What container does FFV1 use? I can't find where this is at in documentation. It mentions the codecs but not corresponding containers when you are using extensions.

I've tried mkv, avi, mp4, etc for FFV1 but I always get

OpenCV: FFMPEG: tag 0x31766666/'ffv1' is not supported with codec id 33 and format '[container]'OpenCV: FFMPEG: fallback to use tag 0x31564646/'FFV1'

such as for mkv which everything tells me FFV1 often uses

OpenCV: FFMPEG: tag 0x31766666/'ffv1' is not supported with codec id 33 and format 'matroska / Matroska'OpenCV: FFMPEG: fallback to use tag 0x31564646/'FFV1'

The resulting MKV file is openable with a video player, and does look lossless, but the file is also not proper, thus no thumbnail or recognized information in properties.