split_video

Split and recode an mp4 video into evenly sized chunks.

Usage:

./split_video [--gop-size 30] [--chunk-size 120] [--skip 123]
              [--length 1200] input_file output_template

where

--gop-size   is the size of a group of pictures
--chunk-size is the size of a chunk in frames
--skip       are the number of frames to skip at the
             beginning of the input file
--length     are the number of frames to encode

Example:

./split_video --gop-size 25 --chunk-size 100 myfile.mp4 chunks/%05d.mp4

will split a video into chunks of size 100, with I-frames every 25 frames.

Notes

ffmpeg itself has been adding functionality for chunking video in recent versions.

For an input file at 25 fps, the following command almost produces 120 frame chunks:

ffmpeg -i input.mp4 -f segment -segment_time 4.8 \
    -segment_format_options movflags=+faststart \
    -force_key_frames '"expr:eq(mod(n,30),0)"' \
    -vcodec h264 -pix_fmt yuv420p \
    -crf 18 \
    -flags +global_header \
    chunks/%05d.mp4

However, I found that occasionally, 119 or 121 frame chunks would be produced, and for my needs, I wanted all chunks to be exactly 120 frames. So I wrote this tool.

Caveats

1. audio information is not preserved
2. only tested on mp4 files, and makes some mp4 specific assumptions
3. assumes fixed frame rate encoding
4. only outputs I and P frames (no B frames)

The code could also use some cleanup, but it works for my use case.

Future Work

I have no plans to extend this right now, but I'll gladly accept pull requests which update the code. Possible extensions

• include audio
• generalize to other video codecs, variable rate encoding
• for mp4, output B frames

Sources

Based on ffmpeg sample code. BSD-style license (see License.txt).