1# External Libraries in Open Screen 2 3Currently, external libraries are used exclusively by the standalone sender and 4receiver applications, for compiling in dependencies used for video decoding and 5playback. 6 7The decision to link external libraries is made manually by setting the GN args. 8For example, a developer wanting to link all the libraries for the standalone 9sender and receiver executables might add the following to `gn args out/Default`: 10 11```python 12is_debug=true 13have_ffmpeg=true 14have_libsdl2=true 15have_libopus=true 16have_libvpx=true 17``` 18 19On some versions of Debian, the following apt-get command will install all of 20the necessary external libraries for Open Screen: 21 22```sh 23sudo apt-get install libsdl2-2.0 libsdl2-dev libavcodec libavcodec-dev 24 libavformat libavformat-dev libavutil libavutil-dev 25 libswresample libswresample-dev libopus0 libopus-dev 26 libvpx5 libvpx-dev 27``` 28 29Similarly, on some versions of Raspian, the following command will install the 30necessary external libraries, at least for the standalone receiver. Note that 31this command is based off of the packages linked in the [sysroot](sysroot.gni): 32 33```sh 34sudo apt-get install libavcodec58=7:4.1.4* libavcodec-dev=7:4.1.4* 35 libsdl2-2.0-0=2.0.9* libsdl2-dev=2.0.9* 36 libavformat-dev=7:4.1.4* 37``` 38 39Note: release of these operating systems may require slightly different 40packages, so these `sh` commands are merely a potential starting point. 41 42Finally, note that generally the headers for packages must also be installed. 43In Debian Linux flavors, this usually means that the `*-dev` version of each 44package must also be installed. In the example above, this looks like having 45both `libavcodec` and `libavcodec-dev`. 46 47## Standalone Sender 48 49The standalone sender uses FFMPEG, LibOpus, and LibVpx for encoding video and 50audio for sending. When the build has determined that [have_external_libs]( 51 ../../cast/standalone_sender/BUILD.gn 52) is set to true, meaning that all of these libraries are installed, then 53the VP8 and Opus encoders are enabled and actual video files can be sent 54to standalone receiver instances. Without these dependencies, the standalone 55sender cannot properly function (contrasted with the standalone receiver, 56which can use a dummy player). 57 58## Standalone Receiver 59 60The standalone receiver also uses FFMPEG, for decoding the video stream encoded 61by the sender, and also uses LibSDL2 to create a surface for decoding video. 62Unlike the sender, the standalone receiver can work without having 63its [have_external_libs](../.../cast/standalone_receiver/BUILD.gn) set to true, 64through the use of its 65[Dummy Player](../../cast/standalone_receiver/dummy_player.h). 66