data for surface 1 can be specified by either 
     
             "SURFACES = 1" 
     
          or 
     
             "SURFACES = "\"Mars MEGDR 128 PIXEL/DEG\"" 
     
          Escaped double quotes are used to delimit the surface name
          because it contains blank characters.
              
          To use data for surfaces 2 and 3 together, any 
          of the following surface lists could be used: 
     
             "SURFACES = 2, 3" 
     
             "SURFACES = \"Mars MEGDR  64 PIXEL/DEG\", 3" 
     
             "SURFACES = 2, Mars_MRO_HIRISE" 
     
             "SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", Mars_MRO_HIRISE" 
            
          An example of a `method' argument that could be constructed 
          using one of the surface lists above is 
    
             "DSK/UNPRIORITIZED/SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", 3"
      
     
          Aberration corrections using DSK data 
          ------------------------------------- 
     
          For irregularly shaped target bodies, the distance between the 
          observer and the nearest surface intercept need not be a 
          continuous function of time; hence the one-way light time 
          between the intercept and the observer may be discontinuous as 
          well. In such cases, the computed light time, which is found 
          using iterative algorithm, may converge slowly or not at all. 
          In all cases, the light time computation will terminate, but 
          the result may be less accurate than expected. 
    

    Code Examples

    The numerical results shown for this example may differ across platforms. The results depend on the SPICE kernels used as input, the compiler and supporting libraries, and the machine specific arithmetic implementation.

       1) Find the phase, solar incidence, and emission angles at the 
          sub-solar and sub-spacecraft points on Mars as seen from the Mars
          Global Surveyor spacecraft at a specified UTC time.
     
          Use both an ellipsoidal Mars shape model and topographic data
          provided by a DSK file. For both surface points, use the "near
          point" and "nadir" definitions for ellipsoidal and DSK shape
          models, respectively.
     
          Use converged Newtonian light time and stellar aberration
          corrections.
    
          The topographic model is based on data from the MGS MOLA DEM 
          megr90n000cb, which has a resolution of 4 pixels/degree. A 
          triangular plate model was produced by computing a 720 x 1440 
          grid of interpolated heights from this DEM, then tessellating 
          the height grid. The plate model is stored in a type 2 segment 
          in the referenced DSK file. 
    
          Use the meta-kernel shown below to load the required SPICE 
          kernels. 
     
             KPL/MK 
     
             File: IlluminationAnglesEx1.tm 
     
             This meta-kernel is intended to support operation of SPICE 
             example programs. The kernels shown here should not be 
             assumed to contain adequate or correct versions of data 
             required by SPICE-based user applications. 
     
             In order for an application to use this meta-kernel, the 
             kernels referenced here must be present in the user's 
             current working directory. 
     
             The names and contents of the kernels referenced 
             by this meta-kernel are as follows: 
     
                File name                        Contents 
                ---------                        -------- 
                de430.bsp                        Planetary ephemeris 
                mar097.bsp                       Mars satellite ephemeris 
                pck00010.tpc                     Planet orientation and 
                                                 radii 
                naif0011.tls                     Leapseconds 
                mgs_ext12_ipng_mgs95j.bsp        MGS ephemeris 
                megr90n000cb_plate.bds           Plate model based on 
                                                 MEGDR DEM, resolution 
                                                 4 pixels/degree. 
     
             \begindata 
     
                KERNELS_TO_LOAD = ( 'de430.bsp', 
                                    'mar097.bsp', 
                                    'pck00010.tpc', 
                                    'naif0011.tls', 
                                    'mgs_ext12_ipng_mgs95j.bsp', 
                                    'megr90n000cb_plate.bds'      ) 
             \begintext 
     
    
       Example code begins here. 
    
