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Fix #232: Make "text alignment" definitions match the rendering
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The rendering rules apply unicode-bidi:plaintext which isolates
each line. Make the definitions for "text alignment" start/end
match the rendering rules. Drop the "paragraph direction" concept
since it was not used for anything that changes the rendered result.
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@zcorpan
zcorpan committed Nov 11, 2015
1 parent 2023c10 commit 7ebfa9a
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76 changes: 18 additions & 58 deletions index.bs
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Expand Up @@ -18,7 +18,7 @@ Test Suite: https://github.com/w3c/web-platform-tests/tree/master/webvtt
Abstract: This specification defines WebVTT, the Web Video Text Tracks format. Its main use is for marking up external text track resources in connection with the HTML <track> element.
Abstract: WebVTT files provide captions or subtitles for video content, and also text video descriptions [[MAUR]], chapters for content navigation, and more generally any form of metadata that is time-aligned with audio or video content.
Boilerplate: omit conformance, omit feedback-header
Ignored Terms: unicode-bidi, direction, color, text-combine-upright
Ignored Terms: unicode-bidi, color, text-combine-upright
Ignored Vars: seconds-frac, selector, fragment
</pre>
Expand Down Expand Up @@ -677,13 +677,12 @@ specification are intended to be easy to follow, and not intended to be performa
setting for a cue, the <a>WebVTT cue position</a> defaults to 50%.</p>

<p class="note">Even for <a lt="WebVTT cue horizontal writing direction">horizontal</a> cues with
right-to-left <i>paragraph direction</i> text, the <a lt="WebVTT cue box">cue box</a> is
positioned from the left edge of the video viewport. This allows defining a rendering space
template which can be filled with either left-to-right or right-to-left <i>paragraph direction</i>
text. If such a <a lt="WebVTT cue box">cue box</a> template is created with <a lt="WebVTT cue
start alignment">start</a> or <a lt="WebVTT cue end alignment">end</a> aligned text, it is best to
also specify a <a lt="WebVTT cue size">size</a> since otherwise the text may flip from one side of
the video viewport to the other.</p>
right-to-left cue text, the <a lt="WebVTT cue box">cue box</a> is positioned from the left edge of
the video viewport. This allows defining a rendering space template which can be filled with
either left-to-right or right-to-left cue text, or both. If such a <a lt="WebVTT cue box">cue
box</a> template is created with <a lt="WebVTT cue start alignment">start</a> or <a lt="WebVTT cue
end alignment">end</a> aligned text, it is best to also specify a <a lt="WebVTT cue size">size</a>
since otherwise the text can flip from one side of the video viewport to the other.</p>

</dd>

Expand Down Expand Up @@ -742,7 +741,7 @@ specification are intended to be easy to follow, and not intended to be performa
of the video (for <a lt="WebVTT cue horizontal writing direction">horizontal</a> cues) or the top
(otherwise), the <a>WebVTT cue position alignment</a> <a lt="WebVTT cue position start
alignment">start value</a> varies between left and top for horizontal and vertical cues, but not
between left and right even for changing <i>paragraph direction</i>.</p>
between left and right for left-to-right and right-to-left cue text.</p>

</dd>

Expand All @@ -760,21 +759,22 @@ specification are intended to be easy to follow, and not intended to be performa
<dd>

<p>An alignment for all lines of text within the <a lt="WebVTT cue box">cue box</a>, in the
dimension of the <a lt="WebVTT cue writing direction">writing direction</a> and the <i>paragraph
direction</i> [[!BIDI]], one of:</p>
dimension of the <a lt="WebVTT cue writing direction">writing direction</a>, one of:</p>

<dl>

<dt><dfn lt="WebVTT cue start alignment">Start alignment</dfn></dt>
<dd>The text is aligned towards the <i>paragraph direction</i> start side of the <a lt="WebVTT
cue box">cue box</a>.</dd>
<dd>The text of each line is individually aligned towards the start side of the box,
where the start side for that line is determined by using the CSS rules for
''unicode-bidi/plaintext'' value of the 'unicode-bidi' property. [[!CSS-WRITING-MODES-3]]</dd>

<dt><dfn lt="WebVTT cue middle alignment">Middle alignment</dfn></dt>
<dd>The text is aligned centered between the box's start and end sides.</dd>

<dt><dfn lt="WebVTT cue end alignment">End alignment</dfn></dt>
<dd>The text is aligned towards the <i>paragraph direction</i> end side of the <a lt="WebVTT cue
box">cue box</a>.</dd>
<dd>The text of each line is individually aligned towards the start side of the box,
where the start side for that line is determined by using the CSS rules for
''unicode-bidi/plaintext'' value of the 'unicode-bidi' property. [[!CSS-WRITING-MODES-3]]</dd>

