Document the /dev/null special case.

[nageru-docs] / futatabi.rst

Futatabi: Real-time instant replay with slow motion
===================================================

Futatabi (after the Japanese word *futatabi*, 再び, meaning “again” or
“for the second time”) is a system for instant replay. Even though Futatabi
is meant to be used with Nageru, shares some code with it and is built from
the same source distribution, it is a separate
application. Futatabi is meant for slow motion for e.g. sports broadcasts, but
can also be used as a more generic multitrack recorder for later editing.

Futatabi supports *interpolated* slow motion, meaning you do not need to buy
a high-speed 120 fps camera to get smooth 60 fps output after slowdown—Futatabi
will automatically synthesize in-between frames for you during playback.
Good interpolation, especially in realtime, is a difficult problem, and not all
content will do equally well. Most content should do quite acceptably, especially
considering that half of the frames will actually be originals, but you
will need to see in practice what actually works for you. Interpolation can
also be used for frame rate conversion (e.g. 50 to 59.94 fps).

Futatabi currently uses a GPU reimplementation of
`Fast Optical Flow using Dense Inverse Search (DIS) <https://github.com/tikroeger/OF_DIS>`_
by Kroeger et al, together with about half of the algorithm from
`Occlusion Reasoning for Temporal Interpolation Using Optical Flow <https://www.microsoft.com/en-us/research/publication/occlusion-reasoning-for-temporal-interpolation-using-optical-flow/>`_
(to do the actual interpolation based on the estimated
optical flow), although this may change in the future.

Since Futatabi is part of the Nageru source distribution, its version number
mirrors Nageru. Thus, the first version of Futatabi is version 1.8.0,
which is the Nageru version when it was first introduced.


System requirements
-------------------

It is strongly recommended to run Futatabi on a separate machine from Nageru,
not the least because you probably want a different person to operate the replay
while the producer is operating Nageru. (See :ref:`coop`.)

Like Nageru, Futatabi uses your GPU for nearly all image processing.
However, unlike Nageru, Futatabi requires a powerful GPU for interpolation;
a GTX 1080 or similar is recommended for interpolated 720p60. (Futatabi
was initially developed on a GTX 950, which is passable, but has little
performance margin.) If you have a slower GPU and are happy with worse
quality, or just wish to test, you can use a faster preset, or turn off
interpolation entirely. Futatabi requires OpenGL 4.5 or newer.

For other required libraries, see :ref:`compile`; when you build Nageru,
you also build Futatabi.


Getting started
---------------

Futatabi always pulls data from Nageru over the network; it doesn't support SDI
input or output. Assuming you have a recent version of Nageru (typically one
that comes with Futatabi), it is capable of sending all of its cameras as one
video stream (see :ref:`futatabiformat`), so you can start Futatabi with

  ./futatabi http://path.to.nageru.host/multicam.mp4

If you do not have a running Nageru installation, see :ref:`sampledata`.

Once you are up and connected, Futatabi will start recording all of the frames
to disk. This process happens continuously for as long as you have disk space,
even if you are playing something back or editing clips, and the streams are
displayed in real time in mini-display as they come in. You make replays in
the form of *clips* (the top list is the clip list), which are then queued into
the *playlist* (the bottom list). Your end result is a live HTTP stream that
can be fed back into Nageru as a video input; by default, Futatabi listens
on port 9096.

Futatabi has the concept of a *workspace*, which defaults to your current
directory (you can change it with the -d option). This holds all of the
recorded frames, all metadata about clips, and preferences set from the menus
(interpolation quality and cue point padding).  If you quit Futatabi and
restart it (or you go down as the result of a power failure or the likes),
it will remember all the state and frames for you. As a special case,
if you give */dev/null* in place of an input URL or file, you can keep using
your workspace without getting any new frames added.

.. _basicui:

Basic UI operations
'''''''''''''''''''

Nageru can be operated using either the keyboard and mouse, or using a MIDI
controller with some help of the mouse. In this section, we will be discussing
the keyboard and mouse only; see :ref:`midi` for details on using MIDI
controllers.

