How to create Frame Sequential / Alternate frames 3D video from two separate one

Question

Platform is Linux, CLI tools is needed.

There are two files: one for left eye another for right eye. I search a way to compose it into single 3D MKV (or any other container) with alternate frame format.

I tried FFmpeg filter framepack and MKV metadata stereo_mode=block_lr:

ffmpeg -i input_l.mkv -i input_r.mkv -filter_complex framepack=frameseq -metadata stereo_mode=block_lr -y out.mkv

But result is short, not all frames repacked/remuxed: input files contains 500 frames, out.mkv contains only 32 one and has doubled FPS.

I expect, that output file will contain 1000 frames.

Any tools are welcomed.

Another related question: is it possible to do such muxing without frame re-encoding? Input files already well-coded and I want just repack them into 3D video format.

PS FFmpeg version 4.2.1

Answer 1

framepack is buggy filter. Use framepack=frameseq,settb=1/50,setpts=N, where 50 is frame rate you want to use, and it should be twice of input framerate.