    
       //
       // IlluminationAngles example 1
       //
    
       //
       // Import JNISpice API classes.
       //
       import spice.basic.*;
    
       //
       // Import the conversion factor DPR.
       //
       import static spice.basic.AngularUnits.*;
    
    
       public class IlluminationAnglesEx1
       {
    
          static{ System.loadLibrary( "JNISpice" ); }
    
          public static void main( String[] args )
          {
             //
             // Local constants
             //
             final String     META  = "IlluminationAngles.tm";
    
             final int        NMETH = 2;
    
             //
             // Local variables
             //
    
             //
             // `method' strings for illumination angle computations.
             //
             String[]         ilumth = { "Ellipsoid", 
                                         "DSK/Unprioritized" };
    
             //
             // `method' strings for sub-spacecraft point and 
             // sub-solar point computations.
             //
             String[]         submth = { "Near Point/Ellipsoid", 
                                         "DSK/Nadir/Unprioritized" };
             int              i;
    
    
             try
             {
                //
                // Load kernels.
                //
                KernelDatabase.load( META );
    
                //
                // Convert the UTC request time string to seconds past 
                // J2000 TDB. 
                //
                String utc  = "2003 OCT 13 06:00:00 UTC";
    
                TDBTime et  = new TDBTime( utc );
    
                System.out.format ( "%n" +
                                    "UTC epoch is %s%n", utc );
    
                //            
                // Assign observer, target, and illumination source
                // names. The acronym MGS indicates Mars Global 
                // Surveyor. See NAIF_IDS for a list of names 
                // recognized by SPICE. 
                //
                // Also set the target body-fixed frame and
                // the aberration correction flag.  
                //
    
                Body target = new Body( "Mars" );
                Body obsrvr = new Body( "MGS"  );
                Body ilusrc = new Body( "Sun"  );
    
                ReferenceFrame       fixref = new ReferenceFrame( "IAU_MARS");
                AberrationCorrection abcorr = new AberrationCorrection( "CN+S" );
    
                for ( i = 0;  i &lt NMETH;  i++ )
                {
                   //
                   // Find the sub-solar point on Mars as 
                   // seen from the MGS spacecraft at `et'. Use the 
                   // "near point" style of sub-point definition 
                   // when the shape model is an ellipsoid, and use 
                   // the "nadir" style when the shape model is 
                   // provided by DSK data. This makes it easy to  
                   // verify the solar incidence angle when 
                   // the target is modeled as an  ellipsoid. 
                   //
                   SubSolarRecord ssolrec 
    
                     = new SubSolarRecord( submth[i], target, et,
                                           fixref,    abcorr, obsrvr );
    
                   Vector3 ssolpt = ssolrec.getSubPoint(); 
    
                   //
                   // Now find the sub-spacecraft point.  
                   //
                   SubObserverRecord sscrec
    
                      = new SubObserverRecord( submth[i], target, et,
                                               fixref,    abcorr, obsrvr );
    
                   Vector3 sscpt = sscrec.getSubPoint();
    
                   //
                   // Find the phase, solar incidence, and emission 
                   // angles at the sub-solar point on Mars as 
                   // seen from MGS at time `et'.  
                   //
                   IlluminationAngles solAngles
    
                      = new IlluminationAngles( ilumth[i], target, ilusrc,
                                                et,        fixref, abcorr,
                                                obsrvr,    ssolpt         );
                   //
                   // Do the same for the sub-spacecraft point. 
                   //
                   IlluminationAngles sscAngles
    
                      = new IlluminationAngles( ilumth[i], target, ilusrc,
                                                et,        fixref, abcorr,
                                                obsrvr,    sscpt          );
                   //
                   // Obtain the illumination angles and flags.
                   // Convert the angles to degrees and write 
                   // the outputs.   
                   //
                   double  sslphs = solAngles.getPhaseAngle()     * DPR;
                   double  sslemi = solAngles.getEmissionAngle()  * DPR;
                   double  sslinc = solAngles.getIncidenceAngle() * DPR;
                   boolean sslvis = solAngles.isVisible();
                   boolean ssllit = solAngles.isLit();
    
                   double  sscphs = sscAngles.getPhaseAngle()     * DPR;
                   double  sscemi = sscAngles.getEmissionAngle()  * DPR;
                   double  sscinc = sscAngles.getIncidenceAngle() * DPR;
                   boolean sscvis = sscAngles.isVisible();
                   boolean ssclit = sscAngles.isLit();
    
