Re: Extract a contour from a 3-D plot.
- To: mathgroup@smc.vnet.net
- Subject: [mg11390] Re: Extract a contour from a 3-D plot.
- From: Allan Hayes <hay@haystack.demon.co.uk>
- Date: Sun, 8 Mar 1998 20:13:13 -0500
- References: <6dj1at$a0m@smc.vnet.net>
Chris Farr wrote:
>
> I would like to extract a contour from a 3-D plot.
>
> I am particularly interested in the contour associated with a value of =
> zero. That is, the curve generated when I set the option in
> ContourPlot = to Contours->{0}.
>
> How can I extract this contour ( I guess the contour would be a =
> collection of XY coordinates) ? What if there are two contours with a
> = value of zero? How can I extract both of them? Thanks in advance.
>
If you intend to use ContourPlot then the first problem is that it
generates a ContourGraphics object which store a matrix of heights,
from which the contour lines are constructed for display - not the
lines themselves. However, we can convert the ContourGraphics object to
a Graphics object which does contain the lines and them use Cases to
extract the lines - all of them:
In[180]:=
ContourPlot[x^2-y^2-1 ,{x,-3,3},{y,-3,3}, Contours ->{0},
ContourShading-> False]
Out[180]=
-ContourGraphics-
In[181]:=
Cases[Graphics[%],Line[_], Infinity]
Out[181]=
{Line[{{-3., 2.82655}, ..., {-3., -2.82655}}], Line[{{3., 2.82655},
..., {3., -2.82655}}]}
Now you can do what you want with the lists of points on the lines.
Allan Hayes
Training and Consulting
Leicester, UK
hay@haystack.demon.co.uk
http://www.haystack.demon.co.uk
voice: +44 (0)116 271 4198
fax: +44 (0)116 271 8642