Selecting one every n frames from a video using ffmpeg

ffmpeg

I was trying to extract one every N frames from a video using ffmpeg. I tried using this command:
ffmpeg -i input.mp4 -vf "select=not(mod(n\,10))" 1_every_10/img_%03d.jpg

I wanted to verify that it is working as expected. So I extracted all frames using:
ffmpeg -i input.mp4 -vf "select=not(mod(n\,1))" all/img_%03d.jpg

And then I tried to see if 2nd image from the first command matches image number 20 from the second command, and it didn't match. Confirmed both visually and using diff command like

diff all/img_020.jpg 1_every_10/img_002.jpg Binary files all/img_020.jpg and 1_every_10/img_002.jpg differ

Anyone knows what could be going on? Thanks!

Best Answer

The image2 muxer defaults to constant frame rate. So if the input is, say, 30 fps, and you select every 10th frame i.e. frames with timestamp 0s, 0.33s, 0.66s.. then ffmpeg will duplicate frames to match the input rate so duplicate 9 frames for every input frame.

Way to avoid that is to set video sync method to passthrough or variable frame rate

e.g.

ffmpeg -i input.mp4 -vf "select=not(mod(n\,10))" -vsync vfr 1_every_10/img_%03d.jpg

This can potentially affect the full extraction if the input is VFR. So, use

ffmpeg -i input.mp4 -vf select -vsync vfr all/img_%03d.jpg

Related Solutions

FFMPEG extract intra-frames I,P,B frames

To extract a certin type of frame use the select filter:

select=eq(pict_type\,<x>)

where <x> is one of the following: pict_type (video only) the type of the filtered frame, can assume one of the following values: I, P, B, S, SI, SP, BI

So for example:

ffmpeg -i <inputfile> -vf '[in]select=eq(pict_type\,B)[out]' b.frames.mp4

To understand more about the output, add the showinfo filter:

ffmpeg -i <inputfile> -vf '[in]select=eq(pict_type\,B),showinfo[out]' b.frames.mp4

FFmpeg – How to Extract Timestamps with the -r Option

What you can do is "simulate" the image writing process by filtering with the fps filter, then using ffprobe to show the timestamps of the generated frames. This means that at 25 fps, the 50th frame (like your 50th image) will have a PTS of 2.00 seconds.

You do it like this:

ffprobe -f lavfi -i "movie=input.mp4,fps=fps=25[out0]" -show_frames -show_entries frame=pkt_pts_time -of csv=p=0

Will output:

0
0.04
0.08
0.12
0.16
...

These are the timestamps for each output image. You can actually combine the list of frames and the timestamps:

ls -1 image-*.jpeg > images.txt
ffprobe -f lavfi -i "movie=input.mp4,fps=fps=25[out0]" -show_frames -show_entries frame=pkt_pts_time -of csv=p=0 > frames.txt
paste images.txt frames.txt > combined.txt

Will create a file with:

image-0001.jpeg 0
image-0002.jpeg 0.04
image-0003.jpeg 0.08
image-0004.jpeg 0.12

Note that this may result in extraneous lines if there are too many frames or too many lines of info output. It seems a little inaccurate there.

Best Answer

Related Solutions

FFMPEG extract intra-frames I,P,B frames

FFmpeg – How to Extract Timestamps with the -r Option

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