                   System.out.format(
    
                      "%n" +
                      "   IlluminationAngle method: %s%n" +
                      "   SubObserverRecord method: %s%n" +
                      "   SubSolarRecord method:    %s%n" +
                      "%n" +
                      "      Illumination angles at the " +
                      "sub-solar point:%n" +
                      "%n" +
                      "      Phase angle            (deg): %15.9f%n" +
                      "      Solar incidence angle  (deg): %15.9f%n" +
                      "      Emission angle         (deg): %15.9f%n" +
                      "      Visible:                        %b%n"   +
                      "      Lit:                            %b%n",
                      ilumth[i],
                      submth[i],
                      submth[i],
                      sslphs,
                      sslinc,
                      sslemi,
                      sslvis,
                      ssllit  );           
    
                   if ( i == 0 )
                   {
                      System.out.format( "%n" +
                                         "         The solar incidence angle "  +
                                         "should be 0.%n"                       +
                                         "         The emission "               +
                                         "and phase angles should be equal.%n"  );
                   }
    
                   System.out.format(
    
                      "%n" +
                      "      Illumination angles at the " +
                      "sub-s/c point:%n" +
                      "%n" +
                      "      Phase angle            (deg): %15.9f%n" +
                      "      Solar incidence angle  (deg): %15.9f%n" +
                      "      Emission angle         (deg): %15.9f%n" +
                      "      Visible:                        %b%n"   +
                      "      Lit:                            %b%n",
                      sscphs,
                      sscinc,
                      sscemi,
                      sscvis,
                      ssclit  );           
    
                   if ( i == 0 )
                   {
                      System.out.format( "%n" +
                                         "         The emission angle "  +
                                         "should be 0.%n"                +
                                         "         The solar incidence " +
                                         "and phase angles should be equal.%n"  );
                   }
    
                }
    
                System.out.println( " " );
             }
             catch ( SpiceException exc )
             {
                exc.printStackTrace();
             }
          }
    
       }
    
    
       When this program was executed on a PC/Linux/gcc/64-bit/Java 1.5 
       platform, the output was:   
    
    
          UTC epoch is 2003 OCT 13 06:00:00 UTC
    
             IlluminationAngle method: Ellipsoid
             SubObserverRecord method: Near Point/Ellipsoid
             SubSolarRecord method:    Near Point/Ellipsoid
    
                Illumination angles at the sub-solar point:
    
                Phase angle            (deg):   138.370270685
                Solar incidence angle  (deg):     0.000000000
                Emission angle         (deg):   138.370270685
                Visible:                        false
                Lit:                            true
    
                   The solar incidence angle should be 0.
                   The emission and phase angles should be equal.
    
                Illumination angles at the sub-s/c point:
    
                Phase angle            (deg):   101.439331040
                Solar incidence angle  (deg):   101.439331041
                Emission angle         (deg):     0.000000002
                Visible:                        true
                Lit:                            false
    
                   The emission angle should be 0.
                   The solar incidence and phase angles should be equal.
    
             IlluminationAngle method: DSK/Unprioritized
             SubObserverRecord method: DSK/Nadir/Unprioritized
             SubSolarRecord method:    DSK/Nadir/Unprioritized
    
                Illumination angles at the sub-solar point:
    
                Phase angle            (deg):   138.387071677
                Solar incidence angle  (deg):     0.967122745
                Emission angle         (deg):   137.621480599
                Visible:                        false
                Lit:                            true
    
                Illumination angles at the sub-s/c point:
    
                Phase angle            (deg):   101.439331359
                Solar incidence angle  (deg):   101.555993667
                Emission angle         (deg):     0.117861156
                Visible:                        true
                Lit:                            false
    

    Version 2.0.0 11-NOV-2016 (NJB)

    Upgraded to support DSK usage.

