streamplot: vectors not pointing in the right direction!

Community matplotlib-users
1

Hi,

I have problems with streamplot

I want to use a 3d vector field in coordinates (x,y,z) stored in a numpy array, and plot slices of it with streamplot.

To test it I wanted to use a vector field with arrows pointed up in the z>0 region and pointed down in the z<0 region.

import numpy as np

import matplotlib.pyplot as plt

from math import * 



max = 100

min = -100





X = np.linspace(min, max, num=100)

Y = np.linspace(min, max, num=100)

Z = np.linspace(min, max, num=100)



N = X.size



#single components in the 3D matrix

Bxa = np.zeros((N, N, N))

Bya = np.zeros((N, N, N))

Bza = np.zeros((N, N, N))





for i, x in enumerate(X):

	for j, y in enumerate(Y):

		for k, z in enumerate(Z):

			Bxa[ i, j, k] = 0.0 #x

			Bya[ i, j, k] = 0.0 #y

			Bza[ i, j, k] = z



#I take a slice close to Y=0

Bx_sec = Bxa[:,N/2,:]

By_sec = Bya[:,N/2,:]

Bz_sec = Bza[:,N/2,:]



fig = plt.figure()

ax = fig.add_subplot(111)

ax.streamplot(X, Z, Bx_sec, Bz_sec, color='b')

ax.set_xlim([X.min(), X.max()])

ax.set_ylim([Z.min(), Z.max()])



plt.show()

But I obtain something that looks like if I have put Bza = x! I tried to invert the order of vectors but it is unuseful!

I attach the picture. Do you understand why? (the code I posted should run)

Gabriele

figure_1.png

2

The documentation for streamplot:

        *x*, *y* : 1d arrays
            an *evenly spaced* grid.
        *u*, *v* : 2d arrays
            x and y-velocities. Number of rows should match length of y, and
            the number of columns should match x.

Note that the rows in u and v should match y, and the columns should match x. I think your u and v are transposed.

Cheers!

Ben Root

···

On Fri, May 22, 2015 at 2:50 AM, Gabriele Brambilla <gb.gabrielebrambilla@…2015…87…> wrote:

Hi,

I have problems with streamplot

I want to use a 3d vector field in coordinates (x,y,z) stored in a numpy array, and plot slices of it with streamplot.

To test it I wanted to use a vector field with arrows pointed up in the z>0 region and pointed down in the z<0 region.

import numpy as np

import matplotlib.pyplot as plt

from math import * 

max = 100

min = -100

X = np.linspace(min, max, num=100)

Y = np.linspace(min, max, num=100)

Z = np.linspace(min, max, num=100)

N = X.size

#single components in the 3D matrix

Bxa = np.zeros((N, N, N))

Bya = np.zeros((N, N, N))

Bza = np.zeros((N, N, N))

for i, x in enumerate(X):

	for j, y in enumerate(Y):

		for k, z in enumerate(Z):

			Bxa[ i, j, k] = 0.0 #x

			Bya[ i, j, k] = 0.0 #y

			Bza[ i, j, k] = z

#I take a slice close to Y=0

Bx_sec = Bxa[:,N/2,:]

By_sec = Bya[:,N/2,:]

Bz_sec = Bza[:,N/2,:]

fig = plt.figure()

ax = fig.add_subplot(111)

ax.streamplot(X, Z, Bx_sec, Bz_sec, color='b')

ax.set_xlim([X.min(), X.max()])

ax.set_ylim([Z.min(), Z.max()])

plt.show()

But I obtain something that looks like if I have put Bza = x! I tried to invert the order of vectors but it is unuseful!

I attach the picture. Do you understand why? (the code I posted should run)

Gabriele

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3

Thanks! you are right!
I didn’t notice this detail

Gabriele

···

On Fri, May 22, 2015 at 3:58 PM, Benjamin Root <ben.root@…1304…> wrote:

The documentation for streamplot:

        *x*, *y* : 1d arrays
            an *evenly spaced* grid.
        *u*, *v* : 2d arrays
            x and y-velocities. Number of rows should match length of y, and
            the number of columns should match x.

Note that the rows in u and v should match y, and the columns should match x. I think your u and v are transposed.

Cheers!

Ben Root

On Fri, May 22, 2015 at 2:50 AM, Gabriele Brambilla <gb.gabrielebrambilla@…287…> wrote:

Hi,

I have problems with streamplot

I want to use a 3d vector field in coordinates (x,y,z) stored in a numpy array, and plot slices of it with streamplot.

To test it I wanted to use a vector field with arrows pointed up in the z>0 region and pointed down in the z<0 region.

import numpy as np

import matplotlib.pyplot as plt

from math import * 

max = 100

min = -100

X = np.linspace(min, max, num=100)

Y = np.linspace(min, max, num=100)

Z = np.linspace(min, max, num=100)

N = X.size

#single components in the 3D matrix

Bxa = np.zeros((N, N, N))

Bya = np.zeros((N, N, N))

Bza = np.zeros((N, N, N))

for i, x in enumerate(X):

	for j, y in enumerate(Y):

		for k, z in enumerate(Z):

			Bxa[ i, j, k] = 0.0 #x

			Bya[ i, j, k] = 0.0 #y

			Bza[ i, j, k] = z

#I take a slice close to Y=0

Bx_sec = Bxa[:,N/2,:]

By_sec = Bya[:,N/2,:]

Bz_sec = Bza[:,N/2,:]

fig = plt.figure()

ax = fig.add_subplot(111)

ax.streamplot(X, Z, Bx_sec, Bz_sec, color='b')

ax.set_xlim([X.min(), X.max()])

ax.set_ylim([Z.min(), Z.max()])

plt.show()

But I obtain something that looks like if I have put Bza = x! I tried to invert the order of vectors but it is unuseful!

I attach the picture. Do you understand why? (the code I posted should run)

Gabriele

One dashboard for servers and applications across Physical-Virtual-Cloud

Widest out-of-the-box monitoring support with 50+ applications

Performance metrics, stats and reports that give you Actionable Insights

Deep dive visibility with transaction tracing using APM Insight.

http://ad.doubleclick.net/ddm/clk/290420510;117567292;y

Matplotlib-users mailing list

Matplotlib-users@lists.sourceforge.net

https://lists.sourceforge.net/lists/listinfo/matplotlib-users