Documentation for [D0LIBRARY.PIXIE.SOURCE]PLFIND.FOR
  
 DISTANCE variables:                       ANGLE variables (in degrees)
 D(1)= Dist from (U1,V1) to (U2,V2)        TH23 = Angle opposite D23
 D(2)= Dist from (U1,V1) to (U3,V3)        TH25 = Angle opposite D25
 D(3)= Dist from (U1,V1) to (U4,V4)        TH14 = angle opposite D(3)
 D(4)= Dist from (U1,V1) to (U5,V5)        PHI1 = Angle opposite DU 
 D23 = Dist from (U2,V2) to (U3,V3)        PHI2 = Angle opposite DV
 D25 = Dist from (U2,V2) to (U5,V5)
 DU  = Proj. of D(4) onto D(1)
 DV  = Proj. of D(4) onto D(2)
  
 RECTANGLE in World coordinates:         DIAMOND in Virtual Coordinates:
  (X3,Y3)               (X1,Y1)                  (U3,V3)       
     ----------------------                         ^
     |                    |                        / \
     |                    |                       /   \ 
     |                    |                      /     \
     |                    |                     /       \
     |                    |             (U1,V1)<   x     > (U4,V4)
     ----------------------                     \(U5,V5)/
  (X4,Y4)                (X3,Y3)                 \     /
                                                  \   /
                                                   \ /
                                                    V 
                                                  (U2,V2) 
  Angles and distances used to find DU,DV

                    ^       ^
                   /       / \
                  /       /   \ 
                 /       /     \
                /       /+++++++\ 
               /       /  TH14   \
             D(2)     /           \
             /       /             \
            /       /               \
           /       / \               \
          /       /   \               \
         /       /     \               \
        /    DV /       \DU            +\
       /       /         \             + \
      /       /           \            +  \
     /       /PHI1     PHI2\       TH23+   \
    v       <---------------X 5th      +    >
             \TH25          | pt       +   /
              \             |          +  /
               \            |          + /
                \           |          +/
                 \          |          /
                  \         |         /
              D(1) \        |        /
                    \       |D25    /
                     \      |      /
                      \     |     /
                       \    |    /
                        \   |   /
                         \  |  /
                          \ | /
                           \|/
                            V        

  Method to check if 5th (PICKED) point is inside the diamond:
         Make a 4 sided polygon by substituing the 5th point
         for the corner of the diamond neareast to it.
         Check the 2 angles it makes with adjacent corners.
         If either of these 2 angles is greater than the corresponding
         angle in the original diamond, the point is outside the
         diamond.
  Variables used in this check:

   IDD1= Index of corner in U,V system closest to 5th point
   IDD2= Index of corner opposite to IDD1
   IDD3,IDD4 = other 2 corners of diamond.
   
   DD(1)= Dist. from 5th point to (U1,V1)
   DD(2)= Dist. from 5th point to (U2,V2)
   DD(3)= Dist. from 5th point to (U3,V3)
   DD(4)= Dist. from 5th point to (U4,V4)

   DG3=Dist. from U(IDD2,V(IDD2) to U(IDD3),V(IDD3)
   DG4=Dist. from U(IDD2,V(IDD2) to U(IDD4),V(IDD4)   

   AL3= Angle between DD(IDD3) and DG3
   AL4= Angle between DD(IDD4) and DG4

  Angles and distances used to check if 5th point is inside diamond:
                          IDD1  
                            ^
                           / \    
                          /   \     
                         /     \     
                        /+++++++\                
                       /  TH14   \           X (5th point)  
                      /           \        # 
                     /             \    #    #
                    /               \       (
                   /              #  \     ( #
                  /            #      \   (     
                 /          #          \(    #
                /        #            ( \   
               /      #             (    \  #
              /    #              (      +\ 
             /  #               (        + \#
       IDD3< #   )          AL4(     TH23+   >IDD4
             \    )             (        +  /
              \    ) AL3         (       + /
               \  )               (      +/
                \)                 (     /
                 \                  (   /
                  \                  (/
               DG3 \                 /DG4
                    \               /
                     \             /
                      \           /
                       \         /
                        \       /
                         \     /
                          \   /
                           \|/
                            V        
                           IDD2