cntrparam

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

Syntax:

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

This command has two functions. First, it sets the values of z for which contours are to be determined. The number of contour levels <n> should be an integral constant expression. <z1>, <z2> ... are real-valued expressions. Second, it controls the appearance of the individual contour lines.

Keywords controlling the smoothness of contour lines:

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--- 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.

Keywords controlling the selection of contour levels:

levels auto--- This is the default. <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).

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 cntrparam levels <n> are ignored.

levels 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 z axis is logarithmic, <increment> will be interpreted as a multiplicative factor, as it is for set ztics, and <end> should not be used.

Keywords controlling the assignment of linetype to contours:

By default the contours are generated in the reverse order specified (unsorted). Thus set cntrparam levels increment 0, 10, 100 will create 11 contours levels starting with 100 and ending with 0. Adding the keyword sorted re-orders the contours by increasing numerical value, which in this case would mean the first contour is drawn at 0.

By default contours are drawn using successive linetypes starting with the next linetype after that used for the corresponding surface. Thus splot x*y lt 5 would use lt 6 for the first contour generated. If hidden3d mode is active then each surface uses two linetypes. In this case using default settings would cause the first contour to use the same linetype as the hidden surface, which is undesirable. This can be avoided in either of two ways. (1) Use set hidden3d offset N to change the linetype used for the hidden surface. A good choice would be offset -1 since that will avoid all the contour linetypes. (2) Use the set cntrparam firstlinetype N option to specify a block of linetypes used for contour lines independent of whatever was used for the surface. This is particularly useful if you want to customize the set of contour linetypes. N <= 0 restores the default.

If the command set cntrparam is given without any arguments specified all options are reset to the default:

      set cntrparam order 4 points 5
      set cntrparam levels auto 5 unsorted
      set cntrparam firstlinetype 0