René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Various issues with miter limits","articleBody":"Noticed this a while ago (while implementing mplcairo) but prompted to report due to #9829:\r\n\r\n```\r\nfrom matplotlib import pyplot as plt, ticker\r\n\r\nfig, axs = plt.subplots(4, 4)\r\nfor i, ax in enumerate(axs.flat):\r\n    ax.xaxis.set_major_locator(ticker.NullLocator())\r\n    ax.yaxis.set_major_locator(ticker.NullLocator())\r\n    ax.plot([-1, 0, 1], [0, 1, 0], lw=i, solid_joinstyle=\"miter\")\r\n    ax.set(xlim=(-15, 15), ylim=(0, 3))\r\n\r\nfig.savefig(\"/tmp/test.png\")\r\nfig.savefig(\"/tmp/test.svg\")\r\nfig.savefig(\"/tmp/test.pdf\")\r\n\r\nplt.show()\r\n```\r\n\r\npdf appears to respect the miter (\"pointed\") joinstyle (funnily this appears to behave in the opposite manner as #9829):\r\n![test-pdf](https://user-images.githubusercontent.com/1322974/33109294-9f8fccd8-cef5-11e7-9f3e-4563f903d8d5.png)\r\nsvg appears to ignore it:\r\n![test-svg](https://user-images.githubusercontent.com/1322974/33109293-9bcf73e6-cef5-11e7-9a30-a9327cde3eac.png)\r\nagg changes its mind half-way:\r\n![test](https://user-images.githubusercontent.com/1322974/33109296-a2d9cb78-cef5-11e7-8bda-a42809d44767.png)\r\n\r\nThe behavior of agg is almost certainly due to a misunderstanding in how the miter limit is set.  See https://www.w3schools.com/tags/canvas_miterlimit.asp and https://www.cairographics.org/manual/cairo-cairo-t.html#cairo-set-miter-limit for the definition of the miter limit.  Note that the two definitions differ: html defines it in absolute lengths whereas cairo defines it relative to the linewidth.\r\n\r\nbackend_agg.h has the line (multiple times)\r\n```\r\nstroke.miter_limit(points_to_pixels(gc.linewidth));\r\n```\r\nwhich suggest that the author thought miter_limit takes an absolute miter limit, but in fact a relative miter limit should have been given (how do I know this? because I can reproduce the same behavior with mplcairo with the same setting, and cairo clearly documents that it uses a relative miter limit :-)).\r\n\r\nSo the current implementation ends up switching from miter join to bevel join at an angle that depends on the linewidth, eek.\r\n\r\n---\r\n\r\nNOTE: when this is fixed, remember to regen the logo per #14939 (which also showcases the issue).","author":{"url":"https://github.com/anntzer","@type":"Person","name":"anntzer"},"datePublished":"2017-11-22T03:58:01.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/9830/matplotlib/issues/9830"}