What Does the SMIL dur Attribute Do?

In Synchronized Multimedia Integration Language (SMIL), the dur (duration) attribute specifies the simple duration of an element, defining exactly how long that media or animation element should play or remain active on its own timeline. By controlling the explicit playback length of elements like audio, video, images, or text streams, dur acts as the fundamental timing mechanism for synchronizing multi-track multimedia presentations.

Defining the Simple Duration

The primary role of the dur attribute is to establish the simple duration of an element, distinct from its active duration or total timeline duration. When a media element is triggered (via a begin time), dur tells the SMIL rendering engine how long to present that item before transitioning, ending, or repeating.

In a SMIL timing graph:

• Explicit Duration: Setting dur="10s" forces the element to play for exactly 10 seconds.
• Continuous Media: For video or audio files that possess an intrinsic length (such as a 30-second clip), setting a smaller dur (e.g., dur="15s") truncates playback, while a larger dur may freeze on the final frame or continue playing silence, depending on media configuration.
• Discrete Media: For static assets with no intrinsic duration, such as text strings or still images, dur provides the necessary playback length to display the asset within a timed layout.

Supported Attribute Values

The dur attribute accepts several distinct value types depending on the author's timing needs:

• Clock Values: Specific time values written in hours, minutes, seconds, or milliseconds (e.g., dur="5s", dur="01:30", dur="250ms").
• "indefinite": Specifies that the element has an unbounded duration and will play continuously until terminated by an external event, a parent container boundary, or an explicit end condition.
• "media": Explicitly directs the engine to use the intrinsic duration of the referenced media file. If applied to static media without intrinsic length, the duration resolves to indefinite.

Interaction with Containers and Timing Attributes

The dur attribute interacts directly with parent timing containers—such as <par> (parallel) and <seq> (sequential)—as well as other timing attributes like begin, end, and repeatCount.

• Sequential Execution (<seq>): In a <seq> container, the dur value dictates exactly when the subsequent child element starts. The next item begins immediately after the current item's simple duration finishes.
• Parallel Execution (<par>): In a <par> container, elements with different dur values run concurrently, allowing tracks of varying lengths (e.g., background music lasting dur="60s" paired with an image carousel where each image has dur="5s") to synchronize along a shared master timeline.
• Overrides with end: If an explicit end attribute condition occurs before dur elapses, the element terminates early. Conversely, if dur elapses before the end condition, the element ends at the duration limit unless extended by repetition parameters.