--- a/608toVTT/608toVTT.html Fri Nov 02 12:15:32 2012 +1100
+++ b/608toVTT/608toVTT.html Fri Nov 02 17:01:05 2012 +1100
@@ -358,6 +358,116 @@
</section>
<section>
+ <h2>Positioning in CEA-708</h2>
+ <p>
+ CEA-708 captions make use of what is called a "Caption Screen", which for WebVTT is the video viewport. CEA-708 defines a grid on top of the video viewport that depends on the dimensions of the video - e.g. it is different for 16:9 video to 4:3 video. For WebVTT we expect that the grid locations are converted to percentage locations in a coordinate system with (0%,0%) at the top left and (100%,100%) on the bottom right. This specification relies on this percentage-based coordinate system.
+ </p>
+
+ <p>
+ Within the Caption Screen, CEA-708 defines "Caption Windows" through which the caption text is rendered. For WebVTT we map CEA-708 "Caption Windows" to <a href="http://www.w3.org/community/texttracks/wiki/MultiCueBox">WebVTT Regions</a>. In CEA-708 there are at most 8 Caption Windows - in WebVTT we can define an unlimited number of regions.
+ </p>
+
+ <p>
+ CEA-708 Caption Windows define a priority between the 8 captions windows with lower priority windows overlaying higher priority windows. Priority values are mapped to the WebVTT "layer" attribute of Regions as follows:
+ </p>
+
+ <table border="1">
+ <thead>
+ <tr>
+ <td>CEA-708 Caption Window Priority</td>
+ <td>WebVTT Region layer attribute</td>
+ </tr>
+ </thead>
+ <tbody>
+ <tr>
+ <td>0</td>
+ <td>7</td>
+ </tr>
+ <tr>
+ <td>1</td>
+ <td>6</td>
+ </tr>
+ <tr>
+ <td>2</td>
+ <td>5</td>
+ </tr>
+ <tr>
+ <td>3</td>
+ <td>4</td>
+ </tr>
+ <tr>
+ <td>4</td>
+ <td>3</td>
+ </tr>
+ <tr>
+ <td>5</td>
+ <td>2</td>
+ </tr>
+ <tr>
+ <td>6</td>
+ <td>1</td>
+ </tr>
+ <tr>
+ <td>7</td>
+ <td>0</td>
+ </tr>
+ </tbody>
+ </table>
+
+ <p>
+ CEA-708 Caption Windows are positioned by specifying one of 9 different "Anchor points" covering a 9 field grid of the window. An Anchor Point is anchored to a specific location on the Caption Screen. Anchor points are mapped to the WebVTT "pin" attribute of Regions as follows:
+ </p>
+
+ <table border="1">
+ <thead>
+ <tr>
+ <td>CEA-708 Caption Window Anchor Point</td>
+ <td>WebVTT Region pin attribute</td>
+ </tr>
+ </thead>
+ <tbody>
+ <tr>
+ <td>0 (top-left corner)</td>
+ <td>pin:0%,0%</td>
+ </tr>
+ <tr>
+ <td>1 (top middle)</td>
+ <td>pin:0%,50%</td>
+ </tr>
+ <tr>
+ <td>2 (top-right corner)</td>
+ <td>pin:0%,100%</td>
+ </tr>
+ <tr>
+ <td>3 (left middle)</td>
+ <td>pin:50%,0%</td>
+ </tr>
+ <tr>
+ <td>4 (middle/center)</td>
+ <td>pin:50%,50%</td>
+ </tr>
+ <tr>
+ <td>5 (right middle)</td>
+ <td>pin:50%,100%</td>
+ </tr>
+ <tr>
+ <td>6 (bottom-left corner)</td>
+ <td>pin:100%,0%</td>
+ </tr>
+ <tr>
+ <td>7 (bottom middle)</td>
+ <td>pin:100%,50%</td>
+ </tr>
+ <tr>
+ <td>8 (bottom-right corner)</td>
+ <td>pin:100%,100%</td>
+ </tr>
+ </tbody>
+ </table>
+
+ </section>
+
+ <section>
<h2>Pop-on Captions</h2>
<p>
CEA-608 has a maximum of 4 rows of captions, which can be placed anywhere in the safe caption area.