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

C     [DOUBLE SURFACE]
C     S1,S2   ARRAYS CONTAINING THE TWO SURFACES
C     Z1,Z2   SPAN OF SURFACE VALUES
C     MX,MY   COMMON DIMENSION OF THE ARRAYS S1 AND S2
C     NX,NY   SECTIONS OF S1 AND S2 ACTUALLY USED
C     US,VS   TOTAL SHEARS IN U AND V DIRECTIONS
C     L       DIRECTION OF VIEW (1=WEST, -1=EAST)
C     IS      SEPARATION OPTION (1=YES , -1=NO)
C     PL      PEN MOVEMENT SUBROUTINE
C     [15-MAY-74]

      EXTERNAL    PL
      LOGICAL*1     P,Q
      DIMENSION   S1(1),S2(1)
      VIRTUAL   X1(351),T1(351),B1(351)
      VIRTUAL    X2(351),T2(351),B2(351)
      VIRTUAL    D(351),E(351),G(351),H(351)
      VIRTUAL    A(351),B(351)
      VIRTUAL    U(201),V1(201),V2(201)
      DATA        M,MK,MA/1,201,351/

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

      N1=0
      N2=0
      N=L*M
      EF=1.0-VS
      EL=FLOAT(L)
      EM=FLOAT(M)
      ZS=EF/(Z2-Z1)
      TE=0.5*(EL+1.0)
      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)))
   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)))
      J=J+L
      EU=EU+EL*DUJ
      IF ((J.GE.1).AND.(J.LE.NX)) GO TO 20

   30 P=L.LT.0
      Q=L.GT.0
      IF (P) KK=MK-K+1
      IF (Q) CALL VISRB (A,B,NA,MA,U,V1,K,U,V2,K,1.0)
      IF (P) CALL VISRB (A,B,NA,MA,U(K),V1(K),KK,U(K),V2(K),KK,1.0)
      IF (Q) CALL VISRB (G,H,NG,MA,U,V1,K,U,V2,K,-1.0)
      IF (P) CALL VISRB (G,H,NG,MA,U(K),V1(K),KK,U(K),V2(K),KK,-1.0)

      IF (IS.LT.0) GO TO 40
      DO 36 II=1,NA
   36 B(II)=EF*B(II)+VS
      DO 38 II=1,NG
   38 H(II)=EF*H(II)

   40 CALL VISRB (D,E,ND,MA,A,B,NA,X1,T1,N1,1.0)
      CALL VISHH (X2,T2,B2,N2,D,E,ND,1,PL)
      CALL VISRB (D,E,ND,MA,G,H,NG,X2,B2,N2,-1.0)
      CALL VISHH (X1,T1,B1,N1,D,E,ND,-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