René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"matplotlib.collections.QuadMesh.set_array() input arg format is weird and undocumented","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\nTrying to give `matplotlib.collections.QuadMesh.set_array()` the same argument as was used by `matplotlib.axes.Axes.pcolormesh()` results in an error. Moreover, the argument format depends on the shading; an argument that is correct for `shading='gouraud'` will be accepted and produce spurious graphics for `shading='flat'`. In my opinion, the user should be able to use the same argument format for the update (via `set_array` or another method if that causes backwards compatibility issues) than used for creation (via `pcolormesh`), and not have to care about the shading when setting the data.\r\n\r\nIt was raised in [a StackOverflow post from 2013](https://stackoverflow.com/questions/18797175/animation-with-pcolormesh-routine-in-matplotlib-how-do-i-initialize-the-data) and the top answer (from 2015) gives workarounds, but I did not find anything in the issue tracker (here).  It should at least be documented, [the current doc](https://matplotlib.org/3.1.1/api/collections_api.html?highlight=collections%20quadmesh%20set_array#matplotlib.collections.QuadMesh.set_array) only says \"Set the image array from numpy array A\".\r\n\r\n\u003c!--A short 1-2 sentences that succinctly describes the bug--\u003e\r\n\r\n**Code for reproduction**\r\n\r\n\u003c!--A minimum code snippet required to reproduce the bug.\r\nPlease make sure to minimize the number of dependencies required, and provide\r\nany necessary plotted data.\r\nAvoid using threads, as Matplotlib is (explicitly) not thread-safe.--\u003e\r\n\r\n```python\r\nimport numpy as np\r\nimport matplotlib.pyplot as plt\r\n\r\n\r\ndef f(X, Y, t=0):\r\n    return np.sin(X+t*np.pi)*np.sin(Y)\r\n\r\nxmin, xmax = 0.0, 4*np.pi\r\nymin, ymax = 0.0, 5*np.pi\r\nNx, Ny = 50, 40\r\nx, y = np.linspace(xmin, xmax, num=Nx), np.linspace(ymin, ymax, num=Ny)\r\nX, Y = np.meshgrid(x, y, indexing='ij')\r\nzstart = f(X, Y)\r\n\r\nrun_number = np.random.rand()\r\n\r\n# Intialize figure stuff\r\nfig = plt.figure()\r\naxs = [\r\n    plt.subplot(221, title='control flat'),\r\n    plt.subplot(222, title='test flat'),\r\n    plt.subplot(223, title='control gouraud'),\r\n    plt.subplot(224, title='test gouraud'),\r\n    ]\r\n\r\ndef drawcmesh(X, Y, z, ax, shading):\r\n    ax.set_xlim(left=xmin, right=xmax)\r\n    ax.set_ylim(bottom=ymin, top=ymax)\r\n    cmesh = ax.pcolormesh(X, Y, z,\r\n                          shading=shading, vmin=-1.5, vmax=1.5, cmap='hot')\r\n    return cmesh\r\n\r\n\r\n\r\nzstart = f(X, Y)\r\nperm_mesh = [\r\n    drawcmesh(X, Y, zstart, axs[1], shading='flat'),\r\n    drawcmesh(X, Y, zstart, axs[3], shading='gouraud')\r\n    ]\r\ndrawcmesh(X, Y, zstart, axs[0], shading='flat')\r\ndrawcmesh(X, Y, zstart, axs[2], shading='gouraud')\r\n\r\ndef update(t):\r\n    z = f(X, Y, t)\r\n\r\n    for i in range(4):\r\n        ax = axs[i]\r\n        if i\u003c2:\r\n            shading='flat'\r\n        else:\r\n            shading='gouraud'\r\n\r\n        if i%2 == 0:\r\n            # Control case: redraw by hand\r\n            axs[i].clear()\r\n            drawcmesh(X, Y, z, axs[i], shading=shading)\r\n            axs[i].set_title('control {}'.format(shading))\r\n\r\n        if i==1:\r\n            # Flat shading\r\n\r\n            # # The following would copy the syntax of plt.pcolormesh() but it\r\n            # # results in an error\r\n            # perm_mesh[0].set_array(z)\r\n\r\n            # The following does not error out but the plot is later incorrect\r\n            perm_mesh[0].set_array(z.ravel())\r\n\r\n            # # The following works\r\n            # perm_mesh[0].set_array(z[:-1, :-1].ravel())\r\n\r\n        if i==3:\r\n            # Gouraud shading\r\n\r\n            # # The following would copy the syntax of plt.pcolormesh() but it\r\n            # # results in an error\r\n            # perm_mesh[1].set_array(z)\r\n\r\n            # The following works\r\n            perm_mesh[1].set_array(z.ravel())\r\n\r\n            # # The following results in an error at draw time:\r\n\r\n            \r\n            # perm_mesh[1].set_array(z[:-1, :-1].ravel())\r\n\r\n# # Uncomment this to check that the initial drawing for the test case is correct\r\n# plt.show(block=True)\r\n\r\nplt.show(block=False)\r\nfor t in np.linspace(0.0, 4.0, num=50):\r\n    update(t)\r\n    fig.canvas.draw()\r\n    fig.canvas.flush_events()\r\n```\r\n\r\n**Results**\r\nCf commented code under `# Flat shading` and `# Gouraud shading` in the source for the various possibilities.\r\n\r\nSpurious graphics for flat shading:\r\n\r\n\u003cimg width=\"561\" alt=\"test-shadings\" src=\"https://user-images.githubusercontent.com/28686753/66308548-b0194600-e907-11e9-97e6-ad61452022d2.PNG\"\u003e\r\n\r\n\r\n\r\n\r\nError given by gouraud shading when dimensions do not match:\r\n\r\n\r\n```\r\n\u003ccurrent file\u003e, line 93, in \u003cmodule\u003e\r\n    fig.canvas.draw()\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\backends\\backend_agg.py\", line 388, in draw\r\n    self.figure.draw(self.renderer)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\artist.py\", line 38, in draw_wrapper\r\n    return draw(artist, renderer, *args, **kwargs)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\figure.py\", line 1709, in draw\r\n    renderer, self, artists, self.suppressComposite)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\image.py\", line 135, in _draw_list_compositing_images\r\n    a.draw(renderer)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\artist.py\", line 38, in draw_wrapper\r\n    return draw(artist, renderer, *args, **kwargs)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\axes\\_base.py\", line 2647, in draw\r\n    mimage._draw_list_compositing_images(renderer, self, artists)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\image.py\", line 135, in _draw_list_compositing_images\r\n    a.draw(renderer)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\artist.py\", line 38, in draw_wrapper\r\n    return draw(artist, renderer, *args, **kwargs)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\collections.py\", line 2037, in draw\r\n    self._meshWidth, self._meshHeight, coordinates)\r\nFile \"\u003canaconda-site-packages\u003e\\matplotlib\\collections.py\", line 1985, in convert_mesh_to_triangles\r\n    c = self.get_facecolor().reshape((meshHeight + 1, meshWidth + 1, 4))\r\nValueError: cannot reshape array of size 7644 into shape (50,40,4)\r\n```\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: Win10\r\n  * Matplotlib version: 3.1.1 (from conda)\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): Qt5Agg\r\n  * Python version: 3.7.4\r\n","author":{"url":"https://github.com/Tigraan","@type":"Person","name":"Tigraan"},"datePublished":"2019-10-07T11:31:38.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/15388/matplotlib/issues/15388"}