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+# External Libraries in Open Screen
+
+Currently, external libraries are used exclusively by the standalone sender and
+receiver applications, for compiling in dependencies used for video decoding and
+playback.
+
+The decision to link external libraries is made manually by setting the GN args.
+For example, a developer wanting to link all the libraries for the standalone
+sender and receiver executables might add the following to `gn args out/Default`:
+
+```python
+is_debug=true
+have_ffmpeg=true
+have_libsdl2=true
+have_libopus=true
+have_libvpx=true
+```
+
+On some versions of Debian, the following apt-get command will install all of
+the necessary external libraries for Open Screen:
+
+```sh
+sudo apt-get install libsdl2-2.0 libsdl2-dev libavcodec libavcodec-dev
+                     libavformat libavformat-dev libavutil libavutil-dev
+                     libswresample libswresample-dev libopus0 libopus-dev
+                     libvpx5 libvpx-dev
+```
+
+Similarly, on some versions of Raspian, the following command will install the
+necessary external libraries, at least for the standalone receiver. Note that
+this command is based off of the packages linked in the [sysroot](sysroot.gni):
+
+```sh
+sudo apt-get install libavcodec58=7:4.1.4* libavcodec-dev=7:4.1.4*
+                     libsdl2-2.0-0=2.0.9* libsdl2-dev=2.0.9*
+                     libavformat-dev=7:4.1.4*
+```
+
+Note: release of these operating systems may require slightly different
+packages, so these `sh` commands are merely a potential starting point.
+
+Finally, note that generally the headers for packages must also be installed.
+In Debian Linux flavors, this usually means that the `*-dev` version of each
+package must also be installed. In the example above, this looks like having
+both `libavcodec` and `libavcodec-dev`.
+
+## Standalone Sender
+
+The standalone sender uses FFMPEG, LibOpus, and LibVpx for encoding video and
+audio for sending. When the build has determined that [have_external_libs](
+  ../../cast/standalone_sender/BUILD.gn
+) is set to true, meaning that all of these libraries are installed, then
+the VP8 and Opus encoders are enabled and actual video files can be sent
+to standalone receiver instances. Without these dependencies, the standalone
+sender cannot properly function (contrasted with the standalone receiver,
+which can use a dummy player).
+
+## Standalone Receiver
+
+The standalone receiver also uses FFMPEG, for decoding the video stream encoded
+by the sender, and also uses LibSDL2 to create a surface for decoding video.
+Unlike the sender, the standalone receiver can work without having
+its [have_external_libs](../.../cast/standalone_receiver/BUILD.gn) set to true,
+through the use of its
+[Dummy Player](../../cast/standalone_receiver/dummy_player.h).