MP4 video ----- fast forward the MOTION parts ?

I'm guessing what you mean is somehow speeding up only parts of a video, based on changes in frame density... percentage of frame change between frames.

If this is a one time operation, just use a video play which allows you to speed up videos, like VLC.

If you're talking about some repetitive task, where you have to do this same processing every day for 8 hours of video, then easiest approach will be to use an ffmpeg script to locate video slices to speed up, then use ffmpeg to re-transcode the entire 8 hours, speeding up parts identified by first script.

Would take a good bit of time to get your script working + once working, likely will work well for every 8 hour slice of footage.

it's possible to do this retrospectively. The idea is you ask software to check each frame and delete any duplicates.  In theory you will only capture movement.  It's much easier to get camera hardware that uses movement detection to activate so the video stream onl contains "interesting" stuff.

You'll need to get to grips with ffmpeg.  There's a bit o a learning curve but if you you video on a regular basis it's worth it.

ffmpeg has a comand line option called "mpdecimate" which does exactly this.

It doesn't really fast-forward - it makes a second file containing only the "interesting" stuff!

Speedup/Slowdown videos are the ffmpeg docs about how to do this.

From your description, a likely workflow will be...

1) Slice up footage into slices, some of which will stay at normal speed. Some will speed up.

Note: Be sure to do your slice with the copy operator for both audio (if you have any) + video.

This will preserved 100% raw bitrate in slices + be very fast. I you do a transcode in this step processing 8 hours of footage may take many days or weeks, depending on many factors.

2) Transcode all normal speed videos into the target container type for your output footage, likely .mp4 or .mkv will be what you use.

In most cases, .mp4 will be best. Might be your footage is already .mp4 so maybe no transcode at all here.

If you use the copy operator, transcodes will be fast.

3) Transcode your speedup footage slices, using the above link.

4) Use the ffmpeg concat operation to merge all the slices together.

5) At this point you have your final footage.

6) If you must store your footage for long amounts of time, then do an HEVC (h.265) transcode on the final footage to create archival footage.

HEVC produces highly compressed, high quality footage at the expensive of a massive amount of time.

If you must do HEVC compression of long stretches of time, like 8 hours of footage, you'll require a machine with many hyperthreaded cores. Also choose the fastest preset (ultrafast to medium) which allows quality you require, where transcodes run in < 24hours...

Because... for an HEVC transcode of 8 hours of footage to finish before the next transcode must start, will require correct selection of hardware + ffmpeg preset to use.