<dt><dfn lt="WebVTT cue left alignment">Left alignment</dfn></dt>
<dd>The text is aligned to the box's left side.</dd>
Expand Down Expand Up @@ -3451,23 +3451,6 @@ manner suiting the user.</p>
already has one child, set |region|'s 'transition-property' to 'top' and 'transition-duration'
to '0.433s'.</p></li>

<!-- The following steps are the subpart of the "apply WebVTT cue settings" algorithm that
applies to regions -->
<li>
<p>Apply the Unicode Bidirectional Algorithm's Paragraph Level steps to the concatenation of
the values of each <a>WebVTT Text Object</a> in |nodes|, in a pre-order, depth-first
traversal, excluding <a lt="WebVTT Ruby Text Object">WebVTT Ruby Text Objects</a> and their
descendants, to determine the <i>paragraph embedding level</i> of the first Unicode paragraph
of the cue. [[!BIDI]]</p>
<p class="note">Within a cue, paragraph boundaries are only denoted by Type B characters, such
as U+000A LINE FEED (LF), U+0085 NEXT LINE (NEL), and U+2029 PARAGRAPH SEPARATOR. (This means
each line of the cue is reordered as if it was a separate paragraph.)</p>
</li>

<li><p>If the <i>paragraph embedding level</i> determined in the previous step is even (the
<i>paragraph direction</i> is left-to-right), let |direction| be "ltr", otherwise, let it be
"rtl".</p></li>

<li><p>Let |offset| be |cue|'s <a lt="cue computed position">computed position</a> multiplied
by |region|'s <a>WebVTT region width</a> and divided by 100 (i.e. interpret it as a percentage
of the region width).</p></li>
Expand Down Expand Up @@ -3526,28 +3509,6 @@ following algorithm.</p>

<ol>

<li>

<p>Apply the Unicode Bidirectional Algorithm's Paragraph Level steps to the concatenation of the
values of each <a>WebVTT Text Object</a> in |nodes|, in a pre-order, depth-first traversal,
excluding <a lt="WebVTT Ruby Text Object">WebVTT Ruby Text Objects</a> and their descendants, to
determine the <i>paragraph embedding level</i> of the first Unicode paragraph of the cue.
[[!BIDI]]</p>

<p class="note">Within a cue, paragraph boundaries are only denoted by Type B characters, such as
U+000A LINE FEED (LF), U+0085 NEXT LINE (NEL), and U+2029 PARAGRAPH SEPARATOR. (This means each
line of the cue is reordered as if it was a separate paragraph.)</p>

</li>

<li>

<p>If the <i>paragraph embedding level</i> determined in the previous step is even (the
<i>paragraph direction</i> is left-to-right), let |direction| be "ltr", otherwise, let it be
"rtl".</p>

</li>

<li><p>If the <a>WebVTT cue writing direction</a> is <a lt="WebVTT cue horizontal writing
direction">horizontal</a>, then let |writing-mode| be "horizontal-tb". Otherwise, if the <a>WebVTT
cue writing direction</a> is <a lt="WebVTT cue vertical growing left writing direction">vertical
Expand Down Expand Up @@ -4087,17 +4048,16 @@ layer as defined in this section. [[!CSS21]]</p>
<ul>
<li>the 'position' property must be set to ''position/absolute''</li>
<li>the 'unicode-bidi' property must be set to ''unicode-bidi/plaintext''</li>
<li>the 'direction' property must be set to |direction|</li>
<li>the 'writing-mode' property must be set to |writing-mode|</li>
<li>the 'top' property must be set to |top|</li>
<li>the 'left' property must be set to |left|</li>
<li>the 'width' property must be set to |width|</li>
<li>the 'height' property must be set to |height|</li>
</ul>

<p>The variables |direction|, |writing-mode|, |top|, |left|, |width|, and |height| are the values
with those names determined by the <a>rules for updating the display of WebVTT text tracks</a> for
the <a>WebVTT cue</a> from whose <a lt="text track cue text">text</a> the <a>list of WebVTT Node
<p>The variables |writing-mode|, |top|, |left|, |width|, and |height| are the values with those
names determined by the <a>rules for updating the display of WebVTT text tracks</a> for the
<a>WebVTT cue</a> from whose <a lt="text track cue text">text</a> the <a>list of WebVTT Node
Objects</a> was constructed.</p>

<p>The 'text-align' property on the (root) <a>list of WebVTT Node Objects</a> must be set to the
Expand Down

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