CODEWITHSUNDEEP
pythonmatplotlibmplot3d
Eric Kightley
Eric Kightley

Reputation: 190

3D plotting: why are coordinate arguments input as 2D arrays?

The mplot3d function plot_surface requires that the coordinates of the points defining the surface be input as three 2D arrays, X,Y,Z. Here is a working example that demonstrates the construction of these arrays. I express the x,y,z coordinates in spherical coordinates parameterized by the angles u and v. 

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

fig = plt.figure()
ax = fig.add_subplot(111, projection = '3d')

# generate the coordinates on the surface by parameterizing 
u = np.linspace(0, 2 * np.pi, 100)
v = np.linspace(0, 2 * np.pi, 100)
x = np.outer(np.cos(u), np.sin(v))
y = np.outer(np.sin(u), np.sin(v))
z = np.outer(np.ones(np.size(u)), np.cos(v))

ax.plot_surface(x, y, z) 
plt.show()

I believe that the coordinates represented by X,Y,Z are precisely the set of all triplets [X[i,j], Y[i,j], Z[i,j]] indexed over i and j. (1) Is this correct?

Many of the functions I have written to construct the real plot take an argument that is a list of triplets, precisely in the form of the triplet above; i.e. points = [[x1,y1,z1], [x2,y2,z2], ..., [xn, yn, zn]]. I anticipated that this input would be accepted by plot_surface, but it is not. As I result I find myself needing to convert between the two representations fairly frequently, e.g. via

points = zip(*[x.flatten() for x in (X,Y,Z)])

I can do so, but I am worried that I ought to be using the 2D array format for my own functions as well. (2) Is there a reason for the array input format other than it is easy to program the construction of these arrays? For example, is it computationally more efficient?

Upvotes: 3

Views: 958

Answers (1)

Russell Zahniser
Russell Zahniser

Reputation: 16364

The surface plot is probably drawn as a triangular mesh. Passing a 2D array of points makes it clear which points should be included in each triangle (probably [i][j]-[i+1][j]-[i][j+1] and [i][j+1]-[i+1][j]-[i+1][j+1]).

Upvotes: 2

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