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Define presentation compositor #61

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Feb 27, 2023
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Define presentation compositor #61

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57bb786
Define presentation compositor (#58)
palemieux Dec 14, 2022
8caafd9
Made Presentation Compositor a definition
palemieux Jan 5, 2023
b05cfa0
Merge remote-tracking branch 'origin/main' into issues/0058-define-pr…
palemieux Jan 10, 2023
b03fcfc
Address https://github.com/w3c/imsc-hrm/pull/61#discussion_r1071132858
palemieux Jan 16, 2023
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31 changes: 17 additions & 14 deletions spec/imsc-hrm.html
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Expand Up @@ -433,8 +433,8 @@ <h2>Architecture</h2>
<a>Intermediate Synchronic Documents</a> E<sub>i</sub>:</p>
<ul>
<li><a>non-empty ISDs</a> are processed using a simple double buffering model: while a <a>non-empty ISD</a>
E<sub>n</sub> is being painted into a Back Buffer, the previous <a>non-empty ISD</a> E<sub>m</sub> is available
for display in a Front Buffer; and</li>
E<sub>n</sub> is being painted into a Back Buffer by the <dfn>Presentation Compositor</dfn>, the previous <a>non-empty
ISD</a> E<sub>m</sub> is available for display in a Front Buffer; and</li>
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<li><a>empty ISDs</a> merely disconnect the Front Buffer from the display when presented,
i.e., presenting an <a>empty ISD</a> is equivalent to presenting nothing.</li>
</ul>
Expand Down Expand Up @@ -462,14 +462,17 @@ <h2>Architecture</h2>
<p>Similarly, Decoded Image Buffers D<sub>n</sub> and D<sub>n-1</sub> store decoded images across <a data-lt=
"Intermediate Synchronic Document">Intermediate Synchronic Documents</a>, allowing images to be copied into the Presentation
Buffer instead of decoded.</p>

<p>Each of the terms <a>Presentation Compositor</a>, Glyph Renderer, Glyph Copier, Image Decoder, and Image Copier
is defined by the algorithmic requirements defined for it in this specification.</p>
</section>

<section id='hypothetical-render-model-general'>
<h2>Algorithm</h2>

<p>The HRM algorithm processes a sequence of <a>Intermediate Synchronic Documents</a> E<sub>n</sub>.</p>

<p>Each successive <a>non-empty ISD</a> E<sub>n</sub> is rendered by the Presentation Compositor using the
<p>Each successive <a>non-empty ISD</a> E<sub>n</sub> is rendered by the <a>Presentation Compositor</a> using the
following steps in order:</p>

<ol>
Expand All @@ -482,7 +485,7 @@ <h2>Algorithm</h2>
<li>paint the text or image subtitle content.</li>
</ol>

<p>The Presentation Compositor begins rendering E<sub>n</sub>:</p>
<p>The <a>Presentation Compositor</a> begins rendering E<sub>n</sub>:</p>

<ul>
<li>at the presentation time of E<sub>m</sub>, where m is the largest non-zero value that is both less than
Expand All @@ -491,7 +494,7 @@ <h2>Algorithm</h2>
<li>at the presentation time of E<sub>n</sub> minus <a>IPD</a>, otherwise.</li>
</ul>

<p class="note">The Presentation Compositor never begins rendering an ISD more than <a>IPD</a> ahead of its
<p class="note">The <a>Presentation Compositor</a> never begins rendering an ISD more than <a>IPD</a> ahead of its
presentation time.</p>

