Font selection: How can I use Roboto?

[kootsoop]

I've been trying (and failing) to select the Roboto font for the text overlay
in the pipeline:
/gst-launch multifilesrc location="start/blank/frame%03d.png" index=1
caps="image/png,framerate=\(fraction\)30/1" ! pngdec ! queue !
ffmpegcolorspace ! videorate ! textoverlay text="Hello out there"
font-desc="Roboto 40" ! x264enc ! mp4mux ! filesink
location="initial-advert.mp4"/
But all it seems to use is the system sans serif font.
/fc-list/ shows that Roboto and all its variant TTF files are installed, but
nothing I do seems to let me use it with gstreamer.
Can anyone offer pointers?
I'm running on 14.04 Ubuntu, and  this is a repeat of a AskUbuntu question.
<http://askubuntu.com/questions/660730/how-do-i-install-fonts-for-use-in-the-font-desc-parameter-of-textoverlay-in>  



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