Drop every even or odd frames using FFmpeg?

Question

Is there an accurate way to take raw video and extract from it a new video that contains only odd frames or even frames (by choice)?

For example:

I have "blah.yuv" with 400 frames (0-399). I want to create "blahOdd.yuv" that contains frames 1-399 (1,3,5,7...399) and "blahEven" that contains frames 0-398 (0,2,4,6...398).

Any ideas how to do it using only FFmpeg?

Solution 1

To work accurately, first convert the video to RAW YUV bitstream (if it is not already) by:

ffmpeg -i input.mp4 -an -vcodec rawvideo -pix_fmt yuv420p rawbitstream.yuv

Next step: The select filter takes an expression, where n is the frame number.

ffmpeg -r 2 -s WxH -i rawbitstream.yuv -filter:v select="mod(n-1\,2)" \
-c:v rawvideo -r 1 -format rawvideo -pix_fmt yuv420p -an odd.yuv

ffmpeg -r 2 -s WxH -i rawbitstream.yuv -filter:v select="not(mod(n-1\,2))" \
-c:v rawvideo -r 1 -format rawvideo -pix_fmt yuv420p -an even.yuv

To have ffmpeg not duplicate frames, you have to force half of the framerate of your input - so you set "2" as the input and "1" to the output. Don't forget to replace the WxH with the actual dimensions of your clip because the raw bitstream doesn't have a header that carries this information.

Instead of the above, another possibility would be to add the setpts filter to set new timestamps for the output. But be careful since it drops frames not accurately. Here, 25 is the actual output frame rate you want:

ffmpeg -i input.mp4 -filter:v select="mod(n-1\,2)",setpts="N/(25*TB)" \
-c:v rawvideo -r 12.5 -format rawvideo -pix_fmt yuv420p -an odd.yuv

ffmpeg -i input.mp4 -filter:v select="not(mod(n-1\,2))",setpts="N/(25*TB)" \
-c:v rawvideo -r 12.5 -format rawvideo -pix_fmt yuv420p -an even.yuv

---

You can of course choose another pixel format (any of ffmpeg -pix_fmts). Make sure that when reading the file you know the pixel size and pixel format:

ffmpeg -f rawvideo -s:v 1280x720 -pix_fmt yuv420p input.yuv …

Solution 2

If your ffmpeg was built with the AviSynth flag, then I believe you can pass an .avs file.

You can check by running ffmpeg and looking for --enable-avisynth in the configuration data.

If it's there you can use it like so: ffmpeg -i blahEven.avs blahEven.yuv.

Where blahEven.avs is simply:

ffvideosource("blah.yuv").SelectEven()

For odd frames, use SelectOdd().

For more advanded usage, see the SelectEvery documentation.

Comments

[slhck]

-vf is an alias of -filter:v. I can't reliably test it now, but will look into this later when I'm back on my machine. Maybe the tinterlace filter can do the same?

[Mark]

I've tried '-r 2 -i blah.yuv -r 1' but it shows me an error Option framerate not found.

[slhck]

Ah sorry.. Scratch that, this doesn't work (anymore?) and only for images.

[slhck]

I think I figured out why. If you use the select filter to drop frames, ffmpeg will still try to create a video with the original input framerate, thus doubling the frames. Try specifying half the frame rate, or using the setpts filter.

[Mark]

The ‘setpts="N/(25*TB)”’ gives me weird frame count. For instance (even): 2, 4, 6, 10, 12, 16, 20, 22 ... (no 8, 14, 18). However I’ve found another way based on the ‘mod(n-1\,2)’. First, in case of compressed file, decode it to raw YUV: ‘-i input.mp4 -an -vcodec rawvideo -pix_fmt yuv420p output.yuv’. With YUVs, input frame rate required, so it also works in this case :) So the second iteration will be ‘-r 2 -s WxH -i output.yuv -vf "select=mod(n-1\,2)" -r 1 -an -vcodec rawvideo -pix_fmt yuv420p outputEven.yuv’. If the file is already YUV, only the second is needed. Works like a charm :)

[slhck]

@Mark I'd be happy for you to suggest an edit to my answer and add the steps that worked for you, thereby correcting any mistakes I made. You can also post your own answer and mark it as accepted :)

[Mark]

This is an excellent alternative, but as I've mentioned - I'm limited to use only ffmpeg. The reason for that is it is a part of an automated system that will create only .bat files, no possibility for addition AVS.

[Mark]

Great, thanks! Very interesting the way it works, I'm new to this system of Q&A :)

[Louis Waweru]

@Mark I was thinking that if --enable-avisynth was there, that it meant AviSynth was built into ffmpeg, but I'm not sure about that.

[Anmol Singh Jaggi]

Can it not be done using the options -framerate/-r ?

[slhck]

@AnmolSinghJaggi No, this just drops frames, but I wouldn't be sure that it's so deterministic (i.e., drops every odd frame). It'll be based on time codes, which may not be accurate.