ffmpeg_extract_subclip returns black frames

[ksharsha]

I am trying to extract particular sub clips from videos and I see that when the duration of the sub clip is shorter (around 2- 3 seconds), only black frames are seen in the subclip. I guess this might be because ffmpeg is unable to locate the key frames. How do I deal with this?

[Zulko]

Hey - That's not enough informations and I've never seen this before, can you provide some code to reproduce the issue ? (and possibly a videofile for which the issue exists too)

[ksharsha]

Hi, I recreated the issue using this gist https://gist.github.com/ksharsha/f28ca2f019070389c38741fb90a077d1 . I am attaching the video file as well. You can see that the audio is still preserved however, the video is completely blank. This is the video that I am trying to extract the subclip from (tstart = 12.334277199999999, tend = 15.97166675). This is the video https://www.youtube.com/watch?v=a-6lVnhqU4w
Videos.zip

[ghost]

It seems like this might help..
https://trac.ffmpeg.org/wiki/Seeking

when I try what is recommended by this link, I do get video, but the video seems to be a bit messed up (like a bad key frame). I'm not very good with ffmpeg, so maybe someone else sees a solution?

[ksharsha]

@Earney true, the problem is with this cmd = [get_setting("FFMPEG_BINARY"),"-y",
"-i", filename,
"-ss", "%0.2f"%t1,
"-t", "%0.2f"%(t2-t1),
"-vcodec", "copy", "-acodec", "copy", targetname]
I am not good with ffmpeg either but looks like the above cmd has to be changed.

[flairtone]

Is this issue resolved? Is there any other way instead of using ffmpeg_extract_subclip to clip videos?

[scrpy]

Looks like PR #579 could fix this.

[gpotter2]

Hi !

There is #848 which is quite clear and well-documented. It seems if fell unoticed, but it does fix this issue, in a better way than #579. (It might not be THE universal fix but it seems very correct)

It would be cool if it could get merged soon :) this issue is getting old, and remains kinda annoying 😄

Regards

[ebenz99]

Updating to say I believe this may still be a problem. Using ffmpeg_extract_subclip for 2-3 second clips and continue to run into this issue. As far as I can tell, the only way to get the frame back is removing the copy/encode options, but this reduces the picture in quality.

[keikoro]

@ebenz99 Should we reopen this issue?

[ebenz99]

Up to you. I ended up just using the workaround I mentioned (removing some options and taking a quality hit), but it might be nice to not have to do that.

[jeff-hykin]

This is an issue for me, I'm using moviepy to automate the refining of a machine learning dataset and the samples need all the quality it can get.

Even with the quality hit, I'm not sure how to remove the copy/encode options, as I'm not passing any copy/encode options to ffmpeg_extract_subclip()

[persas]

Any update on this issue @jeff-hykin ?? I am in the very same position as you were back at the time that you commented on this

[persas]

Which options did you remove @ebenz99 ??

[jeff-hykin]

Because most of my time segments were tiny (less than 1 second), and I couldn't sacrifice on quality, I currently use opencv to load all of the frames, skip to the needed subsection, pull out all the frames, and then create a new video with those frames.

My older method was a direct call to ffmpeg. I gave up on the movie.py library

from subprocess import call,run
from pathlib import Path
import os

def cut_video(source, start, end, output_path, hide_errors=False, errors_to_ignore=None):
    """
    Example:
        exit_code, stdout, stderr = cut_video(source="./myvid.mp4", start=30, end=60, output_path="./myvid_30to60.mp4")
    
    Note:
        - this will not modify the existing video
        - this was tested with ffmpeg 4.3.1
    
    Args:
        source: filepath to the video
        start: seconds (float or int)
        end: seconds (float or int)
        output_path: filepath to new clip
        hide_errors: print or dont print stderr
        errors_to_ignore: list of strings, if string is in the stderr the stderr wont be printed
        
    Returns:
        (exit_code, stdout, stderr)

    Raises:
        Exception: "End time is before start time for "+source
        Exception: "When calling cut_video() it seems ffmpeg is not installed (or not on the path). This was called with "+source
        Exception: "When calling cut_video() it seems the video didn't exist as a file: "+source
    """
    errors_to_ignore = [] if errors_to_ignore is None else errors_to_ignore
    duration = end - start
    # check start/end
    if duration <= 0:
        raise Exception("End time is before start time for "+source)
    # make sure ffmpeg exists (only check once per python process)
    if globals().get("__ffpeg_exists", False):
        command_output = run(["ffmpeg", "-version",], capture_output=True)
        stdout = str(command_output.stdout.decode('UTF-8'))
        if "ffmpeg version" in stdout:
            globals()["__ffpeg_exists"] = True
        else:
            raise Exception("When calling cut_video() it seems ffmpeg is not installed (or not on the path). This was called with "+source)
    # make sure the video exists
    if not os.path.isfile(source):
        raise Exception("When calling cut_video() it seems the video didn't exist as a file: "+source)
        
    # touch the original video so that the file system knows it has been used recently
    Path(source).touch()
    
    # 
    # actually run the command
    # 
    command_output = run(["ffmpeg", "-i", str(source), "-ss", str(start), "-t", str(duration), "-async", "1", str(output_path), "-hide_banner", "-loglevel", "panic"], capture_output=True)
    
    # process the output
    exit_code = command_output.exit_code
    stdout = str(command_output.stdout.decode('UTF-8'))
    stderr = str(command_output.stderr.decode('UTF-8'))
    if not hide_errors and len(stderr) != 0:
        common_errors = [ common_error for common_error in errors_to_ignore if common_error in stderr ]
        if len(common_errors) == 0:
            print('ffmpeg inside cut_video(source='+str(source)+') produced some errors:')
            print(stderr)
    
    return exit_code, stdout, stderr

[keikoro]

Reopening because several people commented since close / problem persists.

[dennymarcels]

I have also encountered this issue. Curiously, it only happens with some of my videos, even if they were films made with the same camera. Also, I noticed that if I request the problematic clip to be a little longer, it eventually renders, but is very accelerated (5x or more, subjectively evaluated).

[OsaAjani]

Thank you for your contributions and for reporting issues in this repository. With the release of v2, which introduces significant changes to the codebase and API, we’ve reviewed the backlog of open PRs and issues. Due to the length of the backlog and the likelihood that many of these are either fixed or no longer applicable, we’ve made the decision to close all previous PRs and issues.

If you believe that any of these are still relevant to the current version or if you'd like to reopen a related discussion, please feel free to create a new issue or pull request, referencing the old one.

Thank you for your understanding and continued support!