STPP subtitles detected but not rendered (DASH live MPD, application/mp4, codecs=stpp)

[openkampus]

Describe the bug

STPP subtitles are detected, selectable, and routed to libass, but no subtitles are rendered when the TTML payload contains backgroundImage attributes.

Kodi logs indicate that TTML parsing is performed, however image-based TTML subtitle features are explicitly not supported, resulting in an empty subtitle output.

Environment

• Kodi: 22 .0-ALPHA2(21.90.702) / flathub
• inputstream.adaptive: 22.3.1
• OS: Linux (x86_64)

Expected behavior
When an STPP (TTML in MP4) subtitle track is selected, subtitles should render correctly during playback.
If image-based TTML is not supported, the subtitle track should be clearly rejected or flagged as unsupported.

Actual behavior

• Subtitle tracks are detected
• Subtitle tracks can be enabled/selected
• No subtitle text or images are rendered
• Playback continues normally with no fatal error

Manifest

<?xml version="1.0" encoding="UTF-8"?>
<MPD profiles="urn:mpeg:dash:profile:isoff-live:2011" type="dynamic" availabilityStartTime="2025-11-24T10:59:42Z" minimumUpdatePeriod="PT4.0S" minBufferTime="PT4.0S" publishTime="2026-01-13T12:56:59.622Z" timeShiftBufferDepth="PT60.0S" suggestedPresentationDelay="PT4.0S" xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:cenc="urn:mpeg:cenc:2013" xmlns:mspr="urn:microsoft:playready" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:mpeg:dash:schema:mpd:2011 DASH-MPD.xsd">
  <Period start="PT0S" id="1">
    <AdaptationSet mimeType="video/mp4" startWithSAP="1" segmentAlignment="true" par="16:9">
      <ContentProtection schemeIdUri="urn:mpeg:dash:mp4protection:2011" value="cenc"/>
      <ContentProtection schemeIdUri="urn:uuid:9a04f079-9840-4286-ab92-e65be0885f95"></ContentProtection>
      <ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed"></ContentProtection>
      <SegmentTemplate timescale="10000000" presentationTimeOffset="0" media="$RepresentationID$_Segment-$Time$.m4v" initialization="$RepresentationID$_init.m4i">
        <SegmentTimeline>
          <S t="43269893296900" d="40000000" r="14"/>
        </SegmentTimeline>
      </SegmentTemplate>
      <Representation width="480" height="270" frameRate="25" codecs="avc1.428016" scanType="progressive" sar="1:1" id="1763981970283item-01item" bandwidth="500000"/>
      <Representation width="640" height="360" frameRate="25" codecs="avc1.4d4020" scanType="progressive" sar="1:1" id="1763981970283item-02item" bandwidth="1000000"/>
      <Representation width="960" height="540" frameRate="25" codecs="avc1.4d4029" scanType="progressive" sar="1:1" id="1763981970283item-03item" bandwidth="2000000"/>
      <Representation width="1280" height="720" frameRate="25" codecs="avc1.640029" scanType="progressive" sar="1:1" id="1763981970283item-04item" bandwidth="3000000"/>
      <Representation width="1920" height="1080" frameRate="25" codecs="avc1.64002a" scanType="progressive" sar="1:1" id="1763981970283item-05item" bandwidth="4000000"/>
    </AdaptationSet>
    <AdaptationSet mimeType="audio/mp4" startWithSAP="1" lang="eng" segmentAlignment="true">
      <ContentProtection schemeIdUri="urn:mpeg:dash:mp4protection:2011" value="cenc"/>
      <ContentProtection schemeIdUri="urn:uuid:9a04f079-9840-4286-ab92-e65be0885f95"></ContentProtection>
      <ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed"></ContentProtection>
      <SegmentTemplate timescale="10000000" presentationTimeOffset="0" media="$RepresentationID$_Segment-$Time$.m4a" initialization="$RepresentationID$_init.m4i">
        <SegmentTimeline>
          <S t="43269893629789" d="39680000"/>
          <S t="43269933309789" d="40106667"/>
          <S t="43269973416456" d="40106666"/>
          <S t="43270013523122" d="40106667"/>
          <S t="43270053629789" d="39680000"/>
          <S t="43270093309789" d="40106667"/>
          <S t="43270133416456" d="40106666"/>
          <S t="43270173523122" d="40106667"/>
          <S t="43270213629789" d="39680000"/>
          <S t="43270253309789" d="40106667"/>
          <S t="43270293416456" d="40106666"/>
          <S t="43270333523122" d="40106667"/>
          <S t="43270373629789" d="39680000"/>
          <S t="43270413309789" d="40106667"/>
          <S t="43270453416456" d="40106666"/>
        </SegmentTimeline>
      </SegmentTemplate>
      <Representation audioSamplingRate="48000" codecs="mp4a.40.5" id="1763981970283item-06item" bandwidth="64000"/>
    </AdaptationSet>
    <AdaptationSet mimeType="audio/mp4" startWithSAP="1" lang="may" segmentAlignment="true">