    Now supports "isLit" and "isVisible" methods. These indicate whether the illumination source and observer, respectively, are visible from the surface point.

    Version 1.0.0 16-JUL-2009 (NJB)

    • Field Detail

      • lit

        private boolean lit
      • visible

        private boolean visible
      • targetEpoch

        private TDBTime targetEpoch
      • surfaceVector

        private Vector3 surfaceVector
      • phaseAngle

        private double phaseAngle
      • incidenceAngle

        private double incidenceAngle
      • emissionAngle

        private double emissionAngle
    • Constructor Detail

      • IlluminationAngles

        public IlluminationAngles​(java.lang.String method,
                                  Body target,
                                  Body ilusrc,
                                  Time t,
                                  ReferenceFrame fixref,
                                  AberrationCorrection abcorr,
                                  Body observer,
                                  Vector3 spoint)
                           throws SpiceException
        Find the illumination angles (phase, solar incidence, and emission) at a specified surface point on a target body, using a specified illumination source; create a record containing the result.The illumination source may be any ephemeris object.

        Values and meanings of the `method' argument are described below.

           method      is a short string providing parameters defining 
                       the computation method to be used. In the syntax 
                       descriptions below, items delimited by brackets 
                       are optional. 
                       
                       `method' may be assigned the following values:    
         
                          "ELLIPSOID" 
          
                             The illumination angle computation uses a 
                             triaxial ellipsoid to model the surface of the 
                             target body. The ellipsoid's radii must be 
                             available in the kernel pool. 
         
         
                          "DSK/UNPRIORITIZED[/SURFACES = &ltsurface list&gt]" 
         
                             The illumination angle computation uses 
                             topographic data to model the surface of the 
                             target body. These data must be provided by 
                             loaded DSK files. 
         
                             The surface list specification is optional. The 
                             syntax of the list is 
         
                                &ltsurface 1&gt [, &ltsurface 2&gt...] 
         
                             If present, it indicates that data only for the 
                             listed surfaces are to be used; however, data 
                             need not be available for all surfaces in the 
                             list. If absent, loaded DSK data for any surface 
                             associated with the target body are used. 
         
                             The surface list may contain surface names or 
                             surface ID codes. Names containing blanks must 
                             be delimited by escaped double quotes, for example 
         
                                "SURFACES = \"Mars MEGDR 128 PIXEL/DEG\""
                                                
                             If multiple surfaces are specified, their names 
                             or IDs must be separated by commas. 
         
                             See the Particulars section below for details 
                             concerning use of DSK data. 
         
         
                       Neither case nor white space are significant in 
                       `method', except within double-quoted strings 
                       representing surface names. For example, the string 
                       " eLLipsoid " is valid.
         
                       Within double-quoted strings representing surface names,
                       blank characters are significant, but multiple
                       consecutive blanks are considered equivalent to a single
                       blank. Case is not significant. So
         
                          \"Mars MEGDR 128 PIXEL/DEG\" 
         
                       is equivalent to  
         
                          \" mars megdr  128  pixel/deg \"
         
                       but not to 
         
                          \"MARS MEGDR128PIXEL/DEG\" 
           
        Parameters:
        method - String
        spoint - Vector3
        target - Body
        observer - Body
        ilusrc - Body
        abcorr - AberrationCorrection
        t - Time
        fixref - ReferenceFrame
        Throws:
        SpiceException - exception
    • Method Detail

      • getPhaseAngle

        public double getPhaseAngle()
        Return the phase angle.Units are radians.

