Proposal to remove env from call to Popen

[anthonyjclark]

The current version of _run_ffmpeg calls Popen with an empty environment:

env: Any = {}
...
return subprocess.Popen(
    ...
    env=env,
  )

This causes a difficult-to-understand error on my machine:

---------------------------------------------------------------------------
BrokenPipeError                           Traceback (most recent call last)
File ~/research/wmr/Transformable-Leg-Wheel-Robot/.venv/lib/python3.11/site-packages/mediapy/__init__.py:1851, in write_video(path, images, **kwargs)
   1850 for image in images:
-> 1851   writer.add_image(image)

File ~/research/wmr/Transformable-Leg-Wheel-Robot/.venv/lib/python3.11/site-packages/mediapy/__init__.py:1755, in VideoWriter.add_image(self, image)
   1754 assert stdin is not None
-> 1755 if stdin.write(data) != len(data):
   1756   self._proc.wait()

BrokenPipeError: [Errno 32] Broken pipe

During handling of the above exception, another exception occurred:

RuntimeError                              Traceback (most recent call last)
Cell In[1], line 32
     29 print(f"Final position: {state.data.qpos[0:3]}")
     31 frames = env.render(rollout)
---> 32 media.show_video(frames, fps=1.0 / env.dt)

File ~/research/wmr/Transformable-Leg-Wheel-Robot/.venv/lib/python3.11/site-packages/mediapy/__init__.py:1961, in show_video(images, title, **kwargs)
   1938 def show_video(
   1939     images: Iterable[_NDArray], *, title: str | None = None, **kwargs: Any
   1940 ) -> str | None:
   1941   """Displays a video in the IPython notebook and optionally saves it to a file.
   1942 
   1943   See `show_videos`.
   (...)   1959     html string if `return_html` is `True`.
   1960   """
-> 1961   return show_videos([images], [title], **kwargs)

File ~/research/wmr/Transformable-Leg-Wheel-Robot/.venv/lib/python3.11/site-packages/mediapy/__init__.py:2043, in show_videos(videos, titles, width, height, downsample, columns, fps, bps, qp, codec, ylabel, html_class, return_html, **kwargs)
   2041   video = [resize_image(image, (h, w)) for image in video]
   2042   first_image = video[0]
-> 2043 data = compress_video(
   2044     video, metadata=metadata, fps=fps, bps=bps, qp=qp, codec=codec
   2045 )
   2046 if title is not None and _config.show_save_dir:
   2047   suffix = _filename_suffix_from_codec(codec)

File ~/research/wmr/Transformable-Leg-Wheel-Robot/.venv/lib/python3.11/site-packages/mediapy/__init__.py:1880, in compress_video(images, codec, **kwargs)
   1878 with tempfile.TemporaryDirectory() as directory_name:
   1879   tmp_path = pathlib.Path(directory_name) / f'file{suffix}'
-> 1880   write_video(tmp_path, images, codec=codec, **kwargs)
   1881   return tmp_path.read_bytes()

File ~/research/wmr/Transformable-Leg-Wheel-Robot/.venv/lib/python3.11/site-packages/mediapy/__init__.py:1849, in write_video(path, images, **kwargs)
   1847   dtype = np.dtype(np.uint16)
   1848 kwargs = {'metadata': getattr(images, 'metadata', None), **kwargs}
-> 1849 with VideoWriter(path, shape=shape, dtype=dtype, **kwargs) as writer:
   1850   for image in images:
   1851     writer.add_image(image)

File ~/research/wmr/Transformable-Leg-Wheel-Robot/.venv/lib/python3.11/site-packages/mediapy/__init__.py:1716, in VideoWriter.__exit__(self, *_)
   1715 def __exit__(self, *_: Any) -> None:
-> 1716   self.close()

File ~/research/wmr/Transformable-Leg-Wheel-Robot/.venv/lib/python3.11/site-packages/mediapy/__init__.py:1773, in VideoWriter.close(self)
   1771   assert stderr is not None
   1772   s = stderr.read().decode('utf-8')
-> 1773   raise RuntimeError(f"Error writing '{self.path}': {s}")
   1774 self._popen.__exit__(None, None, None)
   1775 self._popen = None

RuntimeError: Error writing '/tmp/tmpe9a42is1/file.mp4': Diagnostic { message: "environment variable not found" }
NOTE: If you're looking for the fancy error reports, install miette with the `fancy` feature, or write your own and hook it up with miette::set_hook().

I tracked down the error, and it happens because I used pixi to install ffmpeg. This will install a stub at ~/.pixi/bin/ffmpeg. The _get_ffmpeg_path function will find the stub just fine, but the stub cannot find the actual exectuable when the environment is clobbered.

This is easiest to replicate in bash:

❯ /home/ajc/.pixi/bin/ffmpeg
ffmpeg version 8.0 Copyright (c) 2000-2025 the FFmpeg developers
...
❯ env -i bash --norc
bash-5.1$ printenv
PWD=/home/ajc/research/wmr/Transformable-Leg-Wheel-Robot/packages/twmr/assets
SHLVL=1
_=/usr/bin/printenv
bash-5.1$ ffmpeg
bash: ffmpeg: command not found
bash-5.1$ /home/ajc/.pixi/bin/ffmpeg
Diagnostic { message: "environment variable not found" }
NOTE: If you're looking for the fancy error reports, install miette with the `fancy` feature, or write your own and hook it up with miette::set_hook().

I had a few options:

1. Set the correct path to the executable: media.set_ffmpeg("/home/ajc/.pixi/envs/ffmpeg/bin/ffmpeg")
2. Remove env={} from the call to Popen

I think the second choice is better. The first requires a full, hard-coded path (or resolving from the home directory, which I suppose it OK, but hard to remember after a few months).

I searched through the history of the _run_ffmpeg function and I did not find a reason for clobbering the environment. Please let me know if I am mistaken!

[hhoppe]

The _run_ffmpeg function may have been introduced to allow customization within Google's code tree.
I agree that the default (non-customized) behavior should be to let the environment be preserved for the call to ffmpeg.
Hopefully someone at Google can have a look.

[Conchylicultor]

Thank you for reporting! As @hhoppe said, this was needed due to Google internal only configuration.

I sent a fix in #63 which should be merged soon. Closing this one