Re: ISSUE-302 (line spacing vs line height): Should background of a span be height of text in line or computed lineHeight? [TTML2]

On Thu, Mar 20, 2014 at 5:34 AM, Timed Text Working Group Issue Tracker <
sysbot+tracker@w3.org> wrote:

> ISSUE-302 (line spacing vs line height): Should background of a span be
> height of text in line or computed lineHeight? [TTML2]
>
> http://www.w3.org/AudioVideo/TT/tracker/issues/302
>
> Raised by: Nigel Megitt
> On product: TTML2
>
> 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


Since TTML1 adopts the semantics of XSL-FO for block and line layout, and
since XSL-FO refers to CSS for the semantics of inline style properties
such as background-color, then the CSS property semantics apply.

In this case, background color applied to a span extends to the height of
the inline box generated by the span, and if that box's computed height is
less than the computed line height assigned to the line area, then there
will be gaps between the before/after edges of the inline box and the
containing line box.

See the attached test file as an example.

If we had supported padding on content elements in TTML1, then one could
manually add padding-before/after (top/bottom) to the spans to extend the
height of the inline's background, but unfortunately we didn't add this
until TTML2.


>
> 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.
>
>
>
>
>