Write about CasparCG integration.

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@@ -96,4 +96,74 @@ the next frame will resume playing.
 Integration with CasparCG
 -------------------------
 
-TODO
+`CasparCG <http://casparcg.com/>`_ is an open-source broadcast graphics system,
+originally written by SVT, the Swedish public TV broadcaster. (In this
+context, “graphics” refers mostly to synthetically generated content,
+such as the score box in a sporting match.) With some coaxing, it is possible
+to integrate CasparCG with Nageru, so that Nageru does the mixing of the video
+sources and CasparCG generates graphics—CasparCG can also work as a standalone
+mixer indepedently of Nageru, but this will not be covered here.
+
+The most straightforward use of CasparCG is to use it to generate an overlay,
+which is then taken in as a video input in Nageru. To achieve this, the simplest
+solution is to send raw BGRA data over a UNIX domain socket [#rawvideo]_, which involves
+adding an FFmpeg output to your CasparCG configuration. This can either be done
+by modifying your casparcg.config to open up a socket in your home directory
+(you shouldn't use /tmp on a multi-user machine, or you may open up a security
+hole)::
+
+  <consumers>
+    <ffmpeg>
+      <device>1</device>
+      <path>unix:///home/user/caspar.sock</path>
+      <args>-c:v rawvideo -vf format=pix_fmts=bgra -f nut -listen 1</args>
+    </ffmpeg>
+    <system-audio></system-audio>
+  </consumers>
+
+or by setting it up on-the-fly through ACMP::
+
+  add 1 stream unix:///home/user/caspar.sock -vf format=pix_fmts=bgra -f nut -listen 1
+
+You can then use *unix:///home/user/caspar.sock* as a video input to Nageru on the
+same machine, and then use e.g. *OverlayEffect* to overlay it on your video chains.
+(Remember to set up the video as BGRA and not Y'CbCr, so that you get alpha.)
+
+CasparCG and Nageru does not run with synchronized clocks, so you will not get
+frame-perfect synchronization between graphics and overlay; however, this is normal
+even in a hardware chain, and most overlay graphics does not need to be timed
+to the input more than through a few frames. However, note that it is possible
+that Nageru lags behind CasparCG's graphics production after a while (typically
+on the order of hours) due to such clock skew; the easiest solution to this is
+just to use *change_rate(2.0)* or similar on the input, so that Nageru will consume
+CasparCG's frames as quickly as they come in without waiting any further.
+
+There's also one usability stumbling block: *CasparCG's FFmpeg
+streams are one-shot, and so are FFmpeg's UNIX domain sockets.* This means that,
+in practice, if Nageru ever disconnects from CasparCG for any reason, the socket
+is “used up”, and you will need to recreate it somehow (e.g., by restarting CasparCG).
+Also note that the old socket still lingers in place even after being useless,
+so you will _first_ need to remove it, and CasparCG does not do this for you.
+The simplest way to deal with this is probably to have a wrapper script of some
+sort that orchestrates Nageru, CasparCG and your client for you, so that everything
+is taken up and down in the right order; it may be cumbersome and require some
+tweaking for oyur specific case, but it's not a challenging problem per se.
+
+Nageru does not have functionality to work as a CasparCG client in itself,
+nor can your theme present a very detailed UI to do so. However, do note that
+since the theme is written in unrestricted Lua, so you can use e.g.
+`lua-http <https://github.com/daurnimator/lua-http>`_ to send signals
+to your backend (assuming it speaks HTTP) on e.g. transition changes.
+With some creativity, this allows you to at least bring some loose coupling
+between the two.
+
+In general, the integration between Nageru and CasparCG leaves a bit to be
+desired, and in general, the combination of CasparCG + Nageru will require
+a beefire machine than Nageru alone. However, it also provides a much richer
+environment for graphics, so for many use cases, it will be worth it.
+
+.. [#rawvideo] Good video codecs that support alpha are rare, so as long as CasparCG
+               and Nageru are running on the same machine, raw video is probably your
+               best bet. Even so, note that FFmpeg's muxers are not really made for
+               such large amounts of data that raw HD video produces, so there will
+               be some performance overhead on both sides of the socket.