        Let `phase' refer to the phase angle at `spoint', as seen from `obsrvr' at time `et'. This is the angle between the spoint-obsrvr vector and the spoint-illumination source vector. Units are radians. The range of `phase' is [0, pi]. See Particulars above for a detailed discussion of the definition.

        Returns:
        double
      • getIncidenceAngle

        public double getIncidenceAngle()
        Return the illumination source incidence angle.Units are radians.

        Let `incdnc' refer to the illumination source incidence angle at `spoint', as seen from `obsrvr' at time `et'. This is the angle between the surface normal vector at `spoint' and the spoint-illumination source vector. Units are radians. The range of `incdnc' is [0, pi]. See Particulars above for a detailed discussion of the definition.

        Returns:
        double
      • getSolarIncidenceAngle

        public double getSolarIncidenceAngle()
        Return the solar incidence angle.Units are radians.

        This method is deprecated and is retained for backward compatibility. Use getIncidenceAngle() in place of this method.

        Returns:
        double
      • getEmissionAngle

        public double getEmissionAngle()
        Return the emission angle.Units are radians.

        Let `emissn' refer to emission angle at `spoint', as seen from `obsrvr' at time `et'. This is the angle between the surface normal vector at `spoint' and the spoint-observer vector. Units are radians. The range of `emissn' is [0, pi]. See Particulars above for a detailed discussion of the definition.

        Returns:
        double
      • getTargetEpoch

        public TDBTime getTargetEpoch()
        Return the target epoch.

        Let `trgepc' refer to "target surface point epoch." `trgepc' is defined as follows: letting `lt' be the one-way light time between the observer and the input surface point `spoint', `trgepc' is either the epoch et-lt or `et' depending on whether the requested aberration correction is, respectively, for received radiation or omitted. `lt' is computed using the method indicated by `abcorr'.

        Returns:
        TDBTime
      • getSurfaceVector

        public Vector3 getSurfaceVector()
        Return the observer to surface point vector.

        Let `srfvec' refer to this vector; it is the vector from the observer's position at `et' to the aberration-corrected (or optionally, geometric) position of `spoint', where the aberration corrections are specified by `abcorr'. `srfvec' is expressed in the target body-fixed reference frame designated by `fixref', evaluated at `trgepc'.

        The components of `srfvec' are given in units of km.

        One can use Vector3.norm() to obtain the distance between the observer and `spoint':

           dist = srfvec.norm();
           

        The observer's position `obspos', relative to the target body's center, where the center's position is corrected for aberration effects as indicated by `abcorr', can be computed via the call:

           obspos = spoint.sub(srfvec); 
           
        To transform the vector `srfvec' from a reference frame `fixref' at time `trgepc' to a time-dependent reference frame `ref' at time `et', the method ReferenceFrame.getPositionTransformation(ReferenceFrame,Time,Time) should be called. Let `xform' be the 3x3 matrix representing the rotation from the reference frame `fixref' at time `trgepc' to the reference frame `ref' at time `et'. Then `srfvec' can be transformed to the result `refvec' as follows:
           xform  = fixref.getPositionTransformation( ref, trgepc, et );
           refvec = xform.mxv( srfvec );                                            
           
        Returns:
        Vector3
      • isVisible

        public boolean isVisible()
        Return the visibility flag.

        Let `visible' refer to this flag, which indicates whether the surface point is visible to the observer. `visible' takes into account whether the target surface occults `spoint', regardless of the emission angle at `spoint'. `visible' is returned with the value true if `spoint' is visible; otherwise it is false.

        Returns:
        boolean
      • isLit

        public boolean isLit()
        Return the illumination flag.

        Let `lit' refer to this logical flag, which indicates whether the surface point is illuminated; the point is considered to be illuminated if the vector from the point to the center of the illumination source doesn't intersect the target surface. `lit' takes into account whether the target surface casts a shadow on `spoint', regardless of the incidence angle at `spoint'. `lit' is returned with the value true if `spoint' is illuminated; otherwise it is false.

        Returns:
        boolean