Re: [matplotlib-devel] trisurf plots with independent color data

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I’ve worked out how to create a Poly3DCollection as noted by Maximilian Albert and how to use the set_array method on the collection created by plot_trisurf as noted by Geoffroy Billotey. I have two follow up questions:

1)      Would it be possible to make it so that if the color parameter of plot_trisurf is an array of length n_triangles, that plot_trisurf would internally use set_array to color the triangles with the colors in the color parameter?

2)      Is there a way to color the triangles with gradients? I have an intensity value at each vertex of the triangles so Matlab/octave colors each triangle with a gradient whereas right now I’m taking an average of the values at the vertices and assigning the resulting value to the triangle.

Byron Boulton

From: Geoffroy Billotey [mailto:geo...@gm...]
Sent: Wednesday, January 21, 2015 12:33 PM
To: mat...@li...; Byron K. Boulton
Subject: Re: [matplotlib-devel] trisurf plots with independent color data

Hi,

You can also try to pass a *color* array of size ntri (number of triangles) to the *set_array* method of the collection returned by *plot_trisurf*.

See for instance:
http://stackoverflow.com/questions/24218543/colouring-the-surface-of-a-sphere-with-a-set-of-scalar-values-in-matplotlib/24229480#24229480<http://cp.mcafee.com/d/1jWVIq40USyMCyYUy-qenztPqdQPhOCepshhdETjd7aoVBNOVJ6WpEVj7cK8FCQrFCzBcsOUeKr85i5mPVRw2y7NzbOszP-ndAfz6nAV7DYKrggjZQwM_R-puK-_RXBQSkSu7fcEKsJteOaqJTCel3PWApmU6CQjrVK_9IInojvvpjdTdw0WjSNmFDUKDRcsLwITZ9OH2C9I9ELfIcVqFZmUOwhbP9-nET5TrunGWnQLhoDYvbC9j9BZbQ-puHFuBJfS9sPFXvQbOwnlSqNZpDO8oCMCCqem7HwoCMCCqem7HCOfRttzkJN2hVZp-m1h4jVsSyMMCMr1vF6y0QJGXdo-IPV42W7CvfdbFEwD6l6UKCy1o-IW6y0NapcQgiwxWTMg-4Ph1qFZoCy13pWrsodTc9QClIZK1f8>

2015-01-21 17:02 GMT+01:00 Maximilian Albert <max...@gm...<mailto:max...@gm...>>:
Hi Byron,

This is a bit of a workaround, but you can specify facecolors explicitly by creating a triangulation of your surface explicitly and creating a Poly3DCollection with these facecolors. I'm attaching an example below which is a modified version of the plot_trisurf demo [1] in the matplotlib documentation. It showcases both random colors and a smooth gradient (the latter in the line that's commented out).

I would have thought that it should be possible to pass an argument like "facecolors" to plot_trisurf directly, since the documentation [2] states that "other arguments are passed on to Poly3DCollection". However, I couldn't get this to work quickly. Maybe someone else knows how?

Best regards,
Max

[1] http://matplotlib.org/examples/mplot3d/trisurf3d_demo.html<http://cp.mcafee.com/d/k-Kr6x0SyMCyYUy-qenztPqdQPhOCepshhdETjd7aoVBNOVJ6WpEVj7cK8FCQrFCzBcsOUeKr85i5mPVRw2y7NzbOszP-ndAfz6nAV7DYKrggjZQwM_R-puK-_RXBQSkSu7fcEKsJteOaqJTCel3PWApmU6CQPrVK_9IInojvvpjdTdw0RtCIvmpYy700sYf21_QNlo-ISkcD6l6UKIE2G7GRU76OfRttzkJN2hVZp-m1h4jVsSyMMCMr1vF6y0QJGXdo-IPV42W7CvfdbFEwD6l6UKCy1o-IW6y0NapcQgiwxWTMg-4Ph1qFZoCy13pWrsodCuBsX5L->
[2] http://matplotlib.org/mpl_toolkits/mplot3d/api.html<http://cp.mcafee.com/d/1jWVIq6h8gdEI9ELe8LCzBUTsSztcQsFzCn4kjqdQPhOCepssKrhKCqekNPbyapJ6WpEVj7cK3HCO1kxlI-to0ExYoOYD8Y_BPp3UNBVehV_bCQ44_t8cfZvCnHLLZuVtdBdDxPPabDbnjIyCHtVzBgY-F6lK1FJAS-rLOrb5S4TTSkPtPo0dnpH7RCv8xM075lzNmtqDBpjFyGNZpIEpGQK1NIzZnnoRbsgAuvmvBwkh4-ndEIc9I6MnWhEwdbqKPmfHc-h0KxVDPPiWq89NBhKbFEwmfHexEwciCjd44E8uJY4fxcQgmGvm9EwgSuCT63oeMA92eD>

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

from matplotlib.tri import Triangulation
from mpl_toolkits.mplot3d.art3d import Poly3DCollection

n_angles = 36
n_radii = 8

# An array of radii
# Does not include radius r=0, this is to eliminate duplicate points
radii = np.linspace(0.125, 1.0, n_radii)

# An array of angles
angles = np.linspace(0, 2*np.pi, n_angles, endpoint=False)

# Repeat all angles for each radius
angles = np.repeat(angles[...,np.newaxis], n_radii, axis=1)

# Convert polar (radii, angles) coords to cartesian (x, y) coords
# (0, 0) is added here. There are no duplicate points in the (x, y) plane
x = np.append(0, (radii*np.cos(angles)).flatten())
y = np.append(0, (radii*np.sin(angles)).flatten())

# Pringle surface
z = np.sin(-x*y)

tri = Triangulation(x, y)  # NOTE: This assumes that there is a nice projection of the surface into the x/y-plane!
triangle_vertices = np.array([np.array([[x[T[0]], y[T[0]], z[T[0]]],
                                        [x[T[1]], y[T[1]], z[T[1]]],
                                        [x[T[2]], y[T[2]], z[T[2]]]]) for T in tri.triangles])
midpoints = np.average(triangle_vertices, axis=1)

def find_color_for_point(pt):
    x, y, z = pt
    col = [(y+1)/2, (1-y)/2, 0]
    return col

#facecolors = [find_color_for_point(pt) for pt in midpoints]  # smooth gradient
facecolors = [np.random.random(3) for pt in midpoints]  # random colors

coll = Poly3DCollection(triangle_vertices, facecolors=facecolors, edgecolors='black')

fig = plt.figure()
ax = fig.gca(projection='3d')
ax.add_collection(coll)
ax.set_xlim(-1, 1)
ax.set_ylim(-1, 1)
ax.set_zlim(-1, 1)
ax.elev = 50

plt.show()

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