Start documenting Futatabi.

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+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 (with matching version numbers), 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, although this may change in the future.
+
+
+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.
+
+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 :doc:`compile`; when you build Nageru,
+you also compile Futatabi.
+
+
+Getting started
+---------------
+
+Sample multicamera data
+'''''''''''''''''''''''
+
+Transferring data to and from Nageru
+------------------------------------
+
+Video format specification
+''''''''''''''''''''''''''
+
+Monitoring
+----------
+
+Tally and status talkback
+'''''''''''''''''''''''''
+
+Prometheus metrics
+''''''''''''''''''

diff --git a/index.rst b/index.rst
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    video
    html
    monitoring
+   futatabi
 
 :ref:`search`