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MathGroup Archive 2007

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Re: Contour Lines

  • To: mathgroup at smc.vnet.net
  • Subject: [mg83609] Re: Contour Lines
  • From: "David Park" <djmpark at comcast.net>
  • Date: Sat, 24 Nov 2007 04:17:26 -0500 (EST)
  • References: <fi6b6f$q2e$1@smc.vnet.net>

Let's assume that you are working with Version 6.

f[x_, y_] := x^2 + 2 y^2
plot1 = ContourPlot[f[x, y], {x, -2, 2}, {y, -2, 2}, Contours -> {1},
  ContourShading -> False, PlotPoints -> {30, 30}]

We have to apply Normal to the graphics primitives to obtain the line with 
the actual coordinates in it.

contourline =
  First[Cases[First[plot1] // Normal,
    Line[pts_] -> pts, \[Infinity]]];

We can check it with:

ListPlot[contourline, Joined -> True]


-- 
David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/


"Wyn Evans" <nwe at ast.cam.ac.uk> wrote in message 
news:fi6b6f$q2e$1 at smc.vnet.net...
>
> How do you extract the coordinates of points making up a Contour Line as
> a List?
>
> This has been dealt with before in the archive (see e.g., )
> but the code there seems no longer to work. For example, the following is
> suggested
>
> f[x_, y_] := x^2 + 2y^2
> plot1 = ContourPlot[f[x, y], {x, -2, 2}, {y, -2, 2}, Contours -> {1},
>      ContourShading -> False, PlotPoints -> {30, 30}];
> f[x_, y_] := x^2 + 2y^2
> plot1 = ContourPlot[f[x, y], {x, -2, 2}, {y, -2, 2}, Contours -> {1},
>      ContourShading -> False, PlotPoints -> {30, 30}];
>
> Convert the ContourGraphics to Graphics and then extract the first part
> and you will see how Mathematica represents the contours.
>
> contour = First@Graphics@plot1
> (output omitted)
>
>
> I can see that the data I want (the actual coordinates) is there, but I
> can't figure how to extract it as a matrix or list.
>
>
> 



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