index in pick_event invalid and inconsistent for mplot3d scatter plot (z ordering?)

Bug report

Bug summary

The index returned in the pick_event (event.ind) does not correspond to the index of the source data.
It also changes depending on the view onto the data.

The example contains two points.
When you click on a point, the event.ind is printed out.
The one at (0,0,0) should have index 0, the other index 1.

Rotate the plot so that the second point is further behind (semi transparent). Now it has index 0.
Rotate the plot so that the second point is further in front (full opacity). Now it has index 1.

Code for reproduction

`

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


if __name__ == "__main__":
    fig = plt.figure(constrained_layout=True, figsize=(6,6))

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

    num_points = 2
    data = np.zeros([num_points,3])
    data[:,0] = np.linspace(1,0,num_points)
    print(data)

    path_collection = ax.scatter(xs=data[:,0],
                                 ys=data[:,1],
                                 zs=data[:,2],
                                 picker=True)

    ax.set_xlabel("x")
    ax.set_ylabel("y")
    ax.set_zlabel("z")
    ax.set_xlim(-1,1)
    ax.set_ylim(-1,1)
    ax.set_zlim(-1,1)

    def onpick(event):
        print(event.ind)

    fig.canvas.mpl_connect('pick_event', onpick)

    plt.show()`

Actual outcome

The example contains two points.
When you click on a point, the event.ind is printed out.
The one at (0,0,0) should have index 0, the other index 1.

Rotate the plot so that the second point is further behind (semi transparent). Now it has index 0.
Rotate the plot so that the second point is further in front (full opacity). Now it has index 1.

Expected outcome

The returned event.ind should not depend on view.
For the point in the centre it should always return 0, for the point to the right, it should always return 1.

Matplotlib version

Operating system: macOS Catalina; 10.15.4
Matplotlib version (import matplotlib; print(matplotlib.__version__)): '3.3.4'
Matplotlib backend (print(matplotlib.get_backend())): 'MacOSX'
Python version: Python 3.6.9
Jupyter version (if applicable):
Other libraries:

The axes reverse directions on the selection depending on which points are in front when you rotate the graph. Using matplotlib 3.4.1

#!/usr/bin/env python3

from mpl_toolkits.mplot3d import proj3d
import matplotlib.pyplot as plt

import matplotlib
print('matplotlib: {}'.format(matplotlib.__version__))

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

x = [0, 2, 0,0]
y = [0, 2, 0,2]
z = [0, 2, 2,0]

scatter = ax.scatter(x,y,z,picker=True)

def chaos_onclick(event):
    point_index = int(event.ind)
    print(point_index)

    print("X=",x[point_index], " Y=",y[point_index], " Z=",z[point_index], " PointIdx=", point_index)

fig.canvas.mpl_connect('pick_event', chaos_onclick)
plt.show()

Anyone got a clue where the code is that populates the event.ind? I took linear algebra and I write python code all the time, but finding my way around this project, where should I start?

That's determined in Collection.contains; it's likely that there needs to be an override in Path3DCollection to undo the effect of depth sorting.

I'll take a look. meanwhile this is a work around.

#!/usr/bin/env python3

# much from https://stackoverflow.com/questions/58218482/annotate-3d-scatter-plot-on-pick-event/66926265#66926265

from mpl_toolkits.mplot3d import proj3d
import matplotlib.pyplot as plt
import numpy as np

import matplotlib
print('matplotlib: {}'.format(matplotlib.__version__))

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

x = [0, 2, 0,0]
y = [0, 2, 0,2]
z = [0, 2, 2,0]

scatter = ax.scatter(x,y,z,picker=True)

ax.set_xlabel('X axis')
ax.set_ylabel('Y axis')
ax.set_zlabel('Z axis')


def onMouseMotion(event):
  print(event)

def chaos_onclick(event):

    print(dir(event.mouseevent))
    xx=event.mouseevent.x
    yy=event.mouseevent.y
    
    #magic from https://stackoverflow.com/questions/10374930/matplotlib-annotating-a-3d-scatter-plot
    x2, y2, z2=proj3d.proj_transform(x[0], y[0], z[0], plt.gca().get_proj())
    x3, y3 = ax.transData.transform((x2, y2))
    #the distance
    d=np.sqrt ((x3 - xx)**2 + (y3 - yy)**2)
    
    print ("distance=",d)
    
    
    #find the closest by searching for min distance.
    #All glory to https://stackoverflow.com/questions/10374930/matplotlib-annotating-a-3d-scatter-plot
    imin=0
    dmin=10000000
    for i in range(len(x)):
      #magic from https://stackoverflow.com/questions/10374930/matplotlib-annotating-a-3d-scatter-plot
      x2, y2, z2=proj3d.proj_transform(x[i], y[i], z[i], plt.gca().get_proj())
      x3, y3 = ax.transData.transform((x2, y2))
      #the distance magic from https://stackoverflow.com/questions/10374930/matplotlib-annotating-a-3d-scatter-plot
      d=np.sqrt ((x3 - xx)**2 + (y3 - yy)**2)
      #We find the distance and also the index for the closest datapoint
      if d< dmin:
        dmin=d
        imin=i
        
      #print ("i=",i," d=",d, " imin=",imin, " dmin=",dmin)
    
    # gives the incorrect data point index
    point_index = int(event.ind)

    print("Xfixed=",x[imin], " Yfixed=",y[imin], " Zfixed=",z[imin], " PointIdxFixed=", imin)
    print("Xbroke=",x[point_index], " Ybroke=",y[point_index], " Zbroke=",z[point_index], " PointIdx=", point_index)

fig.canvas.mpl_connect('pick_event', chaos_onclick)
#fig.canvas.mpl_connect('motion_notify_event', onMouseMotion)  # on mouse motion


plt.show()

On the bright side, via #19812 we now carry the z-sorting on the artist so we know enough to do the inversion.

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index in pick_event invalid and inconsistent for mplot3d scatter plot (z ordering?) #19735

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index in pick_event invalid and inconsistent for mplot3d scatter plot (z ordering?) #19735

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snaxxus commented on Mar 31, 2021

snaxxus commented on Apr 1, 2021

QuLogic commented on Apr 2, 2021

snaxxus commented on Apr 2, 2021

tacaswell commented on Apr 2, 2021

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