<figure id='fig-rendering-presentation-time'>
Expand All @@ -500,13 +503,13 @@ <h2>Algorithm</h2>
<figcaption>
illustrates the rendering and presentation of <a>Intermediate Synchronic Documents</a>, where the hatched
areas indicate time spent drawing the associated <a>Intermediate Synchronic Document</a>. For example, the
Presentation Compositor begins rendering E<sub>1</sub> at the presentation time of E<sub>0</sub> since
E<sub>1</sub> is not an <a>empty ISD</a>. In contrast, the Presentation Compositor begins rendering
<a>Presentation Compositor</a> begins rendering E<sub>1</sub> at the presentation time of E<sub>0</sub> since
E<sub>1</sub> is not an <a>empty ISD</a>. In contrast, the <a>Presentation Compositor</a> begins rendering
E<sub>5</sub> at the presentation time of E<sub>5</sub> minus IPD since (i) both E<sub>3</sub> and
E<sub>4</sub> are <a>empty ISDs</a> and the presentation time of E<sub>5</sub> minus that of E<sub>2</sub> is
greater than IPD. Furthermore, E<sub>2</sub> remains in the Front Buffer until the presentation time of
E<sub>5</sub> but is not presented while E<sub>3</sub> and E<sub>4</sub> are presented, during which time the
Front Buffer is not available for display. Finally, the Presentation Compositor begins rendering E<sub>0</sub>
Front Buffer is not available for display. Finally, the <a>Presentation Compositor</a> begins rendering E<sub>0</sub>
at the presentation time of E<sub>0</sub> minus IPD since E<sub>0</sub> is the first <a>Intermediate
Synchronic Document</a>.
</figcaption>
Expand Down Expand Up @@ -549,7 +552,7 @@ <h2>Algorithm</h2>
example, if the Back Buffer is copied twice to Front Buffer between two consecutive video frame boundaries of the
<a>Related Video Object</a>.</p>

<p>It SHALL be an <a>error</a> for the Presentation Compositor to fail to complete painting pixels for
<p>It SHALL be an <a>error</a> for the <a>Presentation Compositor</a> to fail to complete painting pixels for
<a>non-empty ISD</a> E<sub>n</sub> before its presentation time.</p>

<p>The following table specifies the values of <a>IPD</a> and BDraw.</p>
Expand Down Expand Up @@ -639,10 +642,10 @@ <h2>Paint Regions</h2>
<section id='paint-images'>
<h2>Paint Images</h2>

<p>The Presentation Compositor paints into the Back Buffer all visible pixels of <a>presented
<p>The <a>Presentation Compositor</a> paints into the Back Buffer all visible pixels of <a>presented
images</a> of <a>Intermediate Synchronic Document</a> E<sub>n</sub>.</p>

<p>For each <dfn data-cite="IMSC#dfn-presented-image">presented image</dfn>, the Presentation Compositor either:</p>
<p>For each <dfn data-cite="IMSC#dfn-presented-image">presented image</dfn>, the <a>Presentation Compositor</a> either:</p>

<ul>
<li>if an identical image is present in Decoded Image Buffer D<sub>n</sub>, copies the image from Decoded Image Buffer
Expand Down Expand Up @@ -769,7 +772,7 @@ <h2>Paint Text</h2>
</p>

<p>For each <a>glyph</a> associated with a <a>character</a> in a <a>presented region</a> of <a>Intermediate Synchronic Document</a>
E<sub>n</sub>, the Presentation Compositor:</p>
E<sub>n</sub>, the <a>Presentation Compositor</a>:</p>

<ul>
<li>if an identical <a>glyph</a> is present in Glyph Buffer G<sub>n</sub>, copies the <a>glyph</a> from Glyph Buffer
Expand All @@ -787,10 +790,10 @@ <h2>Paint Text</h2>
</ul>

<figure>
<img src="figures/text-render-example-figure-2.png" alt="Example of Presentation Compositor Behavior for Text Rendering">
<img src="figures/text-render-example-figure-2.png" alt="Example of <a>Presentation Compositor</a> Behavior for Text Rendering">

<figcaption>
Example of Presentation Compositor Behavior for Text Rendering
Example of <a>Presentation Compositor</a> Behavior for Text Rendering
</figcaption>
</figure>

Expand Down