Re: Contouring in GnuPlot

[<pl...@pi...>]

On Tue, 31 Jul 2007 23:28:01 +0200, Paul <ps...@dr...> wrote:

>> This will almost certainly need to be done outside gnuplot by
>> preprocessing the data. Catmull-Romm spline would probably be a good
>> choice to interpolate your example contour at 175mm
>
> Hello,
>
>   Do you know of a good software application in linux which will do th=
is
> preprocessing?
>
>

Sorry I dont recall your data. If it's regular in x,y intervals you coul=
d  =

possibly try presenting it as a bitmap to gimp.

I've toyed with this idea a couple of times but never done it.

Your z coordinate would be the "colour" as a greyscale. You could then  =

scale up the image using "cubic" which uses catmull-rom algorithm. This =
 =

would effectively interpolate without affecting your existing points as =
 =

long as you use and integer multiple of the number of points.

One word of warning, the splines tend to over shoot if the data has shar=
p  =

changes in direction within the range of it's four anchors. Like any  =

interpolation, it is fiction. You need to insure that the results make  =

sense.


If you cannot do it with gimp you could probably process your data using=
  =

awk . The spline fit is very simple and fast to execute


/* Catmull-Rom spline - not bad
   * basic intro http://www.mvps.org/directx/articles/catmull/
   * This formula will calculate an interpolated point between pt1 and p=
t2
   * dx=3D0 returns pt1; dx=3D1 returns pt2
   */

static inline gdouble
cubic_spline_fit (gdouble dx,
                   gint    pt0,
                   gint    pt1,
                   gint    pt2,
                   gint    pt3)
{

   return (gdouble) ((( ( - pt0 + 3 * pt1 - 3 * pt2 + pt3 ) * dx +
       ( 2 * pt0 - 5 * pt1 + 4 * pt2 - pt3 ) ) * dx +
       ( - pt0 + pt2 ) ) * dx + (pt1 + pt1) ) / 2.0;
}

This is cut from the Gimp GPL source code in /app/paint-funcs/scale-func=
s.c


Please let me know how you get on . I'm curious to know if the technique=
  =

works.

;)






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