Thread: Re: [Matplotlib-users] plotting a contour from dicrete data

Viraj and Jeff -

Maybe one extension of Jeff's answer.
The process works as long as x, y, and z are 2D arrays of the same size and
shape.
Hence, x and y don't have to form a rectangular grid.
I have used this feature regularly for conformal mapping.
And it makes a lot of sense.
The contour routine simply looks for intersections between x and y values.
Then when it plots it uses the x and y values in the arrays.
So when those are not a rectangular grid, it doesn't care.
It's a cool feature.
I can give an example if you want,

Mark



Viraj Vajratkar wrote:
> > hey guys... i got it... u can use contour(x,y,z)... as in
> > x=load('urfile1.dat'), y=load('urfile2.dat), z=load('urfile3.dat
> > ').... and then type out the above.... for details about the
> > parameters x,y,z see... .
> > http://www.scilab.org/product/man-eng/graphics/contour.htm .... so
> > matplotlib CAN plot a contour from discrete points!!!.... ive tried it
> > and it works...
> Viraj:  That only works because x and y describe a rectangular grid.  If
> x and y described irregularly spaced points, you would need to grid the
> data first using one of the methods described on that Cookbook page.
>
> -Jeff
>

I thought it was cool the first time I saw it.
Just try something simple

from pylab import *
x,y = meshgrid(linspace(-5,5,101),linspace(0,5,101))
h = y
z = x + complex(0,1)*y
znew = z**0.25  # Doing a simple conformal map
xnew = znew.real
ynew = znew.imag
contourf(xnew,ynew,h,linspace(0,5,10))
axis('scaled')

And you get nice contours in a pieslice-shaped domain with an angle of 45
degrees

Mark

From: "Scott Sinclair" <sin...@uk...>
>
> That is very cool, I hadn't thought of it!
>
> So what you're saying is that any transformation (a complex distortion) of
> a regular rectangular grid is fine. The fact that the grid's 'pixels' are
> four sided quadrilaterals satisfies this condition and the contour algorithm
> works...
>
> Cheers,
> Scott
>
> >>> "Mark Bakker" <ma...@gm...> 7/11/2007 11:36 >>>
> Viraj and Jeff -
>
> Maybe one extension of Jeff's answer.
> The process works as long as x, y, and z are 2D arrays of the same size
> and shape.
> Hence, x and y don't have to form a rectangular grid.
> I have used this feature regularly for conformal mapping.
> And it makes a lot of sense.
> The contour routine simply looks for intersections between x and y values.
> Then when it plots it uses the x and y values in the arrays.
> So when those are not a rectangular grid, it doesn't care.
> It's a cool feature.
> I can give an example if you want,
>
> Mark
>

That is very cool, I hadn't thought of it!
=20
So what you're saying is that any transformation (a complex distortion) of =
a regular rectangular grid is fine. The fact that the grid's 'pixels' are =
four sided quadrilaterals satisfies this condition and the contour =
algorithm works...
=20
Cheers,
Scott

>>> "Mark Bakker" <ma...@gm...> 7/11/2007 11:36 >>>
Viraj and Jeff -

Maybe one extension of Jeff's answer.
The process works as long as x, y, and z are 2D arrays of the same size =
and shape.
Hence, x and y don't have to form a rectangular grid.=20
I have used this feature regularly for conformal mapping.=20
And it makes a lot of sense.=20
The contour routine simply looks for intersections between x and y values.
Then when it plots it uses the x and y values in the arrays.=20
So when those are not a rectangular grid, it doesn't care.
It's a cool feature.
I can give an example if you want,

Mark

=20



Viraj Vajratkar wrote:
> hey guys... i got it... u can use contour(x,y,z)... as in
> x=3Dload('urfile1.dat'), y=3Dload('urfile2.dat), z=3Dload('urfile3.dat
> ').... and then type out the above.... for details about the=20
> parameters x,y,z see... .
> http://www.scilab.org/product/man-eng/graphics/contour.htm .... so
> matplotlib CAN plot a contour from discrete points!!!.... ive tried =
it=20
> and it works...
Viraj:  That only works because x and y describe a rectangular grid.  If
x and y described irregularly spaced points, you would need to grid the
data first using one of the methods described on that Cookbook page.=20

-Jeff



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Mark Bakker wrote:
> Viraj and Jeff -
>
> Maybe one extension of Jeff's answer.
> The process works as long as x, y, and z are 2D arrays of the same 
> size and shape.
> Hence, x and y don't have to form a rectangular grid.
> I have used this feature regularly for conformal mapping.
> And it makes a lot of sense.
> The contour routine simply looks for intersections between x and y values.
> Then when it plots it uses the x and y values in the arrays.
> So when those are not a rectangular grid, it doesn't care.
> It's a cool feature.
> I can give an example if you want,
>
> Mark
>
>  
>
>     Viraj Vajratkar wrote:
>     > hey guys... i got it... u can use contour(x,y,z)... as in
>     > x=load('urfile1.dat'), y=load('urfile2.dat), z=load('urfile3.dat
>     > ').... and then type out the above.... for details about the
>     > parameters x,y,z see... .
>     > http://www.scilab.org/product/man-eng/graphics/contour.htm .... so
>     > matplotlib CAN plot a contour from discrete points!!!.... ive
>     tried it
>     > and it works...
>     Viraj:  That only works because x and y describe a rectangular
>     grid.  If
>     x and y described irregularly spaced points, you would need to
>     grid the
>     data first using one of the methods described on that Cookbook page.
>
>     -Jeff
>
>
Mark:  That is cool - didn't know it could do that.  So I guess the 
proper answer is contour requires x and y to describe a *regular*, but 
not recessarily rectilinear, grid.  I should have known, since there is 
an example in basemap (ccsm_popgrid.py) that illustrates this.

-Jeff

-- 
Jeffrey S. Whitaker         Phone : (303)497-6313
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