QuadMesh point in poly by greglucas · Pull Request #22955 · matplotlib/matplotlib

greglucas · 2022-05-02T03:22:41Z

PR Summary

This adds a point-in-quad function to the QuadMesh collection, implemented with Numpy rather than generating all of the paths and passing into Agg's point in poly check.

Change n to check the performance. ~~This still struggles with a 1000 x 1000 mesh for me, but I'm not sure that we can really expect this to scale for generic meshes.~~

import matplotlib.pyplot as plt
import numpy as np
n = 4
x = np.arange(n)
X = x[:, None] * x[None, :]

fig, ax = plt.subplots()
mesh = ax.pcolormesh(X)
mesh.set_mouseover(True)

plt.show()

A concave mesh

import matplotlib.pyplot as plt
import numpy as np

x = [[0, -1], [1, 0]]
y = [[0, 1], [1, -1]]

fig, ax = plt.subplots()
mesh = ax.pcolormesh(x, y, [[0]])
mesh.set_mouseover(True)

plt.show()

PR Checklist

Tests and Styling

Has pytest style unit tests (and pytest passes).
Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

New features are documented, with examples if plot related.
New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
Documentation is sphinx and numpydoc compliant (the docs should build without error).

jklymak · 2022-05-08T18:33:05Z

        self.stale = False

+    def contains(self, event):
+        # docstring inherited


is there a way to fast-path the often-used pcolormesh case of a rectilinear grid (lat and lon are each 1-d)?

Sort-of. I'm not sure it is doable in QuadMesh currently because it is so flexible with the mesh coordinates, so somehow we would need to store some information about what x/y grid was passed in. That case is already handled in PcolorImage though:

matplotlib/lib/matplotlib/image.py

Lines 1318 to 1329 in cb8680f

def get_cursor_data(self, event):

# docstring inherited

x, y = event.xdata, event.ydata

if (x < self._Ax[0] or x > self._Ax[-1] or

y < self._Ay[0] or y > self._Ay[-1]):

return None

j = np.searchsorted(self._Ax, x) - 1

i = np.searchsorted(self._Ay, y) - 1

try:

return self._A[i, j]

except IndexError:

return None

(also note that I still get quite good performance for 400x400 size meshes which is already pretty small boxes for investigating coordinate output)

Thinking about this more generally, I wonder if we could consolidate some of these pcolor-style plots into a more generic Collection that would defer choosing which one of PcolorImage/QuadMesh/PolyCollection to draw until the very end, but leaving all of the options available. For instance, if someone wants shading=gouraud we would choose QuadMesh for the rendering. I think this was sort of the idea behind pcolorfast() (?), but that will return different collections depending on what the input is, which may be somewhat confusing for modifying properties later?

I think in general unifying the three pcolor methods would be great - I am personally not a fan of different ones for different purposes; in general if there is a performance issue, one just rasterizes anyhow and performance is usually more than adequate.

I've added this as an initial condition so we subset quickly to the corners of the quadmesh vertices to see which polys it is in. Then we do a winding check on whatever is left. In this case it would be a single polygon is all that is left to check.

jklymak · 2022-05-24T11:03:29Z

Moved to draft pending rebase. This seems reasonable to me if you say it really helps things. Doesn't particularly help the slowness of the cursor for large pcolormesh to the point where I think the cursor display should be on by default.

greglucas · 2022-05-24T13:39:37Z

I'm not in any rush to get this in and I agree that it doesn't completely help the cursor issue. Thinking about the slow cursor issue, I wonder if it would be helpful to set the mouseover state based on an arbitrary size of the QuadMesh so that the smaller meshes do get it available by default.

if coords.size > 500:
    self.set_mouseover(False)

jklymak · 2022-05-24T13:41:38Z

Interesting idea! I'm not a huge user of this feature, so maybe @anntzer will have a comment....

Check for the bounding boxes of the quad points for an initial fast subsetting of potential candidate polygons, then do a winding number check on the more limited number of candidate polys to see which polygons the point was actually within.

greglucas · 2026-09-15T16:06:57Z

I finally got back around to this and implemented another check to subset the number of polygons we need to check with the more expensive winding check.

Find out which quads are even possible with a simple bounding box check (should subset to much fewer, but allow for diagonal/offset polys to still pass through potentially)
Search the potential candidates with numpy vectorization for winding checks.

With that initial subsetting I can get a 1000x1000 hit check in ~5ms locally.

greglucas added Performance topic: pcolor/pcolormesh labels May 2, 2022

greglucas added this to the v3.6.0 milestone May 2, 2022

greglucas mentioned this pull request May 2, 2022

fix "is" comparison for np.array #22957

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6 tasks

jklymak reviewed May 8, 2022

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jklymak added the status: needs rebase label May 18, 2022

jklymak marked this pull request as draft May 24, 2022 11:02

QuLogic modified the milestones: v3.6.0, v3.7.0 Aug 24, 2022

QuLogic modified the milestones: v3.7.0, v3.8.0 Jan 7, 2023

ksunden modified the milestones: v3.8.0, future releases Aug 9, 2023

anntzer mentioned this pull request Aug 14, 2025

mplcursors with pcolormesh fails anntzer/mplcursors#25

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greglucas force-pushed the quadmesh-point-in-poly branch from 6c22e86 to 2c17cc0 Compare September 15, 2026 16:03

github-actions Bot added status: needs rebase topic: collections and mappables and removed status: needs rebase labels Sep 15, 2026

greglucas marked this pull request as ready for review September 15, 2026 16:03

greglucas removed the status: needs rebase label Sep 15, 2026

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QuadMesh point in poly - #22955

QuadMesh point in poly#22955
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greglucas:quadmesh-point-in-poly

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	def get_cursor_data(self, event):
	# docstring inherited
	x, y = event.xdata, event.ydata
	if (x < self._Ax[0] or x > self._Ax[-1] or
	y < self._Ay[0] or y > self._Ay[-1]):
	return None
	j = np.searchsorted(self._Ax, x) - 1
	i = np.searchsorted(self._Ay, y) - 1
	try:
	return self._A[i, j]
	except IndexError:
	return None

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greglucas commented May 2, 2022 •

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