FFMPEG 4.0 skipping frames

Question

I have noticed today that when I use ffmpeg 4.0 stable to decode some TS files into y4m or ffv1 it occasionally skips some frames. My input file has all the frames burned into the picture and I am extracting some frames at regular intervals. Sometimes it works just fine without any skipped frames but sometimes it skips one or two frames. I am using the following command to decode my input:

ffmpeg -i input.ts -vsync 0 output.y4m

and I am extracting the frames using:

ffmpeg -ss xxx -i output.y4m -vframes 1 xxx.png

I am substituting xxx with 0, 100, 200, etc and sometimes I see 1-2-3 frame dropped which is very annoying since I am trying to evaluate the PSNR afterwards and this is messing up the whole PSNR/SSIM evaluation.

[EDIT] I have executed the command which @Gyan suggested:

ffmpeg -v 99 -loglevel 99 -i test.ts \
-c:v rawvideo -vsync 0 \
-enc_time_base 1/1000 output.nut &> test.log

And here is the log file test.log

Answer 1

Y4M has no timestamps, only framerate in its header. So, if your source has any variability in its framerate, you'll see a shift in apparent timestamp in the Y4M.

e.g.

n     src     y4m
0       0       0
1    0.04    0.04 
2    0.07    0.08
3    0.12    0.12
4    0.21    0.16
5    0.24    0.20
6    0.27    0.24 
...

Over a long period, these perturbations can add up. Here frame #6 in src is at TS of src #5 in Y4M.

Edit: the sample TS file has VFR timestamps.

Use -c:v rawvideo -vsync 0 -vf setpts=N/FRAME_RATE/TB -an and save to .nut.