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SUBROUTINE VISTR (Z0, Z1,S1,S2,S3,Z2, MX,MY, NX,NY, LX,LY, S,
X T, SEP, PL2)
C$ (Triple Surface)
C$ Make a hidden-line drawing of three possibly intersecting
C$ surfaces, hiding those parts of each surface which are
C$ covered by the others.
C$
C$ Z0..........Cutoff value. Only function values, S*(I,J),
C$ above (S .GT. 0.0) or below (S .LT. 0) Z0 are
C$ visible.
C$ S1,S2,S3....Arrays containing the three surfaces. S*(I,J)
C$ = F(X(I),Y(J)), where X(I) = (I-1)*DX and Y(J)
C$ = (J-1)*DY both map onto the interval 0..1.
C$ Z1,Z2.......Span of surface values.
C$ MX,MY.......Actual declared dimensions of the arrays
C$ S1(*,*), S2(*,*), and S3(*,*).
C$ NX,NY.......Sections of S1(*,*), S2(*,*), and S3(*,*)
C$ actually used.
C$ LX,LY.......Increments in X and Y directions (.GT. 0).
C$ Values of LX and LY larger than 1 produce a
C$ coarser mesh on the drawing without losing the
C$ smoothness of the complete surface. LX should
C$ be an integral divisor of NX-1, and LY of NY-1.
C$ If this is not the case, the next smallest
C$ value which satisfies this requirement is used
C$ internally.
C$ S...........=+1.0, graph positive part of function,
C$ =-1.0, graph negative part of function,
C$ = 0.0, graph both positive and negative parts.
C$ If S = 0.0, the cutoff value Z0 has no effect.
C$ T(4,4)......4-D transformation matrix defining orientation
C$ of the surfaces. The window coordinates of a
C$ point (X,Y,Z,1.0) are (U,V,W,H) = (X,Y,Z,1.0)
C$ T. X, Y, and Z are computed in the range 0..1,
C$ and Z is obtained from the function values by
C$ scaling the range Z1..Z2 onto 0..1. A point
C$ (U,V,W,H) is Visible If U/H, V/H, and W/H lie
C$ in the range 0..1.
C$ SEP.........Separation option (.TRUE. = yes, .FALSE. = no).
C$ When separation is requested, the upper horizon
C$ is elevated from the lower horizon, and the
C$ vertical and horizontal coordinates are
C$ rescaled to maintain correct proportions. The
C$ scale factor is 1-abs(T(2,2)), and best results
C$ are obtained when T(2,2) lies in the
C$ approximate range -0.4..+0.4.
C$ PL2.........2-D pen movement subroutine
C$ (01-FEB-82)