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Load a Preview Image Before the Main Content

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Use Case

You have an exceptionally large video to present to site visitors. As each visitor’s connection reliability and bandwidth are unknown and unpredictable, you want to show them a still image preview of the video (which will load more quickly) until the video has finished loading or has sufficiently buffered.

Implementation notes

Across a Manifest and its resources, you may use more than one placeholderCanvas, allowing you to have an authentic placeholderCanvas for each appropriate resource (Collection, Manifest, Canvas, and Range).

The target of the Annotation of a placeholderCanvas should have as its value the id of the placeholderCanvas, not the id of the resource that has the placeholderCanvas.

Always keep in mind the wide latitude given conforming clients: It is up to the client whether and in what sort of UI to display content you place in a placeholderCanvas property. Do not use this property for content that must be displayed. On the other hand, placing content in a placeholderCanvas does communicate to a conforming client that the content, if displayed, should be displayed before the resource to which it is attached.

Restrictions

Each instance of placeholderCanvas may only contain one Canvas, and as such may specifically not contain an additional placeholderCanvas or an accompanyingCanvas.

Example

In the example, the main content is a video of a performance of Donizetti’s L’elisir d’amore and the placeholderCanvas is a still frame from the video. For a site visitor with limited bandwidth, showing the still frame allows them to get an advance look at performance aspects such as costuming, staging, and set design.

Note: The placeholderCanvas property is not yet supported in viewers.

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