it _nearly_ works after I recompiled ffmpeg locally.

but end of file sadly black frames?

FFStream::encode_frame: encode failed.

file: /data/data/com.termux/files/home/av1_.webm

  err: Try again

FFMPEG::mux_video  err: Operation not permitted          FFStream::encode_frame: encode failed.

file: /data/data/com.termux/files/home/av1_.webm

  err: Try again                                                                        FFStream::flush failed

:file:/data/data/com.termux/files/home/av1_.webm      err: Operation not permitted                                                Svt[error]: deinit called without sending EOS!                  Render::render_single: Session finished.                          ** rendered 336 frames in 149.076 secs, 2.254 fps