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I know... there are a lot of questions out there already about reducing the file size with ffmpeg.
But, here's why my question is a bit different, it addresses having difficult content. I think that there's an issue with the significant area of moving sea water in my source video.
From an h624 video off the phone at 35MB, I got a 'compressed' video of 60MB, with

ffmpeg -i water.mp4 -vcodec libx265 -crf 28 small_water.mp4

An even simpler example also produced a much larger output.

ffmpeg -i water.mp4 new_water.mp4

So my question is, what do you do when the video contains a significant amount of fast moving background artefacts? Normal methods do not seem to reduce the size, but I'm assuming, there must be something that works. I suppose it's a similar problem to one with video of F1 racing where the video pans across the spectators while following the cars.

Background; I was converting several phone videos to h625 to shrink them for posting on Whatsapp, all the others shrank as expected, except the 'water' one which doubled in size. So more out of interest for future reference, I'd like to know if there's a solution.

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So my question is, what do you do when the video contains a significant amount of fast moving background artefacts?

Leave it be and do not reencode. You could

  • Reduce its resolution or/and framerate
  • For videos with a lot of noise you could try using a noise removal filter
  • For videos with a lot of motion you could use AviSynth filters to stabilize them

Normal methods do not seem to reduce the size, but I'm assuming, there must be something that works.

H.264 is not as bad as you think it is. Secondly, hardware video encoders introduce their own artefacts which are very difficult to re-encode. Normally you need the source uncompressed video to compress it efficiently.

I've dealt with a situation like yours many times.

https://forum.doom9.org and https://forum.videohelp.com are two primary forums where where people discuss such issues.

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