SVG Animation Profile 2: Replacement Sandwiches

Status: Accepted source profile.

SVG Animation Profile 2 extends SVG Animation Profile 1 by allowing several admitted <animate> elements to target the same resolved element and property. It is cumulative: every source accepted by Profile 1 is accepted by Profile 2 and produces the same sampled values, including the same binary32 bits.

This profile adds no source syntax. It replaces only Profile 0's rule that a target/property pair may have at most one animation. All inherited timing, value, interpolation, target, failure, and trust-boundary rules remain in force. In particular, every admitted effect is still a complete replacement: additive may only be absent or replace, and accumulate may only be absent or none.

SVG Animation Profile 3 keeps this priority model and adds typed addition and repeat-iteration accumulation.

The model follows the SMIL animation sandwich model within Profile 2's deliberately bounded timing grammar.

Replacement sandwich

For one resolved element/property pair, the authored base value is the bottom of a sandwich. Every animation that contributes at the requested sample time is a layer above it. Active and frozen animations both contribute; an animation before its begin or after a fill="remove" active end does not.

Each admitted layer is non-additive. Therefore the highest-priority contributing layer supplies the presentation value and masks all layers below it. If no layer contributes, the property has no sampled override and the authored base value is observed.

Sampling a sandwich still produces at most one effective value for its target/property pair. The sandwich is composition inside the animation program, not a relaxation of the effective-value set's unique-target rule.

Priority

Profile 2 admits one resolved, non-negative begin clock per animation and no restart mechanism. Its priority is consequently fixed for the source snapshot:

1. the animation whose interval begins later has higher priority; and
2. when interval begin times are equal, the animation element appearing later in XML document order has higher priority.

Document order alone is not sufficient: an earlier animation element with a later begin masks a later element whose interval began earlier.

An internal repeat boundary does not begin a new interval and does not change priority. A frozen effect keeps the priority of the interval that produced it. Thus a later temporary replacement may mask a frozen layer, and the frozen layer becomes visible again when the replacement removes.

General SMIL also defines priority for timing dependencies and restarts. Those cases are unreachable here because Profile 2 continues to reject sync-base, event, repeat-event, begin-list, script-triggered, and restart timing. The static pair (interval begin, document order) must not be generalized to those future families; they require priority derived from resolved interval activation.

Sampling procedure

For each target/property sandwich, a processor:

1. validates every layer and its live target, including inactive and masked layers;
2. determines each layer's contribution at the exact requested sample time;
3. selects the highest-priority contributing layer;
4. samples that layer under the inherited keyframe, easing, repeat, and fill rules; and
5. emits its one typed replacement value, or no value when no layer contributes.

Program validation and frame construction remain atomic. An invalid or stale masked layer still rejects Sample; priority cannot hide an invalid program. Sampling remains stateless, so direct seeking and any prior sample sequence produce the same sandwich result at the same time.

Boundaries and fallthrough

The inherited active interval is begin-inclusive and end-exclusive.

• Before a higher layer begins, the next contributing lower layer or base is visible.
• At the higher layer's exact begin, its first keyframe replaces lower layers.
• At an internal repeat boundary, that same layer remains in place and samples its first keyframe for the new iteration.
• At its final active end, fill="remove" stops contributing immediately and reveals the next contributing layer or base.
• At that end, fill="freeze" contributes the exact final keyframe and keeps masking lower layers.

Fallthrough samples a lower animation at the current document time. It does not resume that animation from the point at which it became hidden.

Validation and diagnostics

Profiles 0 and 1 continue to reject a second animation for the same target/property pair. Profile 2 accepts it only when every layer independently conforms to the cumulative profile.

A source processor preserves document order before any canonical grouping by runtime target. If an already resolved runtime target later becomes stale or inapplicable, diagnostics identify every layer in its sandwich because target validity is contribution-independent. Compile-time target-resolution errors identify the offending source site. A sampled effective-state failure identifies the winning contributors that produced the failed value set rather than unrelated inactive or masked layers.

Conformance

Profile 2 adds these laws to the inherited conformance contract:

• every Profile 1 source samples bit-identically under Profile 2;
• a later interval begin wins even when its animation element appears earlier in document order;
• equal interval begins are resolved by later document order;
• before a higher layer begins, a contributing lower layer remains visible;
• at a higher layer's remove end, the lower layer is sampled at the current document time and becomes visible again;
• a higher frozen layer continues to mask lower layers;
• when every layer is inactive or removed, the authored base is observed;
• repeat iteration boundaries do not change layer priority;
• each target/property pair emits at most one effective value;
• arbitrary direct-seek order does not change the result; and
• an invalid masked layer rejects the complete sample.

Boundary tests probe one nanosecond before, exactly at, and one nanosecond after each admitted begin, repeat, and active-end time. Browser differential tests use only times representable by both systems and explicitly seek the SVG timeline; their tolerances do not replace the profile's exact internal oracle.

Deferred families

Profile 2 resolves only replacement sandwiches over the existing scalar property family. Profile 3 separately resolves addition and accumulation. by, lone-to animation, dynamic interval priority, begin lists, sync-base and event timing, restarts, indefinite timing, CSS/Web Animations interaction, transforms, motion paths, colors and paints, more targets and properties, and optimized execution remain outside this profile.