Providing Script-Embedded Text Captions for MediaElement Content

Important Information about Techniques

See Understanding Techniques for WCAG Success Criteria for important information about the usage of these informative techniques and how they relate to the normative WCAG 2.1 success criteria. The Applicability section explains the scope of the technique, and the presence of techniques for a specific technology does not imply that the technology can be used in all situations to create content that meets WCAG 2.1.

Applicability

Note

Microsoft has stopped updating and distributing Silverlight, and authors are encouraged to use HTML for accessible web content.

This technique relates to Success Criterion 1.2.2: Captions (Prerecorded) (Sufficient as a way to meet G87: Providing closed captions).

Description

The objective of this technique is to use text captioning that is embedded in the stream with media displayed in a Silverlight MediaElement, and present that text captioning in a separate Silverlight control or text element.

This particular technique uses scripting files with a TimelineMarkers collection that are embedded directly within the media file. When text captioning is embedded directly in the streams, synchonization of the scripting stream versus the video content stream is done automatically by the MediaElement component. Each time the MarkerReached event fires, that is an indication that a synch point in the video that corresponds to a script marker entry has been reached. Silverlight application authors can obtain the text from the relevant timeline marker entry through their event handler implementations, and can display captions in the user interface area where the text captions are displayed. Typical Silverlight controls that can be used for displaying text captions include TextBlock (nonfocusable), TextBox, or RichTextBox. A typical interface design would place the caption-display control in close proximity to the MediaElement control that is being captioned, for example might place the captions directly underneath the MediaElement "screen".

Script-embedded captions are captions that are stored directly in the media file as metadata, rather than as a separate file. For information about techniques for captions in separate files, see .

Tools

Producing the media file with TimelineMarkers captions directly in embedded scripting can be accomplished using the Microsoft Expression Encoder tool. Online help for the procedure of encoding scripting with text captions in the stream are available in the offline Help file that installs with the Microsoft Expression 4 Encoder products. For more information, see Expression Encoder Pro Overview.

There is a public API for introducing Markers into a WMV file, as part of the Windows Media Format SDK. Using Expression Encoder is the way that the task of directly embedding TimelineMarkers is presented and taught in Microsoft's available instructional material on Silverlight. However, because the mechanism is public, it is possible that other tools exist or will exist that can also produce media with script-encoded TimelineMarkers.

Examples

Example 1: MediaElement handles MarkerReached, displays marker text in existing TextBox

This example has a UI definition in XAML and interaction logic in C#. The following is the basic UI in XAML. This example is deliberately simple and does not include AutomationProperties for identification or user instructions. The most relevant part of this example is that the Silverlight author declares a handler for the event MarkerReached. This event fires potentially hundreds of times, once for each caption in the stream. Each time the event fires, the event handler runs and adds the text to the dedicated TextBox in the user interface. 

<UserControl x:Class="MediaTimelineMarkers.MainPage"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>
   <StackPanel x:Name="LayoutRoot" Background="White">
       <MediaElement MarkerReached="OnMarkerReached"
       HorizontalAlignment="Left"
       Source="/spacetime.wmv"
       Width="300" Height="200" />
       <ScrollViewer>
           <TextBox Name="captionText" Height="40"
           IsReadOnly="true" AcceptsReturn="true"/>
       </ScrollViewer>
   </StackPanel>
 </UserControl>

private void OnMarkerReached(object sender, TimelineMarkerRoutedEventArgs e)
{
   captionText.Focus();
   captionText.SelectedText = e.Marker.Text.ToString() + "\n";
}

This example is shown in operation in the working example of Media Timeline Markers.

Resources

Resources are for information purposes only, no endorsement implied.

Tests

Procedure

  1. Using a browser that supports Silverlight, open an HTML page that references a Silverlight application through an object tag. The application plays media that is expected to have text captioning.
  2. Check that a text area in the user interface shows captions for the media.

Expected Results

# 2 is true.