Thread: [Matplotlib-users] quiver with angles

Im a bit confused of how I should use the "quiver" plotting function.

My data is 2D, a 512x512 data array, where every entry is an angle
(polarimetry).
I also have an array with the degree of polarization which would be nice to
colour-code in to the arrows.
So:
How do I use my angles to control the direction of the arrows, and how do I
get values to control the colour of the arrows?

Cheers,
Magnus

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n.l.o wrote:
> Im a bit confused of how I should use the "quiver" plotting function.
> 
> My data is 2D, a 512x512 data array, where every entry is an angle
> (polarimetry).
> I also have an array with the degree of polarization which would be nice to
> colour-code in to the arrows.
> So:
> How do I use my angles to control the direction of the arrows, and how do I
> get values to control the colour of the arrows?
> 
> Cheers,
> Magnus
> 

Example with ipython -pylab:

x = arange(4)
y = arange(5)
X, Y = meshgrid(x, y)
u = ones_like(X)
v = zeros_like(X)
c = arange(u.size)  # values mapped to colors
angles = (X * 20 + Y * 20).ravel()
quiver(X, Y, u, v, c, angles=angles)
axis([-1, 4, -1, 5])


The .ravel() of the angles is to get around a bug that I fixed a few 
minutes ago in svn.

Eric

I tried the code you supplied and I didn't get it to work with the *angles*
keyword, I got:
"ValueError: shape mismatch: objects cannot be broadcast to a single shape"
I have matplotlib.__version__ = '0.98.5.2'.

Although after thinking about it for a while I did:

from scipy import *
import matplotlib.pyplot as plt

X,Y = meshgrid(arange(64),arange(64)) # a bit bigger just to see what it
looks like
angles = arange(0,4*pi,4.*pi/X.size) # just creating a set of angles

u = cos(angles)
v = sin(angles)

c = arange(u.size) # the colours!
c.shape = X.shape
plt.quiver(X, Y, u, v, c, pivot='middle',minlength=0.04, width=0.05,
headwidth=1,scale=50)

that is, the angles are converted to "u" and "v" parameters.

Anyway. That is how I got it working.



efiring wrote:
> 
> 
> Example with ipython -pylab:
> 
> x = arange(4)
> y = arange(5)
> X, Y = meshgrid(x, y)
> u = ones_like(X)
> v = zeros_like(X)
> c = arange(u.size)  # values mapped to colors
> angles = (X * 20 + Y * 20).ravel()
> quiver(X, Y, u, v, c, angles=angles)
> axis([-1, 4, -1, 5])
> 
> 
> The .ravel() of the angles is to get around a bug that I fixed a few 
> minutes ago in svn.
> 
> Eric
> 
> 
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