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pylab_examples example code: tricontour_smooth_delaunay.pyΒΆ

(Source code, png, hires.png, pdf)

Demonstrates high-resolution tricontouring of a random set of points ;
a matplotlib.tri.TriAnalyzer is used to improve the plot quality.

The initial data points and triangular grid for this demo are:
    - a set of random points is instantiated, inside [-1, 1] x [-1, 1] square
    - A Delaunay triangulation of these points is then computed, of which a
    random subset of triangles is masked out by the user (based on
    *init_mask_frac* parameter). This simulates invalidated data.

The proposed generic procedure to obtain a high resolution contouring of such
a data set is the following:
    1) Compute an extended mask with a matplotlib.tri.TriAnalyzer, which will
    exclude badly shaped (flat) triangles from the border of the
    triangulation. Apply the mask to the triangulation (using set_mask).
    2) Refine and interpolate the data using a
    3) Plot the refined data with tricontour.

from matplotlib.tri import Triangulation, TriAnalyzer, UniformTriRefiner
import matplotlib.pyplot as plt
import as cm
import numpy as np

# Analytical test function
def experiment_res(x, y):
    """ An analytic function representing experiment results """
    x = 2.*x
    r1 = np.sqrt((0.5-x)**2 + (0.5-y)**2)
    theta1 = np.arctan2(0.5-x, 0.5-y)
    r2 = np.sqrt((-x-0.2)**2 + (-y-0.2)**2)
    theta2 = np.arctan2(-x-0.2, -y-0.2)
    z = (4*(np.exp((r1/10)**2)-1)*30. * np.cos(3*theta1) +
         (np.exp((r2/10)**2)-1)*30. * np.cos(5*theta2) +
         2*(x**2 + y**2))
    return (np.max(z)-z)/(np.max(z)-np.min(z))

# Generating the initial data test points and triangulation for the demo
# User parameters for data test points
n_test = 200  # Number of test data points, tested from 3 to 5000 for subdiv=3

subdiv = 3  # Number of recursive subdivisions of the initial mesh for smooth
            # plots. Values >3 might result in a very high number of triangles
            # for the refine mesh: new triangles numbering = (4**subdiv)*ntri

init_mask_frac = 0.0    # Float > 0. adjusting the proportion of
                        # (invalid) initial triangles which will be masked
                        # out. Enter 0 for no mask.

min_circle_ratio = .01  # Minimum circle ratio - border triangles with circle
                        # ratio below this will be masked if they touch a
                        # border. Suggested value 0.01 ; Use -1 to keep
                        # all triangles.

# Random points
random_gen = np.random.mtrand.RandomState(seed=127260)
x_test = random_gen.uniform(-1., 1., size=n_test)
y_test = random_gen.uniform(-1., 1., size=n_test)
z_test = experiment_res(x_test, y_test)

# meshing with Delaunay triangulation
tri = Triangulation(x_test, y_test)
ntri = tri.triangles.shape[0]

# Some invalid data are masked out
mask_init = np.zeros(ntri, dtype=np.bool)
masked_tri = random_gen.randint(0, ntri, int(ntri*init_mask_frac))
mask_init[masked_tri] = True

# Improving the triangulation before high-res plots: removing flat triangles
# masking badly shaped triangles at the border of the triangular mesh.
mask = TriAnalyzer(tri).get_flat_tri_mask(min_circle_ratio)

# refining the data
refiner = UniformTriRefiner(tri)
tri_refi, z_test_refi = refiner.refine_field(z_test, subdiv=subdiv)

# analytical 'results' for comparison
z_expected = experiment_res(tri_refi.x, tri_refi.y)

# for the demo: loading the 'flat' triangles for plot
flat_tri = Triangulation(x_test, y_test)

# Now the plots
# User options for plots
plot_tri = True          # plot of base triangulation
plot_masked_tri = True   # plot of excessively flat excluded triangles
plot_refi_tri = False    # plot of refined triangulation
plot_expected = False    # plot of analytical function values for comparison

# Graphical options for tricontouring
levels = np.arange(0., 1., 0.025)
cmap = cm.get_cmap(name='Blues', lut=None)

plt.title("Filtering a Delaunay mesh\n" +
          "(application to high-resolution tricontouring)")

# 1) plot of the refined (computed) data countours:
plt.tricontour(tri_refi, z_test_refi, levels=levels, cmap=cmap,
               linewidths=[2.0, 0.5, 1.0, 0.5])
# 2) plot of the expected (analytical) data countours (dashed):
if plot_expected:
    plt.tricontour(tri_refi, z_expected, levels=levels, cmap=cmap,
# 3) plot of the fine mesh on which interpolation was done:
if plot_refi_tri:
    plt.triplot(tri_refi, color='0.97')
# 4) plot of the initial 'coarse' mesh:
if plot_tri:
    plt.triplot(tri, color='0.7')
# 4) plot of the unvalidated triangles from naive Delaunay Triangulation:
if plot_masked_tri:
    plt.triplot(flat_tri, color='red')

Keywords: python, matplotlib, pylab, example, codex (see Search examples)