Re: lineHeight="normal"

On Thu, Sep 6, 2012 at 2:21 PM, Kouichi Uchimura <
Kouichi.Uchimura@jp.sony.com> wrote:

> Hi,
>
> I'm studying tts:lineHight attribute and have one comment.
>  (I checked issue58.)
>
> Current text defines:
> If the value of this attribute is normal, then the initial value of the
> style property must be considered to be the same as the largest font size
> that applies to any descendant element.
> I'd like to confirm that
>  whether "any descendant element" includes the affected element itself
> or not.
> My interpretatin is the affected element itself is not included.
>
> Examples are...
>  <p region="r1" tts:lineHeight="normal" tts:fontSize="64px" begin="0s"
> end ="1s">
>   this<br/>
>   span<tts:fontSize="16px">
>     is<br/>
>   span<tts:fontSize="14px">
>     example
> </p>
> This case, lineHeight is 64px.
>  text node "this" and br elem will be enclosed by anonymous span and its
> fontSize is 64px.
> So, max font size of descendants is 64px.
>
> On the other hand,
>  <p region="r1" tts:lineHeight="normal" tts:fontSize="64px" begin="0s"
> end ="1s">
>   span<tts:fontSize="16px">
>     this is<br/>
>   span<tts:fontSize="14px">
>     example
> </p>
> This case, lineHeight is 16px.
>
> Is above understanding correct?
>  If not, suggest to change the text to as like...
> "to be the same as the largest font size that applies to any descendant
> element (includes the affected element itself)."
>

I'm not sure about the origin of the current language, which differs from
both CSS and XSL-FO. XSL-FO has a number of line-stacking-strategy values
(max-height, font-height, line-height) which affect the height of the line
allocation rectangle. The nominal CSS behavior is obtained when stack
strategy is line-height. I suspect that the current language in TTML was an
attempt to simply the extremely complex description found in [1], which,
every time I read it, I can't quite get my head around its meaning. Some
times when I read it I think I understand it, but then I read something
else related to this topic in XSL-FO that abrogates my understanding.

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