v4l2 and obs: added

wiki/obs.md

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+# OBS
+
+[Open Broadcaster Software](https://obsproject.com) - short OBS - is a free
+and open source software for video recording and live streaming.
+
+## Virtual Camera
+
+By using virtual camera the views of OBS can be used as an additional camera
+output, making it possible to add effects and more to the video feed of the
+camera.
+
+### Installation
+
+For Linux systems [V4L2](./linux/v4l2.md) needs to be set up.
+This guide assumes it is set up according to the section in the
+[V4L2 entry](./linux/v4l2.md#v4l2loopback).
+Additionally `obs-v4l2sink` needs to be installed.
+Under `Tools/v4l2sink` select the device path set up in V4L2 and the `YUV420`
+video format.
+Afterwards the virtual camera can be started.