René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Cairo backend discards alpha channel for collections","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\nThe cairo backend ignores the alpha value of artists when rendering Collections such as PatchCollection.\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, also minimizing the number of dependencies required--\u003e\r\n\r\n```python\r\nimport matplotlib\r\nmatplotlib.use('Cairo')\r\nimport matplotlib.pyplot as plt\r\nfrom matplotlib.collections import PatchCollection\r\nimport numpy as np\r\n\r\ncircles = [plt.Circle((i,i), 1.0) for i in np.arange(10)]\r\ncol = PatchCollection(circles, alpha=0.5)\r\nax = plt.gca()\r\nax.add_collection(col)\r\nax.autoscale_view()\r\nplt.savefig('test_cairo_alpha.png')\r\n```\r\n\r\n**Actual outcome**\r\n\r\n![test_cairo_alpha_actual](https://user-images.githubusercontent.com/116013/48202782-17302280-e366-11e8-82bd-ad1e559a6d84.png)\r\n\r\n**Expected outcome**\r\nExpected output when using the default `Qt5Agg` backend:\r\n\r\n![test_cairo_alpha_expected](https://user-images.githubusercontent.com/116013/48202787-1d260380-e366-11e8-952f-037c18ea4521.png)\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: Ubuntu 18.04\r\n  * Matplotlib version: master @ 4dbbd22095e32b77053aab403c442934f18ade71\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): cairo\r\n  * Python version: 3.6.6\r\n  * Jupyter version (if applicable):\r\n  * Other libraries: \r\n","author":{"url":"https://github.com/joh","@type":"Person","name":"joh"},"datePublished":"2018-11-08T13:57:30.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/12773/matplotlib/issues/12773"}