Line Height stacking strategy algorithm - approval needed


As discussed in yesterday's TTWG meeting, as part of the fulfilment of actions 219 [1] and 221 [2] Glenn has edited the TTML2 editor's draft to incorporate an algorithm for computing the value of line height, based on the code used by Mozilla, blink and webkit. This can additionally be considered part of the solution to the line height problem which has also been raised in issues 275 [3] and 284 [4].


The algorithm can be found at [5] in section 8.2.13 tts:lineHeight.

I won't attempt to repeat Glenn's explanation of this algorithm, which I scribed as best I could yesterday, except to note what may appear at first to be a substantive change to some, that the "125% of fontSize" rule does not always apply, but applies only when the font is not associated with "font metrics that specify altitude A, descent D, and line gap G".


This algorithm is absent from CSS but, if this group's members are happy with it, we should communicate it to the CSS WG, to maximise the time for them to raise any potential issues, or indeed choose to adopt it into their own output documentation, which we could then potentially reference. Formally they will report any problems anyway at Last Call stage but I'd rather seek early agreement when possible.


Please notify the group if there are any queries or objections to this algorithm as it stands.

We noted in yesterday's call that it was not possible to verify that IE adopts the same algorithm, so it would be particularly useful if someone from Microsoft could confirm that they are happy with it – Jerry, this probably means you!

Rather than making this an open-ended action I'll close this off and state that we have approval if there are no outstanding objections by the end of our next call on 30th Jan 2014.

This email satisfies Action-260 [6].


[1] Action-219

[2] Action-221

[3] Issue-275

[4] Issue-284

[5] TTML2 Editor's draft, section on lineHeight

[6] Action-260


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Received on Friday, 17 January 2014 14:17:35 UTC