A clip in the clip list consists simply of an in and an out point; it represents
an interval of physical time (although timed to the video clock). A clip in the
playlist contains the same information plus some playback parameters, in particular
which camera (stream) to play back.

Clicking the ”Cue in” button, or pressing the A key, will start a new clip in
the clip list that begins at the current time. Similarly, clicking the ”Cue
out” button, or pressing the S key, will set the end point for said clip.
(If you click a button twice, it will overwrite the previous result.)
Now it is in a state where you can queue it to the play list (mark the camera
you want to use and click the “Queue” button, or Q on the keyboard) for
playing, or you can preview them using the “Preview” button (W on the
keyboard). Previewing can be done either from the clip list the clip list or
the playlist; they will not be interpolated or faded, but will be played back
in the right

You can edit cue points, both in the clip and the playlist, in two ways:
Either use the scroll wheel on the mouse, or hold down the left mouse button
and scrub left or right. (You can hold down Shift to change ten times as fast,
or Alt to change at one-tenth the speed.) You'll see the new cue points as you
change them in the preview window. You can also give clips names; these don't
mean anything, but can be good references to locate a specific kind of clip
for later use. Once you're happy with a playlist, and your producer is ready
to cut to your channel, click on the first clip you want to play back and
click the “Play” button (space on the keyboard); the result will both be
visible in the top screen and go out live over the network to Nageru.

.. _coop:

Working with your producer
''''''''''''''''''''''''''

Generally, we recommend that the producer (Nageru operator) and slow motion
operator (Futatabi operator) sit closely together and can communicate verbally.
Good cooperation between the two is essential to get a good final product;
especially the switches to and from the replays can be a critical point.

The general rule for working together is fairly obvious: The producer should
switch to a replay when there's something to show, and switch away when there's
nothing more to show (or, less ideally, when something live takes priority).
Generally, when you have a playlist ready, inform your producer; they will
count you in (three, two, one, go). At one, start playing so that you have some
margin. If the Nageru theme is set up correctly (see :ref:`talkback`), they will
know how much is queued up so that they can switch back before the replay runs
out, but it doesn't hurt to give a warning up-front. The producer might also
be able to request replays of specific events, or ask you to save something
for later if they can't show it right now (e.g. a foul situation that wasn't called).

Replay workflows
----------------

On top of the basics outlined in :ref:`basicui`, there are many possible
workflows; we'll discuss only two. Try out a few and see which ones fit your
style and type of event.

Repeated cue-in
'''''''''''''''

In many sports, you don't necessarily know when a replay-worthy event has happened
before it's already happened. However, you may reasonably know when something is
*not* happening, and it would be a good time to start a clip if something is happening
immediately afterwards. At these points, you can make repeated cue-ins, ie., start
a clip without finishing it. As long as you keep making cue-ins, the previous one
will be discarded. Once you see that something *is* happening, you can wait until
it's done, and then do a cue-out, which gives you a good clip immediately.

For instance, in a basketball game, you could be seeing a series of uninteresting
passes, clicking cue-in on each of them. However, once it looks like there's an
opportunity for a score, you can hold and see what happens; if the shot happens,
you can click cue-out, and if not, you can go back.

Before playing the clip, you can make adjustments to the in and out points
as detailed above. This will help you trim away any uninteresting lead-ups,
or add more margins for fades. If you consistently find that you have too
little margin, you can use the *cue point padding* feature (either from the
command line using *--cue-point-padding*, or set from the menu). If you set
cue point padding to e.g. two seconds, the cue-in point will automatically be set
two seconds ago when you cue-in, and the cue-out point will be set two seconds
into the future when you cue-out.


Instant clips
'''''''''''''

Like the previous section explained how you generally would know the *start*
of an interesting event (at least if discarding most of the candidates),
you would be even more sure about the *end* of one. Thus, you can simply wait
until something interesting has happened, and then click cue-in immediately
followed by cue-out. This will give you a clip of near zero length, ending
at the right point. Then, edit this clip to set the starting point as needed,
and it's ready to play.

Again, you can use the cue point padding feature to your advantage; if so,
your clips will not be of zero length, but rather of some predefined length
given by your chosen cue point padding.


