A script to convert a video to a desired file size.

ffmpeg-compress.py

#!/usr/bin/env python

# A script to convert a video to a desired filesize.
# Example use case: 8 MB Discord upload limit
#
# Requirements:
# - Python 3
# - ffmpeg should be in PATH
# - ffmpeg-python

import argparse
import os
import ffmpeg
import math


def get_output_path_from_input_path(in_path, postfix):
    in_dir = os.path.dirname(in_path)
    in_basename = os.path.basename(in_path)
    out_name = os.path.splitext(in_basename)[0] + postfix
    out_ext = os.path.splitext(in_basename)[1]
    out_basename = out_name + out_ext
    return os.path.join(in_dir, out_basename)


def convert_mb_to_kbit(size_in_mb):
    return size_in_mb * 8000


def get_video_duration_in_seconds(in_path):
    metadata = ffmpeg.probe(in_path)
    return math.ceil(float(metadata['format']['duration']))


def calc_output_bitrate(desired_file_size_in_kbit,
                        video_duration_in_seconds,
                        audio_bitrate_in_kbit_per_sec):
    return int(desired_file_size_in_kbit / video_duration_in_seconds) - audio_bitrate_in_kbit_per_sec


def null_output_path():
    if os.name == 'nt':
        return 'NUL'
    else:
        return '/dev/null'


def print_ffmpeg_args(stream):
    print('ffmpeg ' + ' '.join(ffmpeg.get_args(stream)))


def as_kbit_ffmpeg_arg(bitrate):
    return str(bitrate) + 'k'


def main():
    parser = argparse.ArgumentParser(description='Convert a video to the desired file size by using ffmpeg (by default 8 MB)')
    parser.add_argument('input', help='Path to the video to be converted')
    parser.add_argument('-o', '--output', help='Path to the final video', type=str)
    parser.add_argument('-s', '--output_size', help='The desired size of the video in MB', type=float, default=8)
    parser.add_argument('-a', '--output_audio_bitrate', help='The desired audio bitrate in kb/s', type=int, default=128)
    parser.add_argument('-r', '--output_height', help='The desired resolution height', type=int)
    args = parser.parse_args()

    if not args.output:
        args.output = get_output_path_from_input_path(args.input, postfix='_small')
    print('Output path: ' + args.output)

    desired_bitrate = calc_output_bitrate(
        convert_mb_to_kbit(args.output_size),
        get_video_duration_in_seconds(args.input),
        args.output_audio_bitrate)
    print('Calculated video bitrate: {} kbit/s'.format(desired_bitrate))

    # Based on https://trac.ffmpeg.org/wiki/Encode/H.264#twopass

    # 1st pass
    ffmpeg_args = {
        'pass': 1,
        'an': None,
        'c:v': 'libx264',
        'b:v': as_kbit_ffmpeg_arg(desired_bitrate),
        'format': 'mp4'
    }
    if args.output_height:
        ffmpeg_args['filter:v'] = 'scale=-1:{}'.format(args.output_height)

    stream = ffmpeg.input(args.input)
    stream = ffmpeg.output(
        stream,
        null_output_path(),
        **ffmpeg_args)
    stream = ffmpeg.overwrite_output(stream)
    print_ffmpeg_args(stream)
    out, _ = ffmpeg.run(stream, capture_stdout=True)
    print(out)

    # 2nd pass
    ffmpeg_args = {
        'pass': 2,
        'c:a': 'aac',
        'b:a': as_kbit_ffmpeg_arg(args.output_audio_bitrate),
        'c:v': 'libx264',
        'b:v': as_kbit_ffmpeg_arg(desired_bitrate),
        'format': 'mp4'
    }
    if args.output_height:
        ffmpeg_args['filter:v'] = 'scale=-1:{}'.format(args.output_height)
    stream = ffmpeg.input(args.input)
    stream = ffmpeg.output(
        stream,
        args.output,
        **ffmpeg_args)
    stream = ffmpeg.overwrite_output(stream)
    print_ffmpeg_args(stream)
    out, _ = ffmpeg.run(stream, capture_stdout=True)
    print(out)


if __name__ == '__main__':
    main()