Thank you all for the suggestions I solved my particular problem the following way
open(`ffmpeg -f rawvideo -pix_fmt gray -s:v 5120x5120 -r 70 -i pipe:0 test.mkv`, "w") do out
for frame in video
img = convert(Image{Gray{U8}}, frame)
write(out, reinterpret(UInt8, data(img)))
end
end
It is not terrible performant but it get’s the job done for now.