No filtering before deinterlacing.
ok just to make sure, you are not doing any filtering before the deinterlacing right? that should be line #2 with your source video input line #1 on your script.
Thanks to pinny, my problem is solved. pinny.
I will still have to read more about filter options (I already tested some options, and previously visible artifact is almost gone), but at least now I know where to start.