67 lines
2.1 KiB
Markdown
67 lines
2.1 KiB
Markdown
# H264 transcoder with RPi4 hardware acceleration
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This is a shell script to transcode video files to h.264.
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It handles audio and subtitles gracefully.
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It uses the Raspberry Pi's hardware acceleration
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(`h264_v4l2m2m` codec)
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to encode the h.264 files.
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I don't know if the ffmpeg code is immature,
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or if I just don't understand ffmpeg,
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but this codec demands some special treatment,
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which this shell script provides.
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# `h264_v4l2m2m` Special Treatment
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Here are the `h264_v4l2m2m` quirks I've identified
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(February 2023):
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* The codec will not write into a `.mkv` container,
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but it will write into a `.mp4` or `.nut` container.
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* The codec does not play well with subrip subtitles,
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making it look like you don't have enough capture buffers.
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You will never have enough capture buffers:
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you have to stop trying to copy over subtitles.
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* `-crf` value, if set, is ignored:
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you must use `-b:v`.
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The solution this script uses is to
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transcode video and audio
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into an intermediate `.nut` file.
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Then it combines the `.nut` file,
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and any subtitles from the original file,
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into a new `.mkv` file.
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Since the "copy" video codec is used in the second step,
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everything works properly.
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Doing things in two steps adds a little bit of time,
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but in my experience,
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the hardware transcoder is so much faster,
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the second step time isn't a problem.
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I see ffmpeg reporting running at `speed=84x` and higher
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on the second step.
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# Thanks
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Thank you to Will Usher,
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who figured out how to get the codec working at all.
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Without Will's
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[helpful blog post](https://www.willusher.io/general/2020/11/15/hw-accel-encoding-rpi4)
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I would still be fighting with ffmpeg trying to make it use hardware acceleration.
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Will also wrote the
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[FBED](https://github.com/Twinklebear/fbed) batch transcoder,
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very similar to this.
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You might try it out if you don't like this script.
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I created this simpler shell script mainly
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to avoid Python package dependencies:
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FBED needed an ffmpeg library that I had to patch
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before it would run.
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I wound up having to fiddle with ffmpeg so much
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dealing with the codec quirks,
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that working in Bourne shell
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made things much simpler
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as I chased down the subtitle problems.
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