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

With attachment.


On Thu, Mar 20, 2014 at 11:11 AM, Glenn Adams <glenn@skynav.com> wrote:

>
>
> 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.
>>
>>
>>
>>
>>
>