HLS top-positioning (styling) subtitles (IMSC1/TTML) not work as expected

Question

Created Sep ’20

Replies 2

Boosts 0

Views 1.1k

Participants 3

Hi there,

Apple devices have issues on showing HLS top-positioning IMSC1 subtitles:
Once top-positioning subtitles are shown, the succeeding subtitles will not be shown any more after the position is changed.
e.g.
expected: bottom->top->top->bottom->bottom->top->...
but: bottom->top->top->nothing till the end.

Some test results are as follows

quickTime : issues as above -- no subs shown after an entry is shown with the top position
safari: all entries are displayed at the bottom. But at least it continues to show subtitles after that.
tvOS: same as QuickTime
iOS: same as safari

Based on
https://developer.apple.com/documentation/http_live_streaming/hls_authoring_specification_for_apple_devices

IMSC1 subtitles (text profile only)

It looks like HLS does not support IMSC1 styling.

Could anyone tell me whether Apple supports such a styling e.g. subtitles positioning?
If not, would Apple support it later?
IMSC1 is a W3C recommendation and the only TTML profile that is part of the CMAF specification. I would like to see that support.

The following is the sample subtitle:

Code Block xml<?xml version="1.0" encoding="utf-8"?>
<tt xml:lang="de" xmlns="http://www.w3.org/ns/ttml"
 xmlns:tts="http://www.w3.org/ns/ttml#styling">
 <head>
  <styling>
   <style xml:id="s1" tts:color="white"/>
  </styling>
  <layout>
   <region xml:id="r1"
    tts:extent="80% 20%"
    tts:origin="10% 70%"
    tts:textAlign="center"
    tts:wrapOption="noWrap"/>
   <region xml:id="r2"
    tts:extent="80% 20%"
    tts:origin="10% 10%"
    tts:textAlign="center"
    tts:wrapOption="noWrap"/>
  </layout>
 </head>
 <body>
  <div region="r1">
  </div>
  <div region="r2">
   <p xml:id="sub48000" begin="00:00:48.000" end="00:00:49.769">
    <span style="s1">26 Angestellte<br/>kamen bei dem GiftmÃ¼ll-Skandal</span>
   </p>
   <p xml:id="sub48001" begin="00:00:49.929" end="00:00:52.000">
    <span style="s1">in Washington Township ums Leben,<br/>Ã¼ber den ausgiebig berichtet wurde.</span>
   </p>
  </div>
 </body>
</tt>

I appreciate for replies.

Boost

Answer 1

pascal_h OP

Apr ’24

Hello, I have same kind of issue, following TTML (embedded in fmp4 fragment, so IMSC1) is not centered, but displayed at the right of the screen, whereas online hls.js web player centers subtitles:

<?xml version="1.0" encoding="utf-8"?> <tt xmlns="http://www.w3.org/ns/ttml" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:xml="http://www.w3.org/XML/1998/namespace" ttp:timeBase="media" ttp:frameRate="25" xml:lang="en"> <head> <metadata> <ttm:title/> </metadata> <styling> <style xml:id="normal" tts:fontFamily="default" tts:fontSize="100%"/> </styling> <layout> <region xml:id="bottom" tts:backgroundColor="transparent" tts:origin="auto" tts:extent="auto" tts:textAlign="center" tts:displayAlign="center"/> </layout> </head> <body style="normal" region="bottom"> <div> There are some things you do not know about The Bosphorus and I will tell you about them The Bosphorus and I will tell you about them It's the strait we like every summer It's the strait we like every summer </div> </body> </tt>

Is there a plan for Apple to fix that ? Thanks

0

Answer 2

zwAlvin OP

Nov ’24

Hello, we also encountered the problem of subtitles not centered. Is there any solution to this problem?

0