Matplotlib "patch" like function

Question

I am trying to plot a 2D finite element mesh using MATPLOTLIB. I have some issues using MAYAVI, so it is not an option.

I would like to know if MATPLOTLIB has any function like "PATCH" in Matlab, which works as follows:

x = [0.0, 0.5, 1.0, 1.0, 1.0, 0.5, 0.0, 0.0];
y = [0.0, 0.0, 0.0, 0.5, 1.0, 1.0, 1.0, 0.5];
v = [0.0, 0.0, 0.0, 0.5, 1.0, 1.0, 1.0, 0.5];
patch(x, y, v)

And produces the plot shown below. In a nutshell, I need a simple patch plotting function, which means that I don't want to use any special library to do so. (In case someone asks why I need this: I teach undergrad students and I am using Python3.6 for coding.)

ImportanceOfBeingErnest · Accepted Answer · 2018-07-03 22:22:39Z

3

Having as little as 8 points to interpolate between, the resulting plot is not well defined. I.e. a plt.scatter(x,y,c=v) would look like

Using a tricontourf plot like

plt.tricontourf(x,y,v)

would look like

and of course you are free to choose as many levels as you like, e.g. 100

plt.tricontourf(x,y,v,100)

While this looks like the plot in question, I don't think it's well suited for educational purposes, where you would rather teach people about the problem of interpolation and different methods to perform such interpolation.

In that sense, keeping data and plotting seperate is probably a better choice. Hence I would rather do the interpolation first and afterwards display the result. Maybe the Contour plot of irregularly spaced data example is a good starting point here. And an example for the case here could look like

x = [0.0, 0.5, 1.0, 1.0, 1.0, 0.5, 0.0, 0.0];
y = [0.0, 0.0, 0.0, 0.5, 1.0, 1.0, 1.0, 0.5];
v = [0.0, 0.0, 0.0, 0.5, 1.0, 1.0, 1.0, 0.5];

import numpy as np
from scipy.interpolate import griddata
import matplotlib.pyplot as plt

xi = np.linspace(min(x), max(x), 100)
yi = np.linspace(min(y), max(y), 100)

zi = griddata((x, y), v, (xi[None,:], yi[:,None]), method='linear')

plt.pcolormesh(xi,yi,zi)

plt.show()

This uses a pcolormesh plot, which is easier to grasp for displaying the data on the regular grid, as it does not perform any interpolation itself.

answered Jul 3, 2018 at 22:22

ImportanceOfBeingErnest

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10 Comments

Gabs Over a year ago

Can I extend this to a 3D mesh?

ImportanceOfBeingErnest Over a year ago

What's the third dimension then?

Gabs Over a year ago

Would be something like (x,y,z) and the scalar field value, v.

ImportanceOfBeingErnest Over a year ago

Sure, I guess you mean something like this example, just with the interpolated values produces in the same manner as in this answer.

Gabs Over a year ago

No... In your example, "Z" is the "high" of the curve. What I need i something like "triangular_mesh" from MAYAVI.

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