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• Precise seeking with ffmpeg

  6 août 2015, par Taras Galchenko

  Let’s say I have an audio file being decoded with ffmpeg. The source format is something like AAC where the audio is split into packets. When seeking to a particular time, it is clear that the time will not fall, most of the time, on the packet border but somewhere within the packet duration. Do I have to seek within packet myself or av_seek_frame does it all by itself and sets up decoding so that the next decoded frame should start at the position I’ve requested ?

  If I use the function av_seek_frame with the flag AVSEEK_FLAG_BACKWARD, I assume that the next packet returned by av_read_frame will be the packet containing the time position I am seeking to. Is that right ?

  If I decode this packet with avcodec_decode_audio4, will the frame returned contain the audio data at the start time of the packet begining or from the time I’ve passed to av_seek_frame ? In the latter case how can I find out the frame/packet timestamp so as to estimate the number of samples to skip in the decoded frame ? The PTS after seek is zero and DTS looks useless either.

  Is it possible to seek with precision to a particular time using ffmpeg ?

• avcodec/motion_est_template : Avoid using last + 1 element of array

  21 octobre 2022, par Andreas Rheinhardt

  avcodec/motion_est_template : Avoid using last + 1 element of array
  For an int array[8][2] using &array[8][0] (which is an int*
  
  pointing to the element beyond the last element of array)
  
  triggers a "runtime error : index 8 out of bounds for type 'int[8][2]'"
  
  from (Clang-)UBSan in the fate-vsynth(1|2|_lena)-snow tests.
  I don't know whether this is really undefined behaviour or does not
  
  actually fall under the "pointer arithmetic with the element beyond
  
  the last element of the array is allowed as long as it is not
  
  accessed" exception". All I know is that the code itself does not
  
  read from beyond the last element of the array.
  Nevertheless rewrite the code to a form that UBSan does not complain
  
  about.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/motion_est_template.c

• Subtitle Overlay Isn't Working, how do I fix it ? [closed]

  27 octobre 2024, par michael tan

  I'm trying to make a program to create clips with subtitles, but instead of the overlaid subtitles syncing with the clip, they just start from the beginning of the movie.

  


  import subprocess
from moviepy.editor import VideoFileClip

def parse_srt(srt_file):
    """Parse the SRT file and return a list of subtitles with their timestamps."""
    subtitles = []
    with open(srt_file, 'r') as f:
        content = f.read().strip().split('\n\n')
        for entry in content:
            lines = entry.split('\n')
            if len(lines) >= 3:
                index = lines[0]
                timestamps = lines[1]
                text = '\n'.join(lines[2:])
                start, end = timestamps.split(' --> ')
                subtitles.append((start.strip(), end.strip(), text.strip()))
    return subtitles

def print_subtitles_in_range(subtitles, start_time, end_time):
    """Print subtitles that fall within the given start and end times."""
    for start, end, text in subtitles:
        start_seconds = convert_srt_time_to_seconds(start)
        end_seconds = convert_srt_time_to_seconds(end)
        if start_seconds >= start_time and end_seconds <= end_time:
            print(f"{start} --> {end}: {text}")


def convert_srt_time_to_seconds(time_str):
    """Convert SRT time format to total seconds."""
    hours, minutes, seconds = map(float, time_str.replace(',', '.').split(':'))
    return hours * 3600 + minutes * 60 + seconds

def create_captioned_clip(input_file, start_time, end_time, srt_file, output_file):
    # Step 1: Extract the clip from the main video
    clip = VideoFileClip(input_file).subclip(start_time, end_time)
    print("Clip duration:", clip.duration)
    temp_clip_path = "temp_clip.mp4"
    clip.write_videofile(temp_clip_path, codec="libx264")

    # Step 2: Parse the SRT file to get subtitles
    subtitles = parse_srt(srt_file)

    # Step 3: Print subtitles that fall within the start and end times
    print("\nSubtitles for the selected clip:")
    print_subtitles_in_range(subtitles, start_time, end_time)

    # Step 2: Add subtitles using FFmpeg
    ffmpeg_command = [
        "ffmpeg",
        "-ss", str(start_time),  # Seek to the start time of the clip
        "-i", input_file,        # Use the original input file for subtitles
        "-vf", f"subtitles='{srt_file}:force_style=Alignment=10,TimeOffset={start_time}'",  # Overlay subtitles
        "-t", str(end_time - start_time),  # Set duration for the output
        output_file
    ]

    print("Running command:", ' '.join(ffmpeg_command))
    subprocess.run(ffmpeg_command, capture_output=True, text=True)

# Define input video and srt file
input_video = "Soul.2020.720p.BluRay.x264.AAC-[YTS.MX].mp4"
subtitle_file = "Soul.2020.720p.BluRay.x264.AAC-[YTS.MX].srt"

# Define multiple clips with start and end times
clips = [
    {"start": (5 * 60), "end": (5 * 60 + 30), "output": "output_folder/captioned_clip1.mp4"},
    {"start": (7 * 60), "end": (7 * 60 + 25), "output": "output_folder/captioned_clip2.mp4"},
]

# Process each clip
for clip_info in clips:
    create_captioned_clip(input_video, clip_info["start"], clip_info["end"], subtitle_file, clip_info["output"])


  


  I thought the subtitles would sync with the clip automatically ; after that didn't work I tried to manually sync them by putting the start time, duration, and an offset, but it still didn't work. The subtitles still start from 0:00 of the movie. There's nothing wrong with the .srt file, it's formatted correctly.