Re: [ttml2] font*-p features

Hi Glenn et al.,

Apologies for the belated reply, I have been on the road, and had not
realized this significant change that happened between TTML1 and TTML2,
i.e. TTML2 applying style properties applying to <p> in addition to <span>,
where they only apply to <span> in TTML1.

I struggle to understand this change since it has not been an issue with
interoperability in TTML1. Do you have a specific test case in mind?

> Only for the purpose of determining the default line height for a
paragraph (used to resolve 'normal'),
> since one has to resolve the font properties to a specific font resource
(in order to obtain the ascent, descent, and line gap (separation) data
from the font).

An implementation determines the height of each line box based on many
factors, e.g. the
styles applied to the <span>s within the line box, tts:lineHeight
specified on <p>, etc... The styles do not need to apply to the <p> for
this to
happen. Can you provide a case where this is not the case? Perhaps specific
code within TTPE?

Best,

-- Pierre


On Mon, May 21, 2018 at 6:40 PM Glenn Adams <glenn@skynav.com> wrote:

> Any comments on this folks? I suggest there are a few different options
that would go along with removing the font*-p features:

> add informative note under each font* property with a font*-p feature
that the specific listing of p was implied (but undocumented) in TTML1 (and
perhaps should be added in TTML1 3ed);
> remove citation of p under application, but add a note like (1) anyway
> instead of using a note, add a special semantics subsection (or
sub-sub-section if already present) that documents the semantics around p
use of the font* property


> On Sun, May 20, 2018 at 9:41 PM, Glenn Adams <glenn@skynav.com> wrote:

>> I'm having second thoughts about adding the font*-p features in
https://github.com/w3c/ttml2/pull/664/commits/640f28ede06c0175e5d260af0790fef913e643ac
.

>> The reason we added these is because we explicitly added 'p' as one of
the elements to which these font* properties applied. Now, why do they
apply? Only for the purpose of determining the default line height for a
paragraph (used to resolve 'normal'), since one has to resolve the font
properties to a specific font resource (in order to obtain the ascent,
descent, and line gap (separation) data from the font).

>> But the thing is: all TTML1 implementations had to use this algorithm
already, so effectively already applied these properties to 'p'. So in
reality, we aren't adding new functionality here (that might warrant having
new feature designators), but only rectifying a missing behavior
specification (in TTML1) that was already implemented in reality (by both
old implementations, like DFXPVW, and new implementations, like IMSCJS,
which maps to CSS (which already applies these font properties to P in this
fashion).

>> I recommend we remove the new font*-p features and instead add notes to
each of the properties that explains that the addition of 'p' to this list
of applied elements serves only to document existing behavior consistent
with TTML1 implementations.

>> However, if someone knows of a TTML1 (or IMSC) implementation that does
not use these semantics for obtaining the default line height of a
paragraph, then we will need to discuss this further.

Received on Sunday, 27 May 2018 15:28:37 UTC