How Does the SMIL seq Element Work?

The <seq> (sequence) timing container in Synchronized Multimedia Integration Language (SMIL) defines a timeline where child elements play strictly one after another in chronological order. Unlike parallel containers that trigger simultaneous playback, the <seq> element establishes automatic temporal dependencies between siblings, advancing playback only after each preceding item concludes. Understanding how SMIL manages implicit start times, active durations, and end conditions within a <seq> container is fundamental for creating structured multimedia playlists and presentations.

Core Mechanics of Sequential Playback

At its foundation, <seq> coordinates the playback flow by evaluating the completion of each nested element. The first child element begins playing immediately when the parent <seq> container becomes active, unless an explicit delay is configured.

Each subsequent child element’s implicit start time is tied directly to the active end of the element immediately preceding it. Once the current media element finishes its active duration—whether through intrinsic file duration or explicit SMIL timing attributes—the SMIL timing engine resolves the next sibling's start condition and initiates its playback.

Implicit vs. Explicit Start Times

In a standard parallel container (<par>), child elements without explicit timing start at time zero relative to the container. Within a <seq> container, the default timing semantics change:

• Implicit Timing: If no begin attribute is declared on a child element, its begin is implicitly defined as prev.end (the exact moment the preceding sibling finishes).
• Positive Offsets: Specifying a positive offset such as begin="2s" introduces a pause or gap between the end of the previous item and the start of the current item.
• Event-Based Overrides: If a child defines an explicit event or syncbase condition (for example, waiting for a user click), playback of that child halts until the condition is met, pausing the overall sequence progression.

Handling Element Duration and container End

The total duration of a <seq> container is determined by the cumulative sum of the active durations of all its children, including any explicit delays between them.

Several key timing attributes influence how the sequence completes:

• dur Attribute on Children: Explicitly setting dur="5s" on an image or text element defines its active duration, allowing static media to hold on screen before the sequence automatically steps forward.
• dur Attribute on <seq>: Setting a hard duration on the <seq> container itself will force the entire sequence to terminate when that duration elapses, even if some children have not yet played.
• endsync Evaluation: Because children play sequentially rather than concurrently, the default endsync="last" attribute naturally resolves when the final sequential child element terminates.
• repeatCount and repeatDur: When repeating loops are applied to a <seq> container, the entire ordered list of media items resets and plays from the first child once the final child completes.