Video playback in LibreOffice Impress 6 on Ubuntu 17.10 [solution]
For a while, I've been struggling with video playback in LibreOffice Impress 184.108.40.206 on Ubuntu 17.10, installed through the LibreOffice PPA. The issue was that videos did play, although with considerable flickering, but always fullscreen.
It took me a while to figure out a solution, but it turned out to be surprisingly simple (but not, as far as I know, documented anywhere): installing the 'bad' gstreamer plugins.
You can do this by executing the following line in a terminal:
sudo apt-get install gstreamer1.0-plugins-bad
For me, this essentially fixed video playback.
There are still (and always have been) issues with video playback in Impress, but most of these can be solved by
transcoding videos into a format that is somewhat lightweight. For example, this works fine for me: