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(gnuplot.info)hidden3d

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 The `set hidden3d` command enables hidden line removal for surface plotting
 (see `splot` (Note: splot )).  Some optional features of the underlying
 algorithm can also be controlled using this command.

 Syntax:
       set hidden3d {defaults} |
                    { {{offset <offset>} | {nooffset}}
                      {trianglepattern <bitpattern>}
                      {{undefined <level>} | {noundefined}}
                      {{no}altdiagonal}
                      {{no}bentover} }
       set nohidden3d
       show hidden3d

 In contrast to the usual display in gnuplot, hidden line removal actually
 treats the given function or data grids as real surfaces that can't be seen
 through, so parts behind the surface will be hidden by it.  For this to be
 possible, the surface needs to have 'grid structure' (see `splot datafile`
 (Note: data-file ) about this), and it has to be drawn `with lines` or `with
 linespoints`.

 When `hidden3d` is set, both the hidden portion of the surface and possibly
 its contours drawn on the base (see `set contour` (Note: contour )) as well
 as the grid will be hidden.  Each surface has its hidden parts removed with
 respect to itself and to other surfaces, if more than one surface is
 plotted.  Contours drawn on the surface (`set contour surface`) don't
 work.  Labels and arrows are always visible and are unaffected.  The key is
 also never hidden by the surface.

 Functions are evaluated at isoline intersections.  The algorithm interpolates
 linearly between function points or data points when determining the visible
 line segments.  This means that the appearance of a function may be different
 when plotted with `hidden3d` than when plotted with `nohidden3d` because in
 the latter case functions are evaluated at each sample.  Please see `set
 samples` and `set isosamples` (Note: isosamples ) for discussion of the
 difference.

 The algorithm used to remove the hidden parts of the surfaces has some
 additional features controllable by this command.  Specifying `defaults` will
 set them all to their default settings, as detailed below.  If `defaults` is
 not given, only explicitly specified options will be influenced: all others
 will keep their previous values, so you can turn on/off hidden line removal
 via `set {no}hidden3d`, without modifying the set of options you chose.

 The first option, `offset`, influences the linestyle used for lines on the
 'back' side.  Normally, they are drawn in a linestyle one index number higher
 than the one used for the front, to make the two sides of the surface
 distinguishable.  You can specify a different line style offset to add
 instead of the default 1, by `offset <offset>`.  Option `nooffset` stands for
 `offset 0`, making the two sides of the surface use the same linestyle.

 Next comes the option `trianglepattern <bitpattern>`.  <bitpattern> must be
 a number between 0 and 7, interpreted as a bit pattern.  Each bit determines
 the visibility of one edge of the triangles each surface is split up into.
 Bit 0 is for the 'horizontal' edges of the grid, Bit 1 for the 'vertical'
 ones, and Bit 2 for the diagonals that split each cell of the original grid
 into two triangles.  The default pattern is 3, making all horizontal and
 vertical lines visible, but not the diagonals.  You may want to choose 7 to
 see those diagonals as well.

 The `undefined <level>` option lets you decide what the algorithm is to do
 with data points that are undefined (missing data, or undefined function
 values), or exceed the given x-, y- or z-ranges.  Such points can either be
 plotted nevertheless, or taken out of the input data set.  All surface
 elements touching a point that is taken out will be taken out as well, thus
 creating a hole in the surface.  If <level> = 3, equivalent to option
 `noundefined`, no points will be thrown away at all.  This may produce all
 kinds of problems elsewhere, so you should avoid this.  <level> = 2 will
 throw away undefined points, but keep the out-of-range ones.  <level> = 1,
 the default, will get rid of out-of-range points as well.

 By specifying `noaltdiagonal`, you can override the default handling of a
 special case can occur if `undefined` is active (i.e. <level> is not 3).
 Each cell of the grid-structured input surface will be divided in two
 triangles along one of its diagonals.  Normally, all these diagonals have
 the same orientation relative to the grid.  If exactly one of the four cell
 corners is excluded by the `undefined` handler, and this is on the usual
 diagonal, both triangles will be excluded.  However if the default setting
 of `altdiagonal` is active, the other diagonal will be chosen for this cell
 instead, minimizing the size of the hole in the surface.

 The `bentover` option controls what happens to another special case, this
 time in conjunction with the `trianglepattern`.  For rather crumply surfaces,
 it can happen that the two triangles a surface cell is divided into are seen
 from opposite sides (i.e. the original quadrangle is 'bent over'), as
 illustrated in the following ASCII art:

                                                               C----B
     original quadrangle:  A--B      displayed quadrangle:     |\   |
       ("set view 0,0")    | /|    ("set view 75,75" perhaps)  | \  |
                           |/ |                                |  \ |
                           C--D                                |   \|
                                                               A    D

 If the diagonal edges of the surface cells aren't generally made visible by
 bit 2 of the <bitpattern> there, the edge CB above wouldn't be drawn at all,
 normally, making the resulting display hard to understand.  Therefore, the
 default option of `bentover` will turn it visible in this case.  If you don't
 want that, you may choose `nobentover` instead.