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sdorof1 opened this issue Mar 2, 2020 · 4 comments · Fixed by #16648
Closed

Wrong drawing Poly3DCollection #16633

sdorof1 opened this issue Mar 2, 2020 · 4 comments · Fixed by #16648
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topic: mplot3d
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v3.2.1

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@sdorof1
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sdorof1 commented Mar 2, 2020

Drawing 3D polygon with the code:

fig = plt.figure()
ax = fig.add_subplot(projection='3d')
ax.set_xlim((0, 5))
ax.set_ylim((0, 5))
ax.set_zlim((0, 5))
x = [1,2,3, 3,2,1]
y = [2,1,2, 2,1,2]
z = [1,1,1, 2,2,2]
v = [list(zip(x, y, z))]
pc = Poly3DCollection(v)
ax.add_collection3d(pc)

Leads to the following result on scene rotation:

2

While the polygon should looks like this:

1

Matplotlib version

  • Operating system: Ubuntu 16.04.6 LTS
  • Matplotlib version: 3.1.2
  • Matplotlib backend (print(matplotlib.get_backend())): module://ipympl.backend_nbagg
  • Python version: 3.8.1
  • Jupyter version (if applicable): 1.0.0
  • Other libraries:
@timhoffm
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timhoffm commented Mar 3, 2020 •
edited
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This is currently a limitation of mplot3d, which is not really a 3D renderer, tries to heuristically create a 3D impression by drawing 2D stuff.

The points a in the (projected) correct position in both cases, as you could see if you just draw the edges but no filling. However, the filling is just a 2D filling of the projection. This is in general not what one would expect from a filled 3D polygon.

Speaking of which, in the general case, creating a filling for a 3D polygon is a non-trivial task and there is no unique solution. Therefore, for now, I'd declare that filling of 3D polygons is not officially supported and whether or not a filled 3D polygon looks right is pure chance.

grafik

@timhoffm timhoffm added this to the unassigned milestone Mar 3, 2020
@anntzer
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anntzer commented Mar 3, 2020

I wouldn't even call this a limitation of mplot3d: the polygon is simply not well defined at all. It could just as well be two parallel triangular faces connected with a rectangle.

In general I think PolyCollection3D should only support triangular faces, possibly with some shortcut to split rectangles into pairs of triangles or more triangulation helpers.

@timhoffm
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timhoffm commented Mar 3, 2020

In general I think PolyCollection3D should only support triangular faces, possibly with some shortcut to split rectangles into pairs of triangles or more triangulation helpers.

I have a slightly different point of view. A polygon is just a sequence of (connected) points. It actually does not have any faces. In 2D you can easily fill the enclosed area, in 3D there is no well-defined contained area.

If you want to have surfaces, you should use plot_trisurf instead. It can already handle triangulation.

@anntzer
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anntzer commented Mar 3, 2020

Indeed, I should have said "don't support filled Poly3DCollections with non-triangles".

@timhoffm timhoffm mentioned this issue Mar 3, 2020
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@tacaswell tacaswell modified the milestones: unassigned, v3.2.1 Mar 13, 2020
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Document filling of Poly3DCollection timhoffm/matplotlib
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@tacaswell @anntzer @timhoffm @sdorof1