SVG Animation Profile 6: Path Geometry

Status: Accepted source profile.

SVG Animation Profile 6 extends SVG Animation Profile 5 with animation of a <path> element's d property. It is cumulative: every source accepted by Profile 5 is accepted by Profile 6 and produces the same sampled values, including the same scalar, transform, and color bits.

All inherited source preservation, timing, keyframe, priority, failure, diagnostic, and trust-boundary rules remain normative unless this document replaces them. This profile adds one typed geometry value family. It does not add a string-valued mutation channel, a second path renderer, or automatic shape matching.

The source behavior follows SVG 2's d property and path interpolation rules and SVG's animation value attributes, within the deliberately bounded compatibility and numeric contract below.

Scope of the extension

Profile 6 adds <path> as an animation target and d as its one newly admitted property:

<path
  id="mark"
  d="M24 96 C24 40 120 40 120 96 C120 152 24 152 24 96 Z"
  fill="#7c3aed"
>
  <animate
    attributeName="d"
    from="M24 96 C24 40 120 40 120 96 C120 152 24 152 24 96 Z"
    to="M24 48 C72 48 120 48 120 96 C120 144 72 144 24 144 Z"
    dur="1200ms"
    fill="freeze"
  />
</path>

The animation may instead be a root-level <animate href="#mark"> under the inherited same-document target rules. The resolved target must be one <path> that materializes one-to-one as one ordinary scene node, and its complete d geometry must retain a one-to-one typed property projection. Source commands are canonicalized for typed compatibility as defined below; later rendering normalization must not replace that animation contract or erase the retained authored source.

Because Profile 6 admits <path> as an ordinary materialized target, the inherited opacity, solid fill, and animateTransform effects may also target it. They retain their existing typed-property semantics and introduce no further path-specific value family. The rectangle-only x, y, width, and height effects remain inapplicable to <path>.

Profile 1's from/to and values forms, key times, linear interpolation, and spline easing apply as narrowed below. Profile 2's replacement sandwich ordering applies. Path animation is replacement-only: additive may be absent or replace, and accumulate may be absent or none. Lone-to, by, additive="sum", and accumulate="sum" are rejected for d.

Profile 6 also admits calcMode="discrete" for d. This admission does not extend discrete calculation to inherited scalar, transform, or paint families.

Admitted path values

Every static d value and every from, to, or values item is first required to be one non-empty path-data string accepted by the complete SVG path-data grammar. It may use:

• absolute or relative move, line, horizontal-line, vertical-line, cubic, smooth-cubic, quadratic, smooth-quadratic, elliptical-arc, and close commands;
• omitted repeated command letters and repeated parameter groups; and
• the separators and numeric spellings admitted by that grammar.

none, an empty string, and a command stream with no drawing segment are outside this profile. Arc flags must be exactly 0 or 1, and arc radii must be non-negative. Source recovery or error correction from wider SVG processing does not repair an animation value.

Profile 6 has one source-number conversion for static and animated path data:

1. Each SVG number is parsed to the nearest finite IEEE 754 binary64 value.
2. Relative-coordinate addition and reflected shorthand controls are evaluated in binary64.
3. Each resulting absolute coordinate is rounded to IEEE 754 binary32 using round-to-nearest, ties-to-even; either signed zero becomes positive zero.
4. A non-finite result rejects the complete source.

This two-stage conversion is normative; parsing a source decimal directly to binary32 is not equivalent at every rounding boundary. Animation does not have a second number parser or recovery path.

The positive root SVG viewport is the fixed reference rectangle for the static path and every keyframe. After ordinary static arc lowering in SVG user space, each binary32 x-coordinate is divided once by the binary32 viewport width and each y-coordinate once by the binary32 viewport height. The resulting tight geometry must fit inside the closed 0 0 1 1 rectangle. A value outside it is rejected. This is a bounded source-profile restriction, not a claim that general SVG clips path geometry to its viewport. A wider profile may select a different stable reference rectangle, but it must select that rectangle once for the complete effect rather than renormalizing each keyframe independently.

Smooth calculation has a narrower command domain. Every smooth keyframe must contain only M, L, H, V, C, S, Q, T, and Z, including their lowercase relative forms. The presence of any A or a command rejects a linear or spline path effect with a diagnostic that identifies the command and recommends calcMode="discrete". Static and explicitly discrete paths continue to admit arcs.

The values attribute continues to split only on semicolons before each item is parsed as path data. XML whitespace around each item is ignored, but whitespace inside an item remains part of the path grammar. One final empty item has Profile 1's trailing-semicolon behavior.

Every parsed keyframe retains both:

1. its source-located authored spelling, for preservation and diagnostics; and
2. an immutable canonical command stream with already parsed parameters, for compatibility and sampling.

The retained source is never regenerated from the canonical stream.

Source topology and canonical geometry

Parsing retains two related structures. The source topology makes implicit command instances explicit while preserving the authored command family:

• additional coordinate pairs after an initial M or m become line instances;
• additional parameter groups become repeated instances of their governing command;
• an omitted command letter inherits the preceding command as SVG requires;
• uppercase and lowercase forms retain one shared family identity; and
• Z and z both become one parameterless close command.

