Quad as triangles#
A contour line within a quad is, by default, a straight line between points on two of its edges.
The quad_as_tri
option divides each quad into four triangles using a virtual point at the
centre (mean x, y of the corner points) and extra contour points are inserted where the contour
level intersects the diagonals so that contour lines are piecewise linear within the triangles.
The z
value of the central point is calculated depending on the z interpolation setting; for
the default z_interp=ZInterp.Linear
this is the mean of the z
values of the corner points.
Corner masked triangles are not affected by this setting, only full unmasked quads.
mpl2005 |
mpl2014 |
serial |
threaded |
|
---|---|---|---|---|
supports_quad_as_tri |
Yes |
Yes |
quad_as_tri
is available for the serial
and threaded
algorithms. It is always disabled
by default, so if required it must be explicitly requested:
>>> cont_gen = contour_generator(quad_as_tri=True, ...)
Note
quad_as_tri
produces more detailed contours, but not necessarily smoother ones.
Here is an example of the difference between quad_as_tri=False
and True
for z
values on
a coarse grid that are given by z = np.sqrt(x**2 + y**2)
. On a finer grid the contours would be
semicircular.
from contourpy import contour_generator
from contourpy.util.mpl_renderer import MplRenderer as Renderer
import numpy as np
x, y = np.meshgrid([-2, -1, 0, 1, 2], [0, 1, 2])
z = np.sqrt(x**2 + y**2)
renderer = Renderer(ncols=2, figsize=(8, 2.4))
for ax in range(2):
quad_as_tri = bool(ax)
renderer.grid(x, y, ax=ax, color="gray", alpha=0.2, quad_as_tri_alpha=ax*0.1)
renderer.title(f"quad_as_tri={quad_as_tri}", ax=ax)
cont_gen = contour_generator(x, y, z, quad_as_tri=quad_as_tri)
for i, level in enumerate(np.arange(0.5, 2.51, 0.5)):
lines = cont_gen.lines(level)
renderer.lines(lines, cont_gen.line_type, ax=ax, color=f"C{i}", linewidth=2)
renderer.z_values(x, y, z, ax=ax, quad_as_tri=quad_as_tri, fmt="0.2f")
renderer.show()
Another situation in which quad_as_tri
may be useful is shown below. The quad has three corners
at the same z
level so without quad_as_tri
contour lines cut across diagonally.
from contourpy import contour_generator
from contourpy.util.mpl_renderer import MplRenderer as Renderer
import numpy as np
x = y = [0, 1]
z = [[0, 0], [0, 4]]
renderer = Renderer(ncols=2, figsize=(6, 3))
for ax in range(2):
quad_as_tri = bool(ax)
renderer.grid(x, y, ax=ax, color="gray", alpha=0.2, quad_as_tri_alpha=ax*0.1)
renderer.title(f"quad_as_tri={quad_as_tri}", ax=ax)
cont_gen = contour_generator(x, y, z, quad_as_tri=quad_as_tri)
for i, level in enumerate(np.arange(0.0, 4.0, 0.4)):
lines = cont_gen.lines(level)
renderer.lines(lines, cont_gen.line_type, ax=ax, color=f"C{i}", linewidth=2)
renderer.z_values(x, y, z, ax=ax, quad_as_tri=quad_as_tri)
renderer.show()
Note
quad_as_tri=True
produces contour lines and filled contours typically containing about three
times as many points as quad_as_tri=False
.