Interpolating Dissimilar SVG Paths with Flubber

This article explores how the open-source library Flubber resolves common challenges in SVG shape morphing by smoothly interpolating dissimilar vector paths. Standard vector interpolation often fails or creates visually chaotic artifacts when morphing between shapes with differing numbers of vertices, complex curves, or disconnected sub-paths. Flubber solves this by intelligently normalizing, resampling, and aligning geometry to produce predictable and aesthetically pleasing transitions.

The Core Challenge of SVG Path Morphing

Vector animations rely on interpolating coordinates between a starting path (\(d_1\)) and an ending path (\(d_2\)). Standard linear interpolation algorithms require a direct 1-to-1 mapping between the command segments and vertices of both paths.

When two SVG paths are dissimilar—such as morphing a simple triangle (3 points) into a detailed star (10 points)—standard tools struggle. Common issues include:

• Vertex mismatch: The animation fails or snaps abruptly when point counts differ.
• Point twisting: Vertices travel across the interior of the shape, creating self-intersecting loops and unnatural distortion.
• Topological differences: Standard interpolators cannot naturally handle transitioning between a single shape and multiple disconnected shapes (like morphing the letter “O” into the letter “B”).

How Flubber Solves Disparity

Flubber overcomes these limitations through several algorithmic steps that prepare and pair vector paths before interpolation begins.

1. Automatic Path Resampling and Subdivision

Rather than requiring designers to manually add matching anchor points in vector software, Flubber automatically resamples both the source and target paths. It computes the perimeter of both paths and subdivides them by inserting additional points along path segments. This guarantees that both paths have an identical number of vertices spaced evenly relative to the geometry.

2. Best-Match Vertex Alignment

Once vertex counts match, Flubber determines the optimal starting index and winding direction (clockwise vs. counter-clockwise) for the paths. By minimizing the sum of squared Euclidean distances between corresponding points, it prevents vertices from traveling unnecessarily far, eliminating the unnatural spinning or folding effects seen in naive interpolators.

3. Shape Triangulation and Interior Mapping

For complex transitions, Flubber uses polygon triangulation. It decomposes source and target shapes into a mesh of triangles. By mapping corresponding triangles between the two shapes, Flubber can smoothly transform complex, concave, or multi-segmented shapes with continuous geometric coherence.

4. Splitting and Merging Topologies

Flubber includes dedicated methods for handling multi-path scenarios:

• flubber.separate(): Morphs a single SVG path into multiple discrete closed paths.
• flubber.combine(): Merges multiple separate SVG paths into a single cohesive path.
• flubber.interpolate(): Standard morphing between two distinct shapes.
• Specialized methods like toCircle(), fromCircle(), toRect(), and fromRect() provide optimized geometric transforms for primitive shapes.

Integrating Flubber into Animation Workflows

Flubber is design-agnostic and returns an interpolation function that accepts a progress parameter \(t\) (ranging from 0 to 1) and outputs an SVG path string (d attribute). This makes it directly compatible with modern web animation tools, including:

• D3.js: Usable inside d3.transition().attrTween() for data-driven visualizations.
• GSAP (GreenSock): Usable inside generic tween callbacks.
• Vanilla JavaScript: Executable directly within a requestAnimationFrame loop.

By automating path normalization, segment distribution, and spatial alignment, Flubber allows developers to create fluid, production-ready SVG morphing animations between completely dissimilar shapes without manual vector tweaking.