.. _sampledata:

Sample multicamera data
'''''''''''''''''''''''

Good multicamera sample video is hard to come by, so it can be hard to
test or train before an actual event. To alleviate this, I've uploaded some
real-world video from the very first event where an early version of Futatabi
was tested. (There are some issues with the JPEG quality, but it should
largely be unproblematic.) You are free to use these for training or
demonstration purposes. Do note that they will not be displayed entirely
correctly in most video players (see :ref:`futatabiformat`), although
they will certainly be watchable.

There are two files:

 * `Trøndisk 2018, finals only (MJPEG, 77 GB) <https://storage.sesse.net/trondisk2018-finals-multicam-mjpeg.mkv>`_:
   The final match, in MJPEG format (about 30 minutes). This can be downloaded
   and then fed directly to Nageru as if it were a real camera stream
   (remember the --slow-down-input option).
 * `Trøndisk 2018, entire tournament (H.264, 74 GB) <https://storage.sesse.net/trondisk2018-multicam-h264.mp4>`_: The entire tournament,
   with no cuts (about 12 hours). However, due to space and bandwidth
   constraints, it has been transcoded to H.264 (with some associated
   quality loss), and needs to be transcoded to MJPEG before Nageru can use it.

Both files are mixed-resolution, with some cameras at 1080p59.94 and some
at 720p59.94 (one even switches between matches, as the camera was replaced).
They contain four different camera angles (overview camera on crane, detail camera
in tripod, two fixed endzone overhead cameras) with differing quality depending
on the camera operators. In short, they should be realistic input material
to practice with.


Transferring data to and from Nageru
------------------------------------

.. _futatabiformat:

Video format specification
''''''''''''''''''''''''''

Futatabi expects to get data in MJPEG format only; though MJPEG is old,
it yields fairly good quality per bit for an intraframe format, supports
4:2:2 without too many issues, and has hardware support through VA-API
for both decode (since Ivy Bridge) and encode (since Skylake). The latter
is especially important for Futatabi, since there are so many high-resolution
streams; software encode/decode of several 1080p60 streams at the same time
is fairly taxing on the CPU if done in software. This means we can easily
send 4:2:2 camera streams back and forth between Nageru and Futatabi without having
to scale or do other lossy processing (except of course the compression itself).

However, JPEG as such does not have any way of specifying things like color
spaces and chroma placement. JFIF, the *de facto* JPEG standard container,
specifies conventions that are widely followed, but they do not match what
comes out of a capture card. Nageru's multicam export _does_ set the appropriate
fields in the output Matroska mux (which is pretty much the only mux that can
hold such information), but there are few if any programs that read them and give
them priority over JFIF's defaults. Thus, if you want to use the multicam stream
for something other than Futatabi, or feed Futatabi with data not from Nageru,
there are a few subtle issues to keep in mind.

In particular:

  * Capture cards typically send limited-range Y'CbCr (luma between 16..235
    and chroma between 16..240); JFIF is traditionally full-range (0..255
    for both). (See also :ref:`synthetictests`.) Note that there is a special
    private JPEG comment added to signal this, which FFmpeg understands.
  * JFIF, like MPEG, assumes center chroma placement; capture cards and most
    modern video standards assume left.
  * JFIF assumes Rec. 601 Y'CbCr coefficients, while all modern HD processing
    uses Rec. 709 Y'CbCr coefficients. (Futatabi does not care much about
    the actual RGB color space; Nageru assumes it is Rec. 709, like for capture
    cards, but the differences between 601 and 709 here are small. sRGB gamma
    is assumed throughout, like in JFIF.)

Many players may also be confused by the fact that the resolution can change
from frame to frame; this is because for original (uninterpolated) frames,
Futatabi will simply output the received JPEG frame directly to the output
stream, which can be a different resolution from the interpolated frames.

Finally, the subtitle track with status information (see :ref:`talkback`) is
not marked as metadata due to FFmpeg limitations, and as such will show up
raw in subtitle-enabled players.


.. _midi:

Using MIDI controllers
----------------------


Monitoring
----------

.. _talkback:

Tally and status talkback
'''''''''''''''''''''''''

Prometheus metrics
''''''''''''''''''