ISSUE-302: Should background of a span be height of text in line or computed lineHeight?

line spacing vs line area height

Should background of a span be height of text in line or computed lineHeight?

State:
OPEN
Product:
TTML2
Raised by:
Nigel Megitt
Opened on:
2014-03-20
Description:
TTML1SE says that lineHeight "is used to specify a style property that defines the inter-baseline separation between line areas" However it is unclear what the height of the line areas is, for the purpose of drawing the background as specified on a span. Is it identical to the computed line height, or is it related to the height of the line areas created based on the text contained within the span, independent of the baseline separation of those line areas?

The XSL-FO line area definition [1] appears to be relevant, though I'm not sure if it resolves the question.

[1] http://www.w3.org/TR/2006/REC-xsl11-20061205/#area-line

We need to address this additionally for any mapping to HTML/CSS because it appears that a simple mapping of TTML concepts into CSS could introduce 'no background paint' areas in between lines.

If the two concepts may be computed to have different values this introduces the possibility that between two lines separated by e.g. a <br/> an empty 'no background paint' area may be introduced. I would argue that this is highly undesirable aesthetically when lineHeight is approximately the same as the line area height, e.g. in the range 80-125%. However if the two concepts must always have identical values this possibility would not arise; instead lines may be spaced far apart (for a large lineHeight) and the entire height would be painted with background colour. This latter option may also be undesirable in some use cases.

This issue is related to Issue-284.
Related Actions Items:
No related actions
Related emails:
  1. ISSUE-380 (Content area width is unclear): The width of content elements' areas is unclear, especially looking at the examples [TTML2] (from sysbot+tracker@w3.org on 2015-03-26)
  2. {minutes} TTWG Meeting 27/3/2014 (from nigel.megitt@bbc.co.uk on 2014-03-27)
  3. Re: {agenda} TTWG Meeting 27/3/2014 (from glenn@skynav.com on 2014-03-27)
  4. Re: {agenda} TTWG Meeting 27/3/2014 (from tmichel@w3.org on 2014-03-27)
  5. {agenda} TTWG Meeting 27/3/2014 (from nigel.megitt@bbc.co.uk on 2014-03-26)
  6. Re: ISSUE-302 (line spacing vs line height): Should background of a span be height of text in line or computed lineHeight? [TTML2] (from nigel.megitt@bbc.co.uk on 2014-03-21)
  7. Re: ISSUE-302 (line spacing vs line height): Should background of a span be height of text in line or computed lineHeight? [TTML2] (from glenn@skynav.com on 2014-03-20)
  8. Re: ISSUE-302 (line spacing vs line height): Should background of a span be height of text in line or computed lineHeight? [TTML2] (from glenn@skynav.com on 2014-03-20)
  9. ISSUE-302 (line spacing vs line height): Should background of a span be height of text in line or computed lineHeight? [TTML2] (from sysbot+tracker@w3.org on 2014-03-20)

Related notes:

Determined from CSSWG that there is no way to force the height of a background for an inline box to exactly match the height of the line box in the case the line box contains multiple inline boxes with different heights.

Glenn Adams, 24 May 2014, 04:26:41

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David Singer <singer@apple.com>, Nigel Megitt <nigel.megitt@bbc.co.uk>, Chairs, Thierry Michel <tmichel@w3.org>, Philippe Le H├ęgaret <plh@w3.org>, Staff Contacts
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