The canonical geometry is the typed command stream sampled and rendered. Each static or keyframe value passes through the same source-number conversion and fixed-reference projection before curve construction:

• relative coordinates are resolved to absolute coordinates in the keyframe's own command stream;
• H and V become absolute line commands;
• S becomes a cubic command with its reflected first control point, and T becomes a quadratic command with its reflected control point; and
• close remains parameterless.

The resulting smooth command sum is closed:

Move(x, y)
Line(x, y)
Quadratic(cx, cy, x, y)
Cubic(c1x, c1y, c2x, c2y, x, y)
Close

Canonicalization removes contours with no drawing segment, but does not reorder drawable contours, insert points, change curve degree, or flatten curves. Arc commands have no canonical smooth form in Profile 6. Explicitly discrete arcs use the same complete-value lowering as the identical static path.

The topology signature is the ordered source-family sequence. Absolute and relative spellings within one family are compatible: L with l, H with h, S with s, and so on. Families do not collapse merely because the renderer shares an expanded primitive: H is not L, V is not L, S is not C, and T is not Q. This distinction is required by SVG's path-data interpolation contract and must survive renderer normalization.

Two keyframes are smoothly compatible exactly when their topology signatures are identical. A smooth values animation requires one shared signature across its complete keyframe list, not merely compatibility around the currently sampled segment. This makes the effect's typed topology independent of sample time.

Smooth path interpolation

Absent calcMode means linear. calcMode="linear" and calcMode="spline" request smooth interpolation. When the keyframe topology is compatible, Profile 1 selects the interval and produces one exact linear or spline-eased segment progress q.

Each corresponding reference-space canonical numeric parameter is interpolated independently:

a + (b - a) × q

The mathematical interpolation result is rounded once to finite IEEE 754 binary32 using round-to-nearest, ties-to-even. There is no intermediate binary floating-point rounding inside interpolation. At an exact key time, the complete stored canonical command stream of that keyframe is used directly; interpolation is not evaluated. Bounds calculation and raster lowering happen only after this typed path value has been produced.

The sampled reference-space geometry is mapped through the fixed viewport-sized box only after interpolation. This projection is geometrically equivalent to interpolating in SVG user coordinates, but the profile does not claim binary32 bit identity with an implementation that interpolates before reference-space projection. Its interpolation bit contract begins with the canonical geometry defined above. Every exact endpoint must equal the canonical geometry produced from the same d value as static source. Paint, fill rule, stroke, opacity, and transform remain separate properties and are unchanged by d animation.

Discrete path effects

Explicit calcMode="discrete" selects complete path keyframes without interpolating commands or parameters. Any collection of individually valid Profile 6 paths is admitted; their topology signatures may differ and they may contain arcs. Each selected keyframe uses the same canonical geometry that the corresponding static path would use.

For N >= 2 values, omitted keyTimes defines the exact offsets

t[i] = i / N, for i = 0 ... N - 1.

This gives one equal-duration interval per value, as required by SMIL discrete calculation. An explicit discrete keyTimes list must contain exactly N exact rational offsets, begin at 0, increase strictly, and remain in [0, 1]. The strict-increase rule deliberately retains Profile 1's unambiguous-boundary discipline even though SVG permits equal neighboring offsets. Unlike smooth calculation, the final offset need not be 1.

At simple progress p, the selected value is the greatest index i for which t[i] <= p. Equality selects the new value. If the final offset is exactly 1, the final value is not selected inside the end-exclusive active interval, but it remains the terminal value used by fill="freeze".

A one-value discrete animation is constant and admits neither keyTimes nor keySplines. For all discrete path animations, keySplines is rejected rather than retained as ignored easing metadata. A two-value discrete from/to animation without keyTimes therefore selects from for 0 <= p < 1/2 and to for 1/2 <= p < 1.

Automatic two-value fallback

SVG 2 assigns discrete interpolation when two d values do not have the same structure. Profile 6 admits that fallback only for the explicit two-value from/to form when both paths are inside Profile 6's non-arc smooth command domain.

When a from/to animation requests linear or spline calculation but its two valid paths have different topology signatures, the complete effect is a discrete replacement:

result = from, when eased segment progress q < 1/2
result = to,   when eased segment progress q >= 1/2

For linear mode, q is the segment-local progress. For spline mode, q is the result of Profile 1's exact spline inversion, so easing can move the document-time switch. Equality selects to. Exact progress zero and one use their stored endpoint streams directly.

No automatic fallback is inferred for the values form, including a two-item values list. A linear or spline values animation with any topology mismatch is a whole-document error whose diagnostic recommends declaring calcMode="discrete". This prevents one mistyped keyframe from silently changing a multi-stage morph into a different global effect.

An arc in either endpoint is also outside automatic fallback. Its diagnostic requires explicit calcMode="discrete", making the whole-value switch visible in authored source rather than accidentally treating a renderer's lowered arc segments as SVG arc parameters.