      <ContentProtection schemeIdUri="urn:mpeg:dash:mp4protection:2011" value="cenc"/>
      <ContentProtection schemeIdUri="urn:uuid:9a04f079-9840-4286-ab92-e65be0885f95"></ContentProtection>
      <ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed"></ContentProtection>
      <SegmentTemplate timescale="10000000" presentationTimeOffset="0" media="$RepresentationID$_Segment-$Time$.m4a" initialization="$RepresentationID$_init.m4i">
        <SegmentTimeline>
          <S t="43269893629789" d="39680000"/>
          <S t="43269933309789" d="40106667"/>
          <S t="43269973416456" d="40106666"/>
          <S t="43270013523122" d="40106667"/>
          <S t="43270053629789" d="39680000"/>
          <S t="43270093309789" d="40106667"/>
          <S t="43270133416456" d="40106666"/>
          <S t="43270173523122" d="40106667"/>
          <S t="43270213629789" d="39680000"/>
          <S t="43270253309789" d="40106667"/>
          <S t="43270293416456" d="40106666"/>
          <S t="43270333523122" d="40106667"/>
          <S t="43270373629789" d="39680000"/>
          <S t="43270413309789" d="40106667"/>
          <S t="43270453416456" d="40106666"/>
        </SegmentTimeline>
      </SegmentTemplate>
      <Representation audioSamplingRate="48000" codecs="mp4a.40.5" id="1763981970283item-07item" bandwidth="64000"/>
    </AdaptationSet>
    <AdaptationSet mimeType="audio/mp4" startWithSAP="1" lang="cmn" segmentAlignment="true">
      <ContentProtection schemeIdUri="urn:mpeg:dash:mp4protection:2011" value="cenc"/>
      <ContentProtection schemeIdUri="urn:uuid:9a04f079-9840-4286-ab92-e65be0885f95"></ContentProtection>
      <ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed"></ContentProtection>
      <SegmentTemplate timescale="10000000" presentationTimeOffset="0" media="$RepresentationID$_Segment-$Time$.m4a" initialization="$RepresentationID$_init.m4i">
        <SegmentTimeline>
          <S t="43269893629789" d="39680000"/>
          <S t="43269933309789" d="40106667"/>
          <S t="43269973416456" d="40106666"/>
          <S t="43270013523122" d="40106667"/>
          <S t="43270053629789" d="39680000"/>
          <S t="43270093309789" d="40106667"/>
          <S t="43270133416456" d="40106666"/>
          <S t="43270173523122" d="40106667"/>
          <S t="43270213629789" d="39680000"/>
          <S t="43270253309789" d="40106667"/>
          <S t="43270293416456" d="40106666"/>
          <S t="43270333523122" d="40106667"/>
          <S t="43270373629789" d="39680000"/>
          <S t="43270413309789" d="40106667"/>
          <S t="43270453416456" d="40106666"/>
        </SegmentTimeline>
      </SegmentTemplate>
      <Representation audioSamplingRate="48000" codecs="mp4a.40.5" id="1763981970283item-08item" bandwidth="64000"/>
    </AdaptationSet>
    <AdaptationSet mimeType="application/mp4" startWithSAP="1" lang="cmn" segmentAlignment="true">
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="subtitle"/>
      <SegmentTemplate timescale="10000000" presentationTimeOffset="0" media="$RepresentationID$_Segment-$Time$.m4s" initialization="$RepresentationID$_init.m4i">
        <SegmentTimeline>
          <S t="43269893296900" d="40000000" r="14"/>
        </SegmentTimeline>
      </SegmentTemplate>
      <Representation codecs="stpp" id="1763981970283item-09item" bandwidth="15000"/>
    </AdaptationSet>
    <AdaptationSet mimeType="application/mp4" startWithSAP="1" lang="eng" segmentAlignment="true">
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="subtitle"/>
      <SegmentTemplate timescale="10000000" presentationTimeOffset="0" media="$RepresentationID$_Segment-$Time$.m4s" initialization="$RepresentationID$_init.m4i">
        <SegmentTimeline>
          <S t="43269893296900" d="40000000" r="14"/>
        </SegmentTimeline>
      </SegmentTemplate>
      <Representation codecs="stpp" id="1763981970283item-10item" bandwidth="15000"/>
    </AdaptationSet>
    <AdaptationSet mimeType="application/mp4" startWithSAP="1" lang="msa" segmentAlignment="true">
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="subtitle"/>
      <SegmentTemplate timescale="10000000" presentationTimeOffset="0" media="$RepresentationID$_Segment-$Time$.m4s" initialization="$RepresentationID$_init.m4i">
        <SegmentTimeline>
          <S t="43269893296900" d="40000000" r="14"/>
        </SegmentTimeline>
      </SegmentTemplate>
      <Representation codecs="stpp" id="1763981970283item-11item" bandwidth="15000"/>
    </AdaptationSet>
  </Period>
</MPD>

