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(gnuplot.info)cntrparam

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 `set cntrparam` controls the generation of contours and their smoothness for
 a contour plot. `show contour` (Note: contour ) displays current settings of
 cntrparam as well as `contour`.

 Syntax:
       set cntrparam {  {linear | cubicspline | bspline}
                       { points <n>} { order <n> }
                       { levels   auto {<n>} | <n>
                                | discrete <z1> {,<z2>{,<z3>...}}
                                | incremental <start>, <incr> {,<end>}
                        }
                      }
       show contour

 This command has two functions.  First, it sets the values of z for which
 contour points are to be determined (by linear interpolation between data
 points or function isosamples.)  Second, it controls the way contours are
 drawn between the points determined to be of equal z.  <n> should be an
 integral constant expression and <z1>, <z2> ... any constant expressions.
 The parameters are:

 `linear`, `cubicspline`, `bspline`---Controls type of approximation or
 interpolation.  If `linear`, then straight line segments connect points of
 equal z magnitude.  If `cubicspline`, then piecewise-linear contours are
 interpolated between the same equal z points to form somewhat smoother
 contours, but which may undulate.  If `bspline`, a guaranteed-smoother curve
 is drawn, which only approximates the position of the points of equal-z.

 `points` (Note: points )---Eventually all drawings are done with
 piecewise-linear strokes. This number controls the number of line segments
 used to approximate the `bspline` or `cubicspline` curve.  Number of
 cubicspline or bspline segments (strokes) = `points` * number of linear
 segments.

 `order`---Order of the bspline approximation to be used.  The bigger this
 order is, the smoother the resulting contour.  (Of course, higher order
 bspline curves will move further away from the original piecewise linear
 data.)  This option is relevant for `bspline` mode only.  Allowed values are
 integers in the range from 2 (linear) to 10.

 `levels`--- Selection of contour levels,  controlled by `auto` (default),
 `discrete`, `incremental`, and <n>, number of contour levels, limited to
  MAX_DISCRETE_LEVELS as defined in plot.h (30 is standard.)

 For `auto`, <n> specifies a nominal number of levels; the actual number will
 be adjusted to give simple labels. If the surface is bounded by zmin and zmax,
 contours will be generated at integer multiples of dz between zmin and zmax,
 where dz is 1, 2, or 5 times some power of ten (like the step between two
 tic marks).

 For `levels discrete`, contours will be generated at z = <z1>, <z2> ... as
 specified; the number of discrete levels sets the number of contour levels.
 In `discrete` mode, any `set cntrparms levels <n>` are ignored.

 For `incremental`, contours are generated at values of z beginning at <start>
 and increasing by <increment>, until the number of contours is reached. <end>
 is used to determine the number of contour levels, which will be changed by
 any subsequent `set cntrparam levels <n>`.

 If the command `set cntrparam` is given without any arguments specified,  the
 defaults are used: linear, 5 points, order 4, 5 auto levels.

 Examples:
       set cntrparam bspline
       set cntrparam points 7
       set cntrparam order 10

 To select levels automatically, 5 if the level increment criteria are met:
       set cntrparam levels auto 5

 To specify discrete levels at .1, .37, and .9:
       set cntrparam levels discrete .1,1/exp(1),.9

 To specify levels from 0 to 4 with increment 1:
       set cntrparam levels incremental  0,1,4

 To set the number of levels to 10 (changing an incremental end or possibly
 the number of auto levels):
       set cntrparam levels 10

 To set the start and increment while retaining the number of levels:
       set cntrparam levels incremental 100,50

 See also `set contour` for control of where the contours are drawn, and `set
 clabel` for control of the format of the contour labels and linetypes.