How to transition smoothly and repeatedly between two videos using command line tools?

Question

I want to output a video using two videos as input, where these two videos fade (or dissolve) into each other in a smooth and repetitive manner, every second or so. I'm assuming a combination of ffmpeg with melt, mkvmerge, or another similar tool might produce the effect I'm after. Basically, I want to use ffmpeg to cut up video A according to a specific interval, discarding every second cut up (automatically). Likewise for video B, but in this case inverting the process to retain the discarded parts. I wish to then interweave these parts.

The file names should be correctly formatted so that I can then concatenate the result using a wild card command argument or batch processing list, as per one of the aforementioned tools. The transition effect (e.g. a "lapse dissolve") isn't absolutely necessary, but it would be great if there were a filter to achieve that too. Lastly, it would also be great if this process could be done with little to no re-encoding, to preserve the video quality.

I've read through this thread and the Melt Framework documentation, in addition to the ffmpeg manual.

Answer 1

Assuming both videos have the same resolution and sample aspect ratio, you can use the blend filter in ffmpeg.

A couple of examples,

---

ffmpeg -i videoA -i videoB -filter_complex \
       "[0][1]blend=all_expr=if(mod(trunc(T),2),A,B);\
        [0]volume=0:enable='mod(trunc(t+1),2)'[a]; [1]volume=0:enable='mod(trunc(t),2)'[b];\
        [a][b]amix"  out.mp4

Straight cuts.

Output:

time,  in seconds,
[0,1) -> videoB
[1,2) -> videoA
[2,3) -> videoB
...
[2N  ,2N+1) -> videoB
[2N+1,2N+2) -> videoA

---

ffmpeg -i videoA -i videoB -filter_complex \
       "[0][1]blend=all_expr='if(mod(trunc(T/2),2),min(1,2*(T-2*trunc(T/2))),max(0,1-2*(T-2*trunc(T/2))))*A+if(mod(trunc(T/2),2),max(0,1-2*(T-2*trunc(T/2))),min(1,2*(T-2*trunc(T/2))))*B';\
        [0]volume='if(mod(trunc(t/2),2),min(1,2*(t-2*trunc(t/2))),max(0,1-2*(t-2*trunc(t/2))))':eval=frame[a]; [1]volume='if(mod(trunc(t/2),2),max(0,1-2*(t-2*trunc(t/2))),min(1,2*(t-2*trunc(t/2))))':eval=frame[b];\
        [a][b]amix"  out.mp4

Each input's video/audio for 2 seconds with a 0.5 second transition.

Output:

time,  in seconds,
[0,0.5) -> videoA fades out 1 to 0 + videoB fades in from 0 to 1
[0.5,2) -> videoB
[2,2.5) -> videoB fades out 1 to 0 + videoA fades in from 0 to 1
[2.5,4) -> videoA
[4,4.5) -> videoA fades out 1 to 0 + videoB fades in from 0 to 1
[4.5,6) -> videoB
[6,6.5) -> videoB fades out 1 to 0 + videoA fades in from 0 to 1
[6.5,8) -> videoA
...
[4N    ,4N+0.5) -> videoA fades out 1 to 0 + videoB fades in from 0 to 1
[4N+0.5,4N+2)   -> videoB
[4N+2  ,4N+2.5) -> videoB fades out 1 to 0 + videoA fades in from 0 to 1
[4N+2.5,4N+4)   -> videoA

Answer 2

Your question is quite vague. As you've acknowledged you've started with some possible tools and got stuck or overwhelmed. Therefore I'd like to answer with a direction and not a final solution.

MLT is the one tool you will need. Under the hood it uses ffmpeg and other tools. But you can put all of the configuration you need into an MLT (XML) configuration file.

If you look at the top of the melt framework's webpage you see a note:

The easiest way to try out and learn MLT is by downloading Shotcut

I strongly suggest you do this and spend a little time getting familiar with Shotcut. Unlike other editors based on MLT, Shotcut directly uses MLT configuration files as its project files.

I suggest you use Shotcut to merge togeather two videos and alternate clips from each. Then save the project and open it in a text editor. You can then use that as an example of how to construct your own MLT config file.