Re: [matplotlib-devel] Strange bug when saving animations (with streamplot)

[Tony Yu <ts...@gm...>]

On Tue, Jul 3, 2012 at 10:22 AM, Ryan May <rm...@gm...> wrote:

> On Tue, Jul 3, 2012 at 9:14 AM, Tony Yu <ts...@gm...> wrote:
> > On Mon, Jul 2, 2012 at 11:42 PM, Tony Yu <ts...@gm...> wrote:
> >> On Sun, Jun 24, 2012 at 10:02 AM, Tony Yu <ts...@gm...> wrote:
> >>>
> >>> I'm having a tough time figuring this out: Saving an animation seems to
> >>> hang when using `streamplot`. The exact same animation runs without
> issue
> >>> when calling show.
> >>>
> >>> On my system, the example below hangs consistently at frame 173.
> However,
> >>> the number of saved frames (before hanging) varies with density of
> stream
> >>> lines (i.e. number of line segments in streamplot): More frames saved
> for
> >>> lower density.
> >>>
> >>> Seems to be a memory or file-size issue, but
> >>>
> >>>    * Memory use doesn't seem to grow. (this suggests that ffmpeg adds
> >>> frames to disk as opposed to memory.)
> >>>    * Saving the animation up to the frame just before it hangs gives
> >>> modest (~1MB) videos.
> >>> Maybe this is a limitation of `ffmpeg`?
> >>>
> >>> Can someone confirm that the example hangs on their system? (Somehow, I
> >>> often run into bugs that are not reproducible.)
> >>>
> >>> In case this is system dependent:
> >>> * OSX 10.6
> >>> * Python 2.6.1 (system install)
> >>> * matplotlib master
> >>> * ffmpeg 0.10
> >>>
> >>> Simple plots seem to work fine, but I should probably try to reproduce
> >>> using something other than streamplot (maybe create the equivalent
> number of
> >>> line and arrow collections).
> >>>
> >>> -Tony
> >>>
> >>> #~~~ Example
> >>> import numpy as np
> >>> import matplotlib.pyplot as plt
> >>> import matplotlib.animation as animation
> >>>
> >>> fig, ax = plt.subplots()
> >>>
> >>> # do nothing function that prints frame
> >>> def update(frame_num):
> >>>     print frame_num
> >>>     return []
> >>>
> >>> Y, X = np.mgrid[:1:100j, :1:100j]
> >>> U = np.ones_like(X)
> >>> V = np.ones_like(X)
> >>> ax.streamplot(X, Y, U, V, density=1)
> >>>
> >>> ani = animation.FuncAnimation(fig, update, save_count=500)
> >>>
> >>> # save hangs at frame 173 (this probably varies by system, if it's
> >>> reproducible).
> >>> ani.save('animation.avi')
> >>> # show animates all frames w/o issue.
> >>> #plt.show()
> >>>
> >>>
> >>
> >> I finally had some time to revisit this issue. It seems that subprocess
> >> PIPEs have fairly limited buffers [1] (also see docs for
> >> `subprocess.Popen.communicate` [2]) and ffmpeg tends to generate a
> decent
> >> amount of output to stderr. A simple solution is to redirect stderr to a
> >> temporary file. This change fixes the issue on my system.
> >>
> >> If this bug is reproducible, I can submit a PR. The temporary file here
> >> could be created using tempfile so it gets cleaned up automatically, or
> >> maybe a more permanent log file for debugging. Thoughts?
> >>
> >> -Tony
> >>
> >> [1]
> >>
> http://thraxil.org/users/anders/posts/2008/03/13/Subprocess-Hanging-PIPE-is-your-enemy/
> >>
> >> [2]
> >>
> http://docs.python.org/library/subprocess.html#subprocess.Popen.communicate
> >>
> >
> > And just to clarify: My original email mentioned that saving would hang
> only
> > when `streamplot` was used to create the figure. It turns out that ffmpeg
> > prints more output (keyframes?) for more complex images, so when I ran
> > simpler plots, they didn't produce enough output to fill the subprocess
> > `PIPE` (although they would eventually).
> >
> > Also, theres's a simpler fix than piping ffmpeg output to a file: Just
> set
> > `-loglevel quiet` in the ffmpeg command.
> >
> > -Tony
>
> It's not a bad idea to have it logged to a file in a temp directory.
> We could potentially just do this if debug is set to verbose, however,
> and just use -loglevel quiet otherwise. Can you manage PR for this or
> do I need to?
>
> Ryan
>
>
Hey Ryan,

If you have time, that'd be great. Otherwise, I should have some time at
the end of the week to submit a PR.

-Tony