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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Rolling image if the FFMpegWriter dpi setting does not match that specified when a figure is created","articleBody":"### Bug report\r\n\r\n**Bug summary**\r\nPlayback of mp4 files written by FFMpegWriter shows a rolling image if the dpi setting does not match that specified when a figure is created. In particular, this happens if the movie frame images are being displayed on the screen, e.g. during creation of the movie. (There is also an annoying flicker.)\r\n\r\n**Code for reproduction**\r\n\r\nAnimation in the code below fails, resulting in a rolling image on playback of the saved mp4 movie.  Either of two modifications indicated in the comments make the saved movie play back normally.\r\n\r\n```python\r\nimport matplotlib.animation\r\nimport numpy\r\nimport pylab\r\n\r\nfig = pylab.figure(figsize=(6,6), dpi=120)\r\nwriter = matplotlib.animation.FFMpegWriter(fps=10)\r\nx = numpy.linspace(0, 6.28, 100)\r\n\r\nwith writer.saving(fig, 'rolling_bug.mp4', dpi=100):           # saving OK if dpi=120\r\n    for offset in numpy.linspace(0, 6.28, 30):\r\n        pylab.plot(numpy.cos(x-offset), color='black')\r\n        # saving OK if the next line is commented out\r\n        pylab.pause(0.01)\r\n        writer.grab_frame()\r\n```\r\nNote that commenting out the pylab.pause() call results in no image being displayed on the screen until the end of the script.\r\n\r\n**Actual outcome**\r\nInspection of the figure vertical dimension in pixels shows that it has changed across the call to grab_frame(), from 600 to 590 in the above example.  The horizontal dimension does not change.\r\nA screen grab from VLC Media Player is shown here:\r\n![rolling_bug_screengrab](https://user-images.githubusercontent.com/7269332/27990577-278e5b28-6410-11e7-8538-123c0c394891.png)\r\n\r\n**Expected outcome**\r\n\r\nThe writer.grab_frame() should not change the vertical dimension of the figure.\r\n\r\nThe flickering did not occur in the last version of Matplotlib I had installed, but I do not know what version that was.  I'm pretty sure the vertical image dimension also stayed constant across the grab_frame() call, since this was (abstracted from) working code from about a year ago.\r\n\r\n**Version info**\r\n\r\nWindows 8.1\r\nMatplotlib 2.0.2\r\nPython 3.6.1\r\nAnaconda 4.4.0\r\n\r\nInstalled from conda\r\n\r\n","author":{"url":"https://github.com/Patrick5","@type":"Person","name":"Patrick5"},"datePublished":"2017-07-09T02:05:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/8852/matplotlib/issues/8852"}