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SUBROUTINE HIDCV (Z0, Z1,ZE,Z2, MX,MY, NX,NY, LX,LY, S,
X SOUTH, EAST, TOP, TILT, INTERP, PL2)
C$ (Corner View)
C$ Make a hidden-line drawing of a surface defined in
C$ cartesian coordinates on a uniform grid. A rather fast
C$ algorithm is used to provide orthographic views from any of
C$ the four corners, with optional tilt of the surface.
C$
C$ Z0......Cutoff value. Only function values, ZE(I,J), above
C$ (S .GT. 0.0) or below (S .LT. 0) Z0 are visible.
C$ ZE......Array containing the surface. ZE(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 array ZE(*,*).
C$ NX,NY...Sections of ZE(*,*) actually used.
C$ LX,LY...Increments in X and Y directions (.GT. 0). Values
C$ of LX and LY larger than 1 produce a coarser mesh
C$ on the drawing without losing the smoothness of the
C$ complete surface. LX should be an integral divisor
C$ of NX-1, and LY of NY-1. If this is not the case,
C$ the next smallest value which satisfies this
C$ requirement is used internally. At present, LX and
C$ LY will be adjusted internally to be of equal size.
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$ SOUTH... .TRUE. - view from southern edge.
C$ .FALSE. - view from northern edge.
C$ EAST.... .TRUE. - view from eastern edge.
C$ .FALSE. - view from western edge.
C$ TOP..... .TRUE. - show top of surface.
C$ .FALSE. - show bottom of surface.
C$ This routine should be called twice, once with each
C$ value of top. This allows changing colors between
C$ calls to help distinguish the sides.
C$ TILT....Angle of tilt in degrees. TILT is positive
C$ counterclockwise looking down the horizontal axis
C$ to the origin. Thus TILT=0.0 corresponds to an
C$ overhead view looking down the positive Z axis
C$ toward the origin, and gives a totally
C$ uninteresting display of the X-Y grid. Negative
C$ TILT angles tip the top part of the rotated surface
C$ away from the observer around the horizontal axis.
C$ TILT=-90.0 corresponds to an edge-on view of the
C$ surface. Recommended values are in the range
C$ -20..-70.
C$ INTERP.. .TRUE. - Interpolate function values using
C$ bivariate surface interpolation.
C$ .FALSE. - Interpolate function values using
C$ simple linear interpolation. This
C$ is the fastest way, and is usually
C$ adequate for grids larger than about
C$ 50 by 50.
C$ PL2.....2-D pen movement subroutine
C$
C$ The algorithm used here is described by
C$
C$ J. Butland, "Surface Drawing Made Simple", Computer Aided
C$ Design, Vol. 11, No. 1 (January, 1979).
C$
C$ It was discovered and implemented several years earlier at
C$ the Bettis Atomic Power Laboratory (pre-1971), and then
C$ improved and rewritten by John Halleck at the University of
C$ Utah, and the SAFRAS program, as it is called, has received
C$ extensive use at the University of Utah Computing Center.
C$ The outline sketch idea implemented in this routine appears
C$ to be original with J. Butland, however. The
C$ Bettis/Halleck version differs from Butland's in that
C$ horizons are computed in left-to-right sweeps, rather than
C$ in vertical strips, which necessitates substantially larger
C$ horizon work areas, but economizes greatly in pen movement.
C$ Butland's algorithm incorporates cubic spline
C$ interpolation, while the Bettis/Halleck version uses linear
C$ interpolation. Provided that a sufficiently dense function
C$ grid is available, the difference in quality is not great.
C$
C$ Because the scan proceeds in strips from front to back, the
C$ Butland algorithm implemented here requires a substantial
C$ amount of pen movement, since the pen must be lifted and
C$ moved from back to front for each stip. It is therefore
C$ not recommended for use on incremental pen plotters, but
C$ will perform quite satisfactorily on CRT and dot-matrix
C$ displays.
C$
C$ (04-FEB-82)