René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"mplot3d: add_collection3d issues","articleBody":"\u003c!--To help us understand and resolve your issue, please fill out the form to the best of your ability.--\u003e\r\n\u003c!--You can feel free to delete the sections that do not apply.--\u003e\r\n\r\n### Bug report\r\n\r\n**Bug summary**\r\n\r\n1. When plotting a parametrized line in 3D using `LineCollection`, a `ValueError` is thrown when `Axes3D.add_collection_3d` upconverts 2D Collections with the `zs` parameter.\r\nIn the MWE below, this is because this line in `_paths_to_3d_segments` tries to broadcast a 2D array (number of line segments on the first axis and segment start \u0026 end on the second) to a 1D array (number of line segments):\r\nhttps://github.com/matplotlib/matplotlib/blob/137edd56a8f85d43c96d3a521a38500ac2977138/lib/mpl_toolkits/mplot3d/art3d.py#L234\r\nCommenting out this line will make the example work.\r\n2. Additionally, `add_collection3d` does not return the `collection` like `add_collection` does (it does not return anything). Hence, you cannot pass the `collection` object to, e.g., a colorbar call:\r\nhttps://github.com/matplotlib/matplotlib/blob/137edd56a8f85d43c96d3a521a38500ac2977138/lib/mpl_toolkits/mplot3d/axes3d.py#L2180-L2186\r\nSimply returning the return value of the call to `super().add_collection` does the trick for me here:\r\n```python\r\ncollection = super().add_collection(col)\r\nreturn collection\r\n```\r\n\r\n**Code for reproduction**\r\n\r\nA minimal working example (including the reason why I use `LineCollection` instead of `plot`, namely a multicolored line) stitched together from two official examples ([1](https://matplotlib.org/3.2.2/gallery/lines_bars_and_markers/multicolored_line.html), [2](https://matplotlib.org/mpl_toolkits/mplot3d/tutorial.html#line-plots)) is:\r\n\r\n```python\r\nimport numpy as np\r\nimport matplotlib.pyplot as plt\r\nfrom matplotlib.collections import LineCollection\r\n\r\ntheta = np.linspace(-4 * np.pi, 4 * np.pi, 100)\r\nz = np.linspace(-2, 2, 100)\r\nr = z**2 + 1\r\nx = r * np.sin(theta)\r\ny = r * np.cos(theta)\r\n\r\npoints = np.array([x, y, z]).T.reshape(-1, 1, 3)\r\nsegments = np.concatenate([points[:-1], points[1:]], axis=1)\r\n\r\nfig = plt.figure()\r\nax = fig.gca(projection='3d')\r\n\r\nnorm = plt.Normalize(0, 2*np.pi)\r\n# 2D LineCollection from x \u0026 y values\r\nlc = LineCollection(segments[:, :, :2], cmap='twilight', norm=norm)\r\nlc.set_array(np.mod(theta, 2*np.pi))\r\n# Add 2D collection at z values to ax\r\nline = ax.add_collection3d(lc, zs=segments[:, :, 2])  # line is None\r\n\r\n# fig.colorbar(line, ax=ax)  # Would fail because add_collection3d does not return the collection\r\n\r\nax.set_xlim(-5, 5)\r\nax.set_ylim(-4, 6)\r\nax.set_zlim(-2, 2)\r\nplt.show()\r\n```\r\n\r\n**Actual outcome**\r\n\r\n```python\r\nTraceback (most recent call last):\r\n\r\n  File \"\u003cipython-input-1-d8492282873b\u003e\", line 23, in \u003cmodule\u003e\r\n    line = ax.add_collection3d(lc, zs=segments[:, :, 2])\r\n\r\n  File \"c:\\users\\hangleiter\\appdata\\local\\continuum\\anaconda3\\envs\\py38\\lib\\site-packages\\mpl_toolkits\\mplot3d\\axes3d.py\", line 2178, in add_collection3d\r\n    art3d.line_collection_2d_to_3d(col, zs=zs, zdir=zdir)\r\n\r\n  File \"c:\\users\\hangleiter\\appdata\\local\\continuum\\anaconda3\\envs\\py38\\lib\\site-packages\\mpl_toolkits\\mplot3d\\art3d.py\", line 332, in line_collection_2d_to_3d\r\n    segments3d = _paths_to_3d_segments(col.get_paths(), zs, zdir)\r\n\r\n  File \"c:\\users\\hangleiter\\appdata\\local\\continuum\\anaconda3\\envs\\py38\\lib\\site-packages\\mpl_toolkits\\mplot3d\\art3d.py\", line 234, in _paths_to_3d_segments\r\n    zs = np.broadcast_to(zs, len(paths))\r\n\r\n  File \"\u003c__array_function__ internals\u003e\", line 5, in broadcast_to\r\n\r\n  File \"C:\\Users\\hangleiter\\AppData\\Local\\Continuum\\anaconda3\\envs\\py38\\lib\\site-packages\\numpy\\lib\\stride_tricks.py\", line 182, in broadcast_to\r\n    return _broadcast_to(array, shape, subok=subok, readonly=True)\r\n\r\n  File \"C:\\Users\\hangleiter\\AppData\\Local\\Continuum\\anaconda3\\envs\\py38\\lib\\site-packages\\numpy\\lib\\stride_tricks.py\", line 125, in _broadcast_to\r\n    it = np.nditer(\r\n\r\nValueError: input operand has more dimensions than allowed by the axis remapping\r\n```\r\n\r\n**Expected outcome**\r\nThis is the result if both of the above fixes are applied:\r\n![grafik](https://user-images.githubusercontent.com/32895729/85722691-d596eb80-b6f2-11ea-88b5-222dad3ee73d.png)\r\n\r\n\u003c!--A description of the expected outcome from the code snippet--\u003e\r\n\u003c!--If this used to work in an earlier version of Matplotlib, please note the version it used to work on--\u003e\r\n\r\n**Matplotlib version**\r\n\u003c!--Please specify your platform and versions of the relevant libraries you are using:--\u003e\r\n  * Operating system: `Windows`\r\n  * Matplotlib version: `3.2.2`\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): `module://ipykernel.pylab.backend_inline`\r\n  * Python version: `3.8.3`\r\n  * Numpy version: `1.18.5`\r\n\r\nMatplotlib is installed via `conda:defaults`.","author":{"url":"https://github.com/thangleiter","@type":"Person","name":"thangleiter"},"datePublished":"2020-06-25T13:13:42.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":8},"url":"https://github.com/17755/matplotlib/issues/17755"}