René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`_path.get_extents` does not correctly handle bezier curves","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\n\u003c!--A short 1-2 sentences that succinctly describes the bug--\u003e\r\n\r\nCurrently, `.Path` extents are calculated by `_path.h` as simply the `Bbox` of the control points.\r\n\r\nFor bezier curves, the convex hull of the control points can be easily shown to contain the path. So these extents are a correct upper bound. However, especially for cubic curves, this upper bound will often not be even approximately equal. (See example below).\r\n\r\nSome thoughts:\r\n\r\n1) Doing this correctly is required in order to correctly implement \"width\", \"bbox-width\", and \"bbox-area\" for `MarkerStyle.normalization` as initially proposed in #15703 and discussed in #16772. \r\n2) Presumably this hasn't been a problem in the past because in practice, matplotlib primarily deals with lines, where the control points are exactly the extents. \r\n3) It might make sense to leave the current `Path.get_extents` code, and implement a `Path.get_exact_extents` to be used upon `MarkerStyle.__init__` to pre-calculate/cache the `Bbox` of that marker.\r\n\r\nThis solution prevents incurring a (presumably significant) performance penalty for something that clearly has not been a big issue in the past, allows us to still quickly compute the Bbox of a collection of markers by adding the appropriate padding to the Bbox of the marker centers, and doesn't change existing API.\r\n\r\nSomeone who's more familiar with the `Agg` internals is free to translate my Python code if we decide it's worth it to implement a \"fast\" version of the \"correct\" solution for use on all `Path`'s. \r\n\r\nI will submit a PR to implement `Path.get_exact_extents` Python-side (not much work), but I wanted to open this up for discussion. \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 matplotlib.pyplot as plt\r\nfrom matplotlib.path import Path\r\nfrom matplotlib.transforms import Bbox\r\nfrom matplotlib import patches\r\n\r\nhard_curve = Path([[0, 0],\r\n                   [4, 0], [-3, 1], [1, 1]],\r\n                  [Path.MOVETO,\r\n                   Path.CURVE4, Path.CURVE4, Path.CURVE4])\r\nreported_extents = hard_curve.get_extents()\r\nactual_extents = Bbox([[-0.3, 0], [1.3, 1]])\r\nfig, ax = plt.subplots()\r\npatch = patches.PathPatch(hard_curve, facecolor='orange', lw=2)\r\nax.add_patch(patch)\r\nax.set_xlim(-1, 2)\r\nax.set_ylim(-0.1, 1.1)\r\nax.set_title(f\"Path extents are {reported_extents};\\nShould be ~{actual_extents}.\")\r\n```\r\n\r\n**Actual outcome**\r\n\r\n\u003c!--The output produced by the above code, which may be a screenshot, console output, etc.--\u003e\r\n![issue-image](https://user-images.githubusercontent.com/1475390/77058298-7b1f1e00-6992-11ea-8999-749fc7573b27.png)\r\n\r\n```\r\n# If applicable, paste the console output here\r\n#\r\n#\r\n```\r\n\r\n**Expected outcome**\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\nSee title of plot.\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: Debian 9 Jessie\r\n  * Matplotlib version: master\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): qt5agg\r\n  * Python version: 3.7.3\r\n  * Jupyter version (if applicable): N/A\r\n  * Other libraries: N/A\r\n\r\n\r\n\u003c!--Please tell us how you installed matplotlib and python e.g., from source, pip, conda--\u003e\r\n\u003c!--If you installed from conda, please specify which channel you used if not the default--\u003e\r\n\r\n","author":{"url":"https://github.com/brunobeltran","@type":"Person","name":"brunobeltran"},"datePublished":"2020-03-19T10:39:19.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/16830/matplotlib/issues/16830"}