TTML sidecar files in DASH manifests are not decoded

When TTML subtitles in a DASH manifest are distributed as standalone "sidecar" XML files VLC fails to initialize the decoder and does not show the subtitles.

Tested with revision 4.0.0-dev-10001-g4f11b288

Can be reproduced with:

  $ ./vlc https://dash.akamaized.net/dash264/CTA/imsc1/IT1-20171027_dash.mpd

By looking at the logs it seems that VLC succeeds in parsing the adaptation set from the manifest and even retrieves the data and looks for a decoder but it fails to find a suitable one.

I checked that the TTML decoder is indeed able to handle that data by downloading the subtitles and loading the file manually in VLC and in this case the data gets parsed and the subtitles are shown.

Command for the crosscheck:

  $ wget https://dash.akamaized.net:443/dash264/CTA/imsc1/IT1-T-20171026.ttml
  $ ./vlc -vv https://dash.akamaized.net/dash264/CTA/imsc1/IT1-20171027_dash.mpd --sub-file IT1-T-20171026.ttml

Is the issue about selecting the decoder?

Thanks, Antonio

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