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[VTT] continued discussion in the CSS WG

From: David Singer <singer@apple.com>
Date: Tue, 31 Mar 2015 09:13:59 -0700
To: public-texttracks@w3.org, Public TTWG List <public-tt@w3.org>
Message-id: <49DC9637-4254-4D5C-A055-CE3EA7E67A95@apple.com>

> Begin forwarded message:
> 
> From: Alan Stearns <stearns@adobe.com>
> To: Simon Pieters <simonp@opera.com>, David Singer <singer@apple.com>, Bert Bos <bert@w3.org>
> Cc: "www-style@w3.org" <www-style@w3.org>, Randy Edmunds <redmunds@adobe.com>
> Subject: Re: Agenda+ review 1st WD of WebVTT
> Date: March 31, 2015 at 07:40:00 PDT
> 
> On 3/31/15, 4:26 AM, "Simon Pieters" <simonp@opera.com> wrote:
> 
>> (Again move technical discussion to the public list....)
>> 
>> On Tue, 31 Mar 2015 13:19:30 +0200, Simon Pieters <simonp@opera.com>
>> wrote:
>> 
>>> On Mon, 30 Mar 2015 23:22:58 +0200, Alan Stearns <stearns@adobe.com>
>>> wrote:
>>> 
>>>> 
>>>> My comment for the collection is either on WebVTT or CSS Text level 4.
>>>> 
>>>> The
>>>> definitions for line balancing should be rationalized, and probably a
>>>> note
>>>> should be added to both that the definition may only hold for Latin
>>>> text.
>>>> 
>>>> In WebVTT section 6.1 [1], step 11 of the algorithm for obtaining CSS
>>>> boxes says:
>>>> 
>>>> -----
>>>> any line breaks inserted by the user agent
>>>> for the purposes of line wrapping must be
>>>> placed so as to minimize Δ across each run of
>>>> consecutive lines between preserved newlines
>>>> in the source. Δ for a set of lines is defined
>>>> as the sum over each line of the absolute of
>>>> the difference between the line's length and
>>>> the mean line length of the set.
>>>> 
>>>> -----
>>>> 
>>>> In Text level 4 section 5.1 [2], the definition of text-wrap:balance
>>>> says:
>>>> 
>>>> -----
>>>> 
>>>> Line boxes are balanced when the standard deviation from
>>>> the average inline-size consumed is reduced over the block
>>>> 
>>>> (including lines that end in a forced break).
>>>> 
>>>> -----
>>>> 
>>>> 
>>>> I’d be happy to adopt WebVTT’s second sentence if that’s deemed better,
>>>> but I’m not that happy about the first sentence. If you assume a forced
>>>> break is always a paragraph boundary, then different line lengths
>>>> before
>>>> and after the break are fine. But if you consider a forced break to not
>>>> break apart the paragraph, then different line lengths before and after
>>>> the break are bad.
>>> 
>>> I think it would be good if WebVTT used text-wrap:balance instead of
>>> its  
>>> own prose to handle line balancing, so UAs can have a single
>>> implementation for both WebVTT and CSS.
>>> 
>>> I don't have a strong opinion on what the rule should be, but for CSS
>>> it  
>>> would be good if it allows an implementation to balance many lines of
>>> text with acceptable performance (e.g. O(n^2) is not acceptable).
>>> 
>>> Also see https://www.w3.org/Bugs/Public/show_bug.cgi?id=19458
> 
> On performance, Randy Edmunds demonstrated a proposal a while back [1]
> where the algorithm runs at most two layout passes. One of the reasons I
> used the word “reduced” rather than “minimized” is to allow some variation
> in the ways that different browsers can achieve the balanced effect.
> 
> Browser interop does not (and I believe it can not) include identical line
> breaks in the non-balanced case, so I don’t think it makes any sense to
> require ideal breaks when balancing. The general result should merely show
> more balance (when possible), and we can construct some obvious test cases
> as a baseline for any algorithm to pass.
> 
> Thanks,
> 
> Alan
> 
> [1] https://lists.w3.org/Archives/Public/www-style/2013Jan/0597.html
> 

David Singer
Manager, Software Standards, Apple Inc.
Received on Tuesday, 31 March 2015 16:14:30 UTC

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