I have some cheap HDMI stick which i use with my notebook to turn it into a second screen (vnc is too slow and net io hungry for my case)

```
mplayer tv:// -tv driver=v4l2:device=/dev/video0:width=1280:height=720:fps=60:outfmt=MJPG
```

If you just watch easy the webcam output (and can go in fullscreen and having linear fulter), use q4vl2 which is available in v4l-utils package