command = [
    "ffmpeg",
    "-loglevel", "error",
    "-y",
    "-pix_fmt", "gray",
    "-s", "%dx%d" % (data.shape[1], data.shape[0]),
    "-r", "30",
    "-c:v", "rawvideo",
    "-f", "rawvideo",
    "-i", "-", # Input from stdin
    "-crf", str(crf),
    "-c:v", "libx264",
    "-preset", preset,
    "-f", "matroska",
    "-threads", "1"
    "-" # Output to stdout
]