SUBROUTINE  VISTR (Z1,S1,S2,S3,Z2,NX,MX,NY,MY,US,VS,VD,L,IS,PL)

C     [TRIPLE SURFACE]
C     US,VS   TOTAL SHEARS IN U AND V DIRECTIONS
C     VD      VERTICAL DISPLACEMENT BETWEEN FUNCION SEGMENTS
C     L       DIRECTION OF VIEW (1=WEST, -1=EAST)
C     IS      SEPARATION OPTION (1=YES,  -1=NO)
C     PL      PEN MOVEMENT SUBROUTINE
C     [16-MAY-74]

      EXTERNAL    PL
      DIMENSION   S1(1),S2(1),S3(1)
      DIMENSION   G1(275),G2(275),G3(275),H1(275),H2(275),H3(275)
      DIMENSION   U1(275),U2(275),U3(275),F1(275),F2(275),F3(275)
      DIMENSION   X1(275),X2(275),X3(275)
      DIMENSION   B1(275),T1(275),B2(275),T2(275),B3(275),T3(275)
      DIMENSION   U(275),V1(275),V2(275),V3(275)
      DATA        M,MK/1,275/

      IX(J,I)=(I-1)*MX+J
      SC(Z)=ZS*(Z-Z1)

      N1=0
      N2=0
      N3=0
      N=L*M
      DD=2.0*VD
      EF=1.0-2.0*VD
      EL=FLOAT(L)
      EM=FLOAT(M)
      TE=0.5*(EL+1.0)
      ZS=(1.0-VS)/(Z2-Z1)
      DUI=-(EL*US)/FLOAT(MY-1)
      DUJ=(1.0-US)/FLOAT(MX-1)
      DVI=VS/FLOAT(MY-1)

      I0=(NY+1-M*(NY-3))/2
      J0=(NX+1-L*(NX-1))/2
      K0=((MK+1)*(1-N))/2
   10 K=K0
      I=MAX0(MIN0(I0,NY+1),0)
      J=J0
      EU=TE*US+DUI*FLOAT(I-1)+DUJ*FLOAT(J-1)
      VE=      DVI*FLOAT(I-1)
   20 IF ((I.LT.1).OR.(I.GT.NY)) GO TO 22
      K=MAX0(MIN0(K+N,MK),1)
      U(K)=EU
      V1(K)=VE+SC(S1(IX(J,I)))
      V2(K)=VE+SC(S2(IX(J,I)))
      V3(K)=VE+SC(S3(IX(J,I)))
   22 I=I-M
      EU=EU-DUI
      VE=VE-EM*DVI
      IF ((I.LT.1).OR.(I.GT.NY)) GO TO 30
      K=MAX0(MIN0(K+N,MK),1)
      U(K)=EU
      V1(K)=VE+SC(S1(IX(J,I)))
      V2(K)=VE+SC(S2(IX(J,I)))
      V3(K)=VE+SC(S3(IX(J,I)))
      J=J+L
      EU=EU+EL*DUJ
      IF ((J.GE.1).AND.(J.LE.NX)) GO TO 20

   30 IF (L.GT.0) GO TO 32
      NK=MK-K+1
      DO 31 KK=1,NK
      II=K+KK-1
      U(KK)=U(II)
      V1(KK)=V1(II)
      V2(KK)=V2(II)
      V3(KK)=V3(II)
   31 II=II+1
      K=NK

   32 CALL VISRB (G1,H1,NG1,MK,U,V1,K,U,V2,K,1.0)
      CALL VISRB (U3,F3,NU3,MK,G1,H1,NG1,U,V3,K,1.0)
      CALL VISRB (G1,H1,NG1,MK,U,V2,K,U,V3,K,-1.0)
      CALL VISRB (G2,H2,NG2,MK,U,V3,K,U,V1,K,-1.0)
      CALL VISRB (G3,H3,NG3,MK,U,V1,K,U,V2,K,-1.0)
      CALL VISRB (U1,F1,NU1,MK,G1,H1,NG1,G2,H2,NG2,1.0)
      CALL VISRB (U2,F2,NU2,MK,U1,F1,NU1,G3,H3,NG3,1.0)
      CALL VISRB (U1,F1,NU1,MK,G3,H3,NG3,U,V3,K,-1.0)

      IF (IS.LT.0) GO TO 40
      DO 36 KK=1,NU1
   36 F1(KK)=EF*F1(KK)
      DO 37 KK=1,NU2
   37 F2(KK)=EF*F2(KK)+VD
      DO 38 KK=1,NU3
   38 F3(KK)=EF*F3(KK)+DD

   40 CALL VISRB (G1,H1,NG1,MK,U1,F1,NU1,X2,B2,N2,-1.0)
      CALL VISHH (X1,B1,T1,N1,G1,H1,NG1, 1,PL)
      CALL VISRB (G1,H1,NG1,MK,U2,F2,NU2,X1,T1,N1, 1.0)
      CALL VISRB (G2,H2,NG2,MK,G1,H1,NG1,X3,B3,N3,-1.0)
      CALL VISHH (X2,B2,T2,N2,G2,H2,NG2,-1,PL)
      CALL VISRB (G1,H1,NG1,MK,U3,F3,NU3,X2,T2,N2, 1.0)
      CALL VISHH (X3,B3,T3,N3,G1,H1,NG1, 1,PL)

      I0=I0+M
      IF ((I0.GE.0).AND.(I0.LE.NY+1)) GO TO 10
      J0=J0+L
      IF ((J0.GE.1).AND.(J0.LE.NX))   GO TO 10
      RETURN

      END