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Deprecate use of pixel units unless tts:extent on root element is in pixels. #330

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skynavga opened this issue May 24, 2017 · 3 comments
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@skynavga
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At present, TTML1 allows a document to use pixel units when expressing <length> terms, even when the author has not defined the resolution of the document coordinate space by specifying a pixel based tts:extent on the <tt> element. I propose that this behavior be deprecated in TTML2, as it leads to potential interoperability problems when presenting TTML content. Note that a number of profiles, including IMSC1 and EBU-TT, require that a pixel based tts:extent be present when pixel units are used in content.

@palemieux
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ttp:pixelAspectRatio should be similarly deprecated when tts:extent is not specified in px on tt

@skynavga
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skynavga commented May 24, 2017

OMG, we are in agreement! One of our reputations is certain to suffer. 🙂

@skynavga skynavga changed the title deprecate use of pixel units unless tts:extent on root element is in pixels Deprecate use of pixel units unless tts:extent on root element is in pixels May 25, 2017
@skynavga skynavga changed the title Deprecate use of pixel units unless tts:extent on root element is in pixels Deprecate use of pixel units unless tts:extent on root element is in pixels. May 25, 2017
@skynavga skynavga self-assigned this Jun 10, 2017
@skynavga skynavga added this to the Editor's CR Work List milestone Jan 8, 2018
@css-meeting-bot
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The Working Group just discussed Deprecate use of pixel units unless tts:extent on root element is in pixels. ttml2#330, and agreed to the following resolutions:

  • RESOLUTION: Implement this as per the above discussion.
The full IRC log of that discussion <nigel> Topic: Deprecate use of pixel units unless tts:extent on root element is in pixels. ttml2#330
<nigel> github: https://github.com//issues/330
<nigel> Glenn: Everyone agrees with this, I just want to get confirmation.
<nigel> .. This will probably cause more deprecation warnings than anything other deprecation we are making.
<nigel> Nigel: How do you deprecate the absence of something?
<nigel> Pierre: Require issuing a warning if pixel units are used but no tts:extent is present on tt.
<nigel> Nigel: Where in the spec?
<nigel> Cyril: On the `<length>` unit definition.
<nigel> Pierre: And on tts:extent.
<nigel> .. Or on tt?
<nigel> .. I'm happy either way.
<nigel> Glenn: Probably on the tt element.
<nigel> Nigel: Can we please also have a note on the `<length>` unit to warn about this behaviour?
<nigel> Glenn: Sure that's reasonable.
<nigel> RESOLUTION: Implement this as per the above discussion.

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