SVG Animation Profile 5: Solid Fill Paints

Status: Accepted source profile.

SVG Animation Profile 5 extends SVG Animation Profile 4 with solid-color animation of a <rect> element's fill property. It is cumulative: every source accepted by Profile 4 is accepted by Profile 5 and produces the same sampled values, including the same binary32 bits and transform-list topology.

All inherited source preservation, timing, interpolation, priority, composition, failure, diagnostic, and trust-boundary rules remain normative unless this document replaces them. This profile adds one typed value family; it does not add another animation element or a parallel color-only scene property.

The source behavior follows SVG's paint properties, animation value attributes, and addition attributes, within a deliberately narrower color grammar and deterministic numeric contract.

Scope of the extension

Profile 5 adds fill to the admitted property set for <animate> targeting a <rect>:

<rect id="tile" x="24" y="24" width="160" height="96" fill="#152238">
  <animate
    attributeName="fill"
    values="#7c3aed;#2563ebcc;#06b6d4"
    keyTimes="0;0.55;1"
    calcMode="spline"
    keySplines="0.2 0.8 0.2 1;0.4 0 0.6 1"
    dur="1200ms"
    repeatCount="2"
    fill="freeze"
  />
</rect>

The two uses of fill have distinct roles: attributeName="fill" selects the paint property, while the animation element's fill="freeze" controls post-interval timing behavior.

Profile 1's from/to and values forms, key times, linear interpolation, and spline interpolation apply to solid-fill keyframes. Profile 3's additive and accumulate forms apply componentwise. Profile 4's lone-to form is also admitted:

<animate attributeName="fill" to="#ffffff80" dur="1s" />

calcMode remains limited to linear and spline. Discrete and paced calculation remain deferred. <animateTransform> continues to target only transform and cannot target a paint property.

Admitted color literals

Every from, to, or values item for fill must be one hexadecimal sRGB literal in this closed grammar:

color ::= "#" hex hex hex
        | "#" hex hex hex hex
        | "#" hex hex hex hex hex hex
        | "#" hex hex hex hex hex hex hex hex
hex   ::= [0-9A-Fa-f]

After inherited XML-whitespace trimming, no additional characters are allowed. The forms expand as follows:

The eight-digit form uses CSS/SVG's red-green-blue-alpha order, not an alpha-red-green-blue storage order. Letter case has no semantic effect.

Named colors, transparent, currentColor, CSS color functions, system colors, none, context paint, and url(...) paint servers are rejected as animation values. This is a source-profile boundary, not a statement that those spellings are semantically incapable of producing colors.

Every admitted literal denotes four exact integer channel values in straight, unpremultiplied sRGB. RGB channels remain meaningful when alpha is zero. The RGB integers are sRGB-encoded component values and are not linearized before arithmetic. color-interpolation, color-interpolation-filters, and an output surface's color space do not change the Profile 5 interpolation space.

Paint-list projection

The semantic target is the existing Fills property: an ordered list of typed paints in painter order, first paint bottommost and last paint topmost. fill animation does not create a second color property beside that list and does not target a color field nested inside one paint.

One sampled solid-fill value projects to the complete list

[ Solid(active = true, color = RGBA, blend = normal) ]

Alpha is carried by the solid color. The animation does not manufacture a separate opacity layer. Replacing fill replaces the complete ordered list, not only its first or last member. Consequently a contributing replacement can cover an empty, multi-paint, gradient, or image-backed authored list when the active static source profile can represent that list; when the effect no longer contributes, the authored list is observed unchanged.

Addition remains a typed color operation. It adds RGBA channel values and projects one solid paint; it never concatenates paint lists, adds another paint layer, or modifies the authored list in place.

Compatible underlying values

A replacement-only fill sandwich does not use its authored base as a color operand. It therefore requires the target and complete Fills projection, but it does not require the authored list itself to be a singleton solid.

An additive="sum" or lone-to fill layer does consume the lower sandwich result. Every possible base operand for such a sandwich must therefore be exactly one active, normal-blend solid paint. Its four straight-sRGB channels are the typed underlying value. An empty list, several paints, a gradient, image, inactive solid, non-normal blend, or unresolved resource is not silently flattened or sampled into a color.

As in Profiles 3 and 4, this compatibility requirement is validated for the authored base even when a replacement currently masks it or the dependent layer is inactive. Program validity cannot depend on the requested sample time. Every fill effect produced by Profile 5 is itself a compatible singleton solid, so higher additive and lone-to layers can consume it.

For a lone-to fill effect, the live lower RGBA value U(t), authored target T, and inherited eased progress q(t) produce, componentwise,

R(t) = U(t) + (T - U(t)) × q(t)

U(t) is the result already folded at the same sample time, not a color captured at compilation, interval begin, or first playback. The layer remains non-additive and non-cumulative exactly as Profile 4 requires. At a frozen final end it contributes the exact authored target color.

Straight-RGBA numeric contract

Profile 5 uses one exact channel domain through the complete fill sandwich. In this section, q means the final exact segment progress selected by the inherited calculation mode: linear segment progress or spline-eased progress. For each channel c in red, green, blue, and alpha:

1. A literal supplies an exact integer c in [0, 255].
2. Keyframe interpolation evaluates a + (b - a) × q exactly using the inherited selected and eased progress q.
3. accumulate="sum" evaluates V + k × T exactly, using the terminal keyframe channel T and zero-based repeat index k.
4. additive="sum" adds the effect channel to the exact lower channel.
5. Lone-to interpolation consumes the exact lower channel by the formula above.

