[FFmpeg-user] Asking for help with filter chain concat troubles

[wilken]

Hello ffmpegs,

I hope I have come to the right place. I've got a problem concatenating 
webcam videos through an automatically generated filter chain.

The idea is to combine a series of webcam videos from a webconference, 
filling the gaps between webcam videos with still images.
The still images are the last frame of the previous video respectively.

The problem is that at sometimes, the concat command will start dropping 
all frames at some point during a video.
This seems to be related to one of the videos changing resolution 
mid-stream, as this changing point is where frames are starting to be 
dropped (or at least it seems that way to me).

Here is an example of the command in question:

ffmpeg -i 
35837_35835_0_c91a9aa8_89cb_4672_837f_3cf24ebc1a1d_image_video.mp4 -i 
medium-w_my42crqamdtp-1594284917876.webm
-i 35840_35839_0_2a59b51c_e889_47d7_91b0_76fa9a7ad1ae_image_video.mp4 -i 
medium-w_my42crqamdtp-1594284935540.webm
-i 35843_35842_0_2b508777_bb2f_4270_8864_0a62b4fb40b9_image_video.mp4 -i 
medium-w_my42crqamdtp-1594284947574.webm
-i 35846_35845_0_9b312ddc_3883_461e_bb03_36ac12c871fa_image_video.mp4 -i 
medium-w_my42crqamdtp-1594284964346.webm
-i 35849_35848_0_805c9ebd_ccd5_4f4f_8754_20c2f32afb41_image_video.mp4 -i 
medium-w_my42crqamdtp-1594284993207.webm
-i 35852_35851_0_463f246d_ad51_4592_a840_40ccebdf7d1f_image_video.mp4 
-filter_complex
"[0:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[b];
[1:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[c];
[2:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[d];
[3:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[e];
[4:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[f];
[5:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[g];
[6:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[h];
[7:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[i];
[8:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[j];
[9:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[k];
[10:v]scale=iw*min(1280/iw\,720/ih):ih*min(1280/iw\,720/ih),pad=1280:720:(ow-iw)/2:(oh-ih)/2,setdar=1.7777778[l];
[b][c][d][e][f][g][h][i][j][k][l]concat=n=11:v=1:a=0[v]" -map [v] -c:a 
flac -c:v libx264 -crf 10 -preset fast output.mp4

Running the command looks like this: https://pastebin.com/vW7K2SGG

Thanks for your time,
Arne Wilken