Function parseBorderRadius

src/elements/element.arc.ts:66–89

* Parse border radius from the provided options

(arc: ArcElement, innerRadius: number, outerRadius: number, angleDelta: number)

 * Parse border radius from the provided options
 */
function parseBorderRadius(arc: ArcElement, innerRadius: number, outerRadius: number, angleDelta: number) {
  const o = toRadiusCorners(arc.options.borderRadius);
  const halfThickness = (outerRadius - innerRadius) / 2;
  const innerLimit = Math.min(halfThickness, angleDelta * innerRadius / 2);

  // Outer limits are complicated. We want to compute the available angular distance at
  // a radius of outerRadius - borderRadius because for small angular distances, this term limits.
  // We compute at r = outerRadius - borderRadius because this circle defines the center of the border corners.
  //
  // If the borderRadius is large, that value can become negative.
  // This causes the outer borders to lose their radius entirely, which is rather unexpected. To solve that, if borderRadius > outerRadius
  // we know that the thickness term will dominate and compute the limits at that point
  const computeOuterLimit = (val) => {
    const outerArcLimit = (outerRadius - Math.min(halfThickness, val)) * angleDelta / 2;
    return _limitValue(val, 0, Math.min(halfThickness, outerArcLimit));
  };

  return {
    outerStart: computeOuterLimit(o.outerStart),
    outerEnd: computeOuterLimit(o.outerEnd),
    innerStart: _limitValue(o.innerStart, 0, innerLimit),
    innerEnd: _limitValue(o.innerEnd, 0, innerLimit),
  };
}

/**
 * Convert (r, 𝜃) to (x, y)