/**
 * Flow: what turns a connection into a direction.
 *
 * An arc says two places are related. It does not say which way anything moves,
 * and on a hub map every route leaves the same airport, so the reader cannot even
 * infer it. Beads travelling along the path say it without a legend, an arrowhead
 * or a second encoding.
 *
 * The mechanism is one dashed companion path per route, animated on
 * `stroke-dashoffset`, and the reason it is that rather than a dot moved from
 * JavaScript is arithmetic: a dash pattern only has to advance by one period to
 * loop seamlessly, so the duration is `spacing / speed` and the route's own
 * length