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From: Karl Ratzsch <mail.kfr@gmx.net>
Newsgroups: comp.graphics.apps.gnuplot
Subject: Re: Samples, isosamples, and how they affect contour lines
Date: Fri, 13 Jul 2018 12:39:51 +0200
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Am 13.07.2018 um 09:22 schrieb nawridlz@gmail.com:
> Can we go through a simple example? Consider once again the following case:
> 
>     set samples 6
>     set isosamples 21
>     set table $surface
>     splot x ** 2 + y ** 2
>     unset table
>     set contour
>     set cntrparam levels discrete 80
>     set view map
>     splot $surface
> 
> Now, you wrote:
> 
>> gnuplot takes the grid values in x-direction, and adds an interpolated
>> contour point wherever the connection between these two points crosses
>> the y-grid.

> If I understand correctly, we consider two grid points consecutive in x-direction, for example (-6, -8) and (-2, -8). Now, we add a contour point between them, according to the plot at (-3.5, -8). Why exactly at (-3.5, -8)?

That is where a line parallel to the x-axis at y=-8 and z=80 goes
through that the badly sampled surface of z = x^2 + y2. At least that is
how I understand the contour generation algorithm.