Replacement composition

One sampled d value replaces the target's complete path geometry. It does not merge contours with the lower path, replace only a command range, or mutate the authored geometry.

Profile 2 orders multiple replacement effects on the same target/property pair. The highest-priority active or frozen path effect contributes; a post-interval remove falls through to the next contributing effect or the authored base path, and freeze retains that effect's terminal keyframe. Smooth compatibility is checked within each effect only. A replacement effect's keyframes do not have to share topology with the authored base or a lower replacement effect.

The selected geometry is one effective value. Every geometry-aware consumer, including bounds, clipping where applicable, damage, cache identity, display construction, and pixels, observes that value rather than the authored base. A consumer whose declared contract is box-based may continue to consume the box; this profile does not require point-in-path hit testing.

Exact endpoints and no shape matching

Path endpoints are typed values, not reconstructed strings. Exactness means that at an authored key time or terminal freeze:

• the command topology is exactly the stored canonical topology;
• every numeric parameter has exactly the stored binary32 bits produced by ordinary static canonicalization of that keyframe; and
• a discrete arc endpoint is exactly the canonical geometry produced by that same path as a static value.

The retained source spelling remains independently unchanged. Equivalent whitespace, comma usage, repeated-command elision, or decimal spelling is not part of effective geometry identity.

Profile 6 performs no point resampling, contour pairing, contour reordering, winding reversal, path-length alignment, Bézier degree conversion, or "best" morph inference. Static and discrete arc lowering is an ordinary rendering normalization, never an interpolation domain. Other geometry-changing operations can be useful authoring tools, but they require an explicit export or normalization step outside animation sampling.

Validation and diagnostics

Profile 6 inherits whole-document atomic validation. In addition to inherited failures, compilation or sampling rejects at the applicable boundary:

• a d target that is not one identity-preserved <path>;
• an empty, none, malformed, non-finite, or non-drawing path value;
• an invalid arc flag or negative arc radius;
• any arc command in a linear or spline effect, with guidance to use explicit discrete calculation;
• a linear or spline values list whose source-family topology is not uniform;
• path keyTimes that violate the selected smooth or discrete rules;
• keySplines on a discrete path effect;
• a lone-to, by, additive, or cumulative path effect;
• a failure to preserve exact endpoint parameters or the complete effective geometry projection; or
• non-finite geometry produced while interpreting an otherwise finite relative command stream.

A path-syntax diagnostic identifies the source-located animation, value item, and path-data offset when available. A topology diagnostic identifies the differing keyframe, the first differing source-family command index, and the expected and observed command kinds. For a mismatched values form, it also states that calcMode="discrete" is the admitted explicit alternative. A target diagnostic distinguishes lost node identity from lost d property identity.

All layers and live targets are validated, including inactive and masked layers. Failure produces neither a partial effective-value set nor a partial frame. An implementation resource limit is reported as such and must not be misrepresented as malformed SVG path data.

Conformance

Profile 6 adds these laws to the inherited conformance contract:

• every Profile 5 source samples bit-identically under Profile 6;
• parent and same-document href targeting of the same path agree;
• command elision and repeated groups expand deterministically; absolute and relative modes share a family, while shorthand families remain distinct;
• every compatible numeric parameter follows the inherited exact linear or spline progress, with exact keyframe streams at equality;
• arcs remain valid static and explicitly discrete values but are rejected from smooth effects before conic lowering can affect semantics;
• explicit discrete values use one interval per value and switch exactly at their implicit or authored offsets;
• incompatible from/to paths fall back at eased progress 1/2, with equality selecting to;
• an incompatible smooth values list fails atomically and recommends explicit discrete calculation;
• replacement and freeze preserve complete path topology, while remove reveals the lower path without mutation;
• no resampling or contour matching participates in sampling; and
• bounds, damage, cache identity, draw-list construction, and pixels all observe the same sampled geometry, while query follows its declared box or path-narrowphase policy over that resolved frame.

Browser-differential vectors

Browser comparison remains separate from the profile's exact-bit oracle. Every browser vector explicitly seeks the SVG timeline and records the browser build, source, exact requested time, computed path value when exposed, and pixels. At minimum, the differential corpus contains:

Only vectors whose source lies in the shared browser/profile surface support a compatibility claim. Legacy browser behavior that rejects incompatible path lists, a browser's serialization choices, and floating-point raster tolerance are reported as differences; they do not alter Profile 6's source or exact sampling contract.

Deferred families

Profile 6 resolves compatible path interpolation and explicit discrete path replacement only.

Paced path-data interpolation, smooth arc-parameter and arc-flag interpolation, additive or cumulative path geometry, lone-to, by, none transitions, automatic point or contour matching, path morphing between basic-shape elements, motion paths, path-length and trim animation, markers, animated clipping paths or masks, CSS d animation and Web Animations interaction remain outside this profile.

<set> and general calcMode="discrete" support for other property families also remain deferred. Profile 6's discrete path effect establishes a typed selection rule; it does not silently make arbitrary scene values animatable.