These operations use straight channels independently. RGB is not multiplied or divided by alpha, and alpha is not a separate post-composition scalar. There is no channel clamp, eight-bit quantization, or binary floating-point rounding between keyframes, repeat accumulation, or sandwich layers.

After the complete target/property sandwich has been folded, each channel is projected exactly once:

bounded   = min(255, max(0, channel))
quantized = floor(bounded + 1/2)

The four quantized integers form the final straight RGBA8 solid color. Thus a half-channel tie rounds upward. Exact keyframe endpoints survive unchanged when no other layer composes over them. Unbounded intermediate arithmetic is intentional: a higher lone-to layer can consume a lower value above 255 before the one final clamp.

This color contract replaces Profile 3's per-operation binary32 rounding only for the fill value family. The inherited scalar and transform families keep their existing numeric contracts.

Ordering and boundaries

Profile 2's priority and Profile 3's sandwich procedure apply unchanged. A contributing replacement fill is a cutoff for all lower fill layers and for the authored paint list. Higher additive and lone-to effects fold over its exact RGBA value in low-to-high priority order.

At an exact internal repeat boundary, simple progress resets to zero and the repeat index increments before channel accumulation. At the final active end, fill="remove" falls through to the next contributing fill layer or authored paint list, while fill="freeze" retains the terminal effect value with the final repeat's accumulation. The animation timing attribute does not alter the paint-list topology described above.

Validation and diagnostics

Profile 5 inherits whole-document atomic validation. In addition to inherited failures, compilation or sampling rejects at the applicable boundary:

• a fill keyframe outside the four admitted hexadecimal forms;
• none, a CSS color function or keyword, context paint, or a paint-server reference used as an animated value;
• a fill target whose complete ordered-paint projection or one-to-one node identity was lost during materialization;
• an additive or lone-to fill sandwich whose authored base is not exactly one active, normal-blend solid paint; or
• an attempt to treat a gradient stop, stroke member, image source, or other nested paint field as this profile's fill target.

A color diagnostic identifies the source-located animation, offending value item, resolved target, and fill property, and lists the four accepted forms. An underlying-value diagnostic reports the required singleton-solid topology and the observed paint-list topology. Resource and multi-paint values are reported as unsupported typed values, not as malformed colors.

All layers and live targets are validated, including inactive and masked layers. Failure produces neither a partial effective-value set nor a partial frame. Exact rational channel evaluation has no finite-overflow failure; implementation resource limits must be reported independently and must not be misrepresented as a source-profile violation.

Conformance

Profile 5 adds these laws to the inherited conformance contract:

• every Profile 4 source samples bit-identically under Profile 5;
• all four hexadecimal forms expand to the required RGBA8 endpoints, including CSS/SVG alpha-last order;
• red, green, blue, and alpha use the same straight-channel interpolation, accumulation, and addition rules;
• an RGB channel paired with zero alpha is preserved through interpolation rather than discarded by premultiplication;
• a replacement fill emits exactly one active, normal-blend solid paint and replaces the complete lower paint list;
• when no fill effect contributes, the authored ordered paint list is revealed without mutation or normalization;
• color addition is componentwise and never appends a paint-list member;
• additive and lone-to sandwiches reject every non-singleton-solid authored base, even while a replacement masks it;
• lone-to consumes the exact live lower RGBA value at the same sample time;
• no intermediate layer clamps or quantizes a channel; each completed sandwich clamps and quantizes every channel exactly once;
• an exact half-channel rounds upward, and exact endpoints remain exact; and
• typed query results and pixels agree with the same final singleton-solid Fills value at every keyframe, repeat, and fill boundary.

A canonical rounding vector interpolates from #00000000 to #01010101 at progress 0.5; its final color is #01010101. A late-clamp vector begins at #f00000, adds the constant color #20000000, then applies a higher lone-to effect toward #000000 at progress 0.5. The required red result is 136: the exact lower value 272 reaches the lone-to layer before final projection. Clamping the lower layer first would incorrectly produce 128.

Browser differential tests remain separate from this exact oracle. They use only the source overlap admitted by both systems, seek the SVG timeline explicitly, and record browser color serialization and pixel tolerances. A browser's wider CSS color parser, float arithmetic, or intermediate color projection cannot override this profile's grammar or exact-channel rules.

Deferred families

Profile 5 resolves one solid-color fill family. It does not define general interpolation between arbitrary paint lists or paint variants.

stroke, fill-opacity, stroke-opacity, stop-color, gradient-stop animation, gradients, patterns, images, context paint, URL paint servers, switching to or from none, multiple animated fill paints, paint visibility or blend animation, named and functional color syntax, wider color spaces, color-interpolation, discrete and paced calculation, and CSS/Web Animations interaction remain outside this profile.

Gradient stops and stroke paints are nested inside resource or stroke aggregates whose members need durable target identity before independent animation can be defined. Arbitrary paint-kind and resource switching also needs a global discrete-animation primitive. Those concerns are not modeled as color interpolation and are not approximated by Profile 5.