Relevant log output
The warning appears repeatedly during playback.
<inputstream.adaptive>: ParseTagBody: The "backgroundImage" attribute to show subtitles images is not supported.

Feature request

To improve behavior and user experience, please consider adding support for image-based TTML subtitles (backgroundImage) in STPP streams, enabling proper rendering of such subtitles.

[CastagnaIT]

since Kodi dont support TTML
to enable the display of the most common TTML subtitles in text format
InputStream Adaptive provides a very basic "on the fly" conversion of TTML text to SRT format
this OFC will ignore almost all TTML features, included the images that cannot be processed in any way

If image-based TTML is not supported, the subtitle track should be clearly rejected or flagged as unsupported.

there is no way to reject these subtitles streams
because the manifest dont provide metadata which specifies that subtitles segments in MP4 TTML use images only

To improve behavior and user experience, please consider adding support for image-based TTML subtitles (backgroundImage) in STPP streams, enabling proper rendering of such subtitles.

at present this is not possible
because on kodi its missing the TTML format support (with his MP4 decoding support)

anyway for a TTML decoder is not required to fully support all TTML features,
because they are optional according to the TTML specifications, since TTML a very complex format,
this is to say that a possible TTML implementation on Kodi may still not support images

so before requesting this feature here, you must first request this support on Kodi
indicating support for MP4 metadata containing image formats
this forum section its the right place: https://forum.kodi.tv/forumdisplay.php?fid=9

only after that Kodi implementation, this request can be reconsidered here

[openkampus]

Thank you for the clarification.

Given that proper support for image-based TTML subtitles requires Kodi core support first, this request is currently out of scope for InputStream Adaptive.
I will close this request on my side and follow up with a feature request to Kodi.
Thank you.