CODEWITHSUNDEEP
ffmpegh.264filesizeframe-rate
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Reputation: 449

ffmpeg drop frames on purpose to lower filesize

Our security system records and archives our IP cameras streams with ffmpeg -use_wallclock_as_timestamps 1 -i rtsp://192.168.x.x:554/mpeg4 -c copy -t 60 my_input_video.avi

I run it with crontab every minute so it creates videos of 60 seconds (~15Mb) for each camera every minute. When an intrusion occurs, the camera sends a picture through FTP and a script called by incrontab:

1- forwards immediately the picture by email

2- selects the video covering the minute the intrusion occured, compress it with h264 (to ~2,6Mb) and sends it by email

It is working really well but if a thief crosses the path of various cameras, the connection to the SMTP server is not fast enough so video emails are delayed. I'd like to compress the videos even more to avoid that. I could lower the resolution (640x480 to 320x240 for example) but sometimes 640x480 is handy to zoom on something which looks to be moving...

So my idea is to drop frames in the video in order to lower the filesize. I don't care if the thief is walking like a "stop motion Lego" on the video, the most important is I know there is someone so I can act.

mediainfo my_input_video.avi says Frame rate = 600.000 fps but it is of course wrong. FPS sent by IP cameras are always false because it varies with the network quality; this is why i use "-use_wallclock_as_timestamps 1" in my command to record the streams.

with ffmpeg -i my_input_video.avi -vcodec h264 -preset ultrafast -crf 28 -acodec mp3 -q:a 5 -r 8 output.avi the video is OK but filesize is higher (3Mb)

with ffmpeg -i my_input_video.avi -vcodec h264 -preset ultrafast -crf 28 -acodec mp3 -q:a 5 -r 2 output.avi the filesize is lower (2,2Mb) but the video doesn't work (it is blocked at the first frame).

Creating a mjpeg video (mjpeg = not interlaced frames) in the middle of the process (first exporting to mjpeg with less frames and then exporting to h264) creates same results.

Do you know how I can get my thief to walk like a "stop motion Lego" to lower the filesize to a minimum?

Thanks for any help

Upvotes: 2

Views: 7053

Answers (1)

jeremy
jeremy

Reputation: 4314

What are your constraints file size wise? 2.6MB for 60 seconds of video seems pretty reasonable to me, thats about 350kbps, which is pretty low for video quality.

You need to specify the video bitrate -b:v 125000 (125kbps, should drop you to about 900kb) to control the bitrate/s you want the video encoded at. Your not giving FFMpeg enough hints as to how you want the video handled, so its picking arbitrary values you don't like. As you drop the frame rate, its just using up the buffers allocating more bits to each frame. (one big thing you need to keep in mind with this is, as you stretch the video out over a longer time period the more likely the scene will change significantly require an I frame (full encoded frame vs frame based on previous frame) so reducing the frame rate will help some, but may not help as much as you'd think).

Your "(it is blocked at the first frame)." is most likely an issue with you trying to start decoding a stream when it is not at an I frame and not an issue with your settings.

Upvotes: 1

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