[imsc] image based subtitle positioning

Dear all,

As the public review period nears its end, I'd like to highlight a positive addition to the imsc spec to facilitate image based subtitling.

A basic image based subtitling script file will normally contain:
Incue/Outcue
Image name/url
Image position
Optionally, image size may be specified, and the overall size of the canvas may be specified.


Imsc by inclusion of backgroundimage just about allows for these.  But the specification of image size and position is very convoluted.

For image based subtitling, position and size of the image in relation to the video is paramount.


It would be really good if tts:origin and tts:extent were enabled on div for image based subtitling; at the moment I cannot see how they are allowed.  Please correct me and include a sample in the document if I am wrong.
It would also be good to be explicit about image scaling.  Ideally, the image should be scaled to match the specified image extent (with some notes that if this scaling is close to 1:1 after taking into account player size, etc., then the decoder may prefer not to scale to retain quality).


This modification would make image based subtitling in imsc a relatively simple and easy to understand construct.  It makes it almost as simple to write as current extant image + script formats, and (I would imagine) make it relative easy to parse.



Modified example from http://en.wikipedia.org/wiki/User:Cwmwenallt/SMPTE-TT (i'm not claiming this was correct to start with!).


<tt xmlns:smpte="http://www.smpte-ra.org/schemas/2052-1/2010/smpte-tt"
   xmlns="http://www.w3.org/ns/ttml"
   xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
   xmlns:tts='http://www.w3.org/ns/ttml#styling'
   xml:lang="en">
<head>
   <layout>
     <region xml:id="imageRegion" tts:color="transparent" tts:origin="0% 0%" tts:extent="100% 100%" >
       <set begin="0.19305s" end="0.21581s" tts:origin="0px 2px" tts:extent="4px 8px" />
       <set begin="5.89876s" end="8.09467s" tts:origin="230px 50px" tts:extent="243px 58px" />
       <set begin="8.20106s" end="10.1922s" tts:origin="202px 50px" tts:extent="302px 64px" />
       <set begin="10.3032s" end="12.2943s" tts:origin="180px 402px" tts:extent="341px 32px" />
     </region>
   </layout>
</head>
<body>
   <div region="imageRegion">
     <div begin="0.19305s" end="0.21581s" smpte:backgroundImage="Subtitles_EN/SPU0.png">
       <p>[Example SMPTE-TT file]</p>
     </div>
     <div begin="5.89876s" end="8.09467s" smpte:backgroundImage="Subtitles_EN/SPU1.png">
       <p>Hello Wikipedia</p>
     </div>
     <div begin="8.20106s" end="10.1922s" smpte:backgroundImage="Subtitles_EN/SPU2.png">
       <p>This is a basic Example</p>
     </div>
     <div begin="10.3032s" end="12.2943s" smpte:backgroundImage="Subtitles_EN/SPU3.png" >
       <p>of pop on style captioning with preformatted background images</p>
     </div>
   </div>
 </body>
</tt>


Becomes (I did not add the required namespace):

<tt xmlns:smpte="http://www.smpte-ra.org/schemas/2052-1/2010/smpte-tt"
   xmlns="http://www.w3.org/ns/ttml"
   xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
   xmlns:tts='http://www.w3.org/ns/ttml#styling'
   xml:lang="en">
<head>
   <layout>
     <region xml:id="imageRegion" tts:color="transparent" tts:origin="0% 0%" tts:extent="100% 100%" >
     </region>
   </layout>
</head>
<body>
   <div region="imageRegion">
     <div begin="0.19305s" end="0.21581s" smpte:backgroundImage="Subtitles_EN/SPU0.png" tts:origin="0px 2px" tts:extent="4px 8px" >
       <ittm:altText>[Example SMPTE-TT file]</ittm:altText>
     </div>
     <div begin="5.89876s" end="8.09467s" smpte:backgroundImage="Subtitles_EN/SPU1.png" tts:origin="230px 50px" tts:extent="243px 58px" >
       <ittm:altText>Hello Wikipedia</ittm:altText>
     </div>
     <div begin="8.20106s" end="10.1922s" smpte:backgroundImage="Subtitles_EN/SPU2.png" tts:origin="202px 50px" tts:extent="302px 64px" >
       <ittm:altText>This is a basic Example</ittm:altText>
     </div>
     <div begin="10.3032s" end="12.2943s" smpte:backgroundImage="Subtitles_EN/SPU3.png" tts:origin="180px 402px" tts:extent="341px 32px" >
       <ittm:altText>of pop on style captioning with preformatted background images</ittm:altText>
     </div>
   </div>
 </body>
</tt>



Best regards,

Simon Hailes.
p.s. please reply direct if you have any comments/questions.  I don't monitor the mailing lists....

Simon Hailes | Chief Technology Officer | Screen
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Received on Wednesday, 8 October 2014 08:12:27 UTC