Round Corners

Shape operation that replaces sharp line-segment corners with smooth circular arcs. Works on polygonal shapes and primitive outlines.

Category: Shape Ops Menu path: Shape Ops > Round Corners

Ports

Port	Type	Direction	Description
shape_in	shape	input	Source shape
out	shape	output	Shape with rounded corners

Parameters

Param	Type	Default	Description
radius	scalar	10	Corner arc radius in comp pixels. Clamped to half of the shorter adjacent segment length, so very large values degenerate into maximum-roundness (arcs passing through segment midpoints) rather than overlapping neighboring corners
selectAttribute	string	""	Optional per-vertex attribute name used to filter which corners round. Empty (default) rounds every eligible corner. When set, only corners whose scalar attribute value is >= selectThreshold are rounded. Missing or non-scalar attribute → treated as 0.0 (so a typo'd name rounds nothing, matching PointDelete's convention)
selectThreshold	scalar	0.5	Threshold compared against the per-vertex attribute. Only used when selectAttribute is non-empty

Expose Channels

When enabled (E button on node header), adds input ports that override params via edge connections:

Port	Type	Overrides
radius_in	scalar	radius

Useful for driving the corner radius from a Time ramp, a Math expression, or another scalar source.

How It Works

For each interior vertex where two straight line segments meet, Round Corners:

1. Measures the two adjacent segment directions.
2. Inset the vertex back by radius along each tangent — producing two new points.
3. Connects the new points with a cubic bezier arc, using control-handle length r × (4/3) × tan(α/4) where α is the exterior angle (the industry-standard circular-arc bezier approximation).
4. Substitutes the original vertex with these two new inset points + arc in the output path.

Curved segments (already bezier) pass through unchanged. Collinear joints (dot product > 0.9999) are skipped — no rounding needed. Open-path endpoints aren't rounded (no outgoing segment to pair with).

The radius is clamped per-corner to min(incoming_length, outgoing_length) / 2, so the inset never goes past a segment midpoint. This means asking for a very large radius just produces maximally-rounded corners; it won't introduce self-intersections.

Per-point attributes on the original shape are duplicated onto both inset points (the inset points inherit from the original corner vertex). The index attribute is rewritten as a clean 0..N sequence matching the new point count.

Usage Examples

Basic: Rounded rectangle

Rectangle → RoundCorners(radius: 20) → DrawShape. The rectangle's four corners become arcs.

Basic: Rounded polygon

Polygon(sides: 6) → RoundCorners → DrawShape. Each corner of the hexagon softens.

Creative: Stroke offset

Run the shape through RoundCorners before DrawShape's stroke to avoid harsh miter joins at low stroke widths.

Creative: Progressive rounding with keyframes

Animate radius from 0 → max over time. The corners smoothly soften into arcs. Since radius is clamped internally, there's no "flip-out" artifact when the animated value exceeds the corner's max.

Selective rounding: only outer star spikes

Polygon (sides: 10, alternating inset for a star) → ShapeAttributes (Custom target, attribute select, value 1 on odd vertices, 0 on even) → RoundCorners (selectAttribute: select, selectThreshold: 0.5) → DrawShape. Only the outer spikes round; the inner concave corners stay sharp.

Animated reveal: corners round one at a time

ShapeAttributes (Custom attribute keyed off a Time-driven counter) → RoundCorners (selectAttribute set). Drive the per-vertex selection mask from a keyframed scalar to roll rounding around the path.

Tips

• Most primitives (Rectangle, Polygon, Line) already have a cornerRadius param built in — Round Corners is most useful on custom shapes (EditableShape, imported SVG, or the output of shape-op chains like ShapeAttributes).
• Combine with ResampleShape upstream to force straight-line approximation on curved inputs before rounding — gives uniform rounding on any shape.
• Path-level attributes (strokeWidth, opacity, color on path.attributes) are preserved untouched. Only point-level attributes are duplicated into new inset points.
• Very small radii (< 1 px) are clamped to zero to avoid numerical artifacts.

Related Nodes

• Rectangle / Polygon — have built-in cornerRadius param that achieves the same effect cheaper at the primitive stage
• ResampleShape — force polylinear approximation upstream to round a curved shape
• ShapeDeform — per-vertex deformation; Round Corners is the structural cousin that changes topology