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Re: Agenda+ review 1st WD of WebVTT

From: Alan Stearns <stearns@adobe.com>
Date: Tue, 31 Mar 2015 14:40:00 +0000
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>
Message-ID: <D13FFC44.25812%stearns@adobe.com>
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>
>> 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
>>> 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
>> 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
>> 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.



[1] https://lists.w3.org/Archives/Public/www-style/2013Jan/0597.html

Received on Tuesday, 31 March 2015 14:40:29 UTC

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