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SUBROUTINE HZNSK (Z0, Z1,ZE,Z2, MX,MY, NX,NY, LX,LY, S,
X SOUTH, EAST, TOP, TILT, PL2)
C$ (Corner View - Outline Sketch)
C$ Make a hidden-line outline sketch 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),
C$ above (S .GT. 0.0) or below (S .LT. 0) Z0 are
C$ 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).
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. At present, LX and LY will be
C$ 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
C$ each value of top. This allows changing colors
C$ between calls to help distinguish the sides.
C$ TILT........Angle of tilt in degrees. Tilt is positive
C$ counterclockwise looking down the horizontal
C$ axis to the origin. Thus TILT=0.0 corresponds
C$ to an overhead view looking down the positive Z
C$ axis toward the origin, and gives a totally
C$ uninteresting display of the X-Y grid.
C$ Negative tilt angles tip the top part of the
C$ rotated surface away from the observer around
C$ the horizontal axis. TILT=-90.0 corresponds to
C$ an edge-on view of the surface. Recommended
C$ values are in the range -20..-70.
C$ PL2.........2-D pen movement subroutine, usually PL2CA
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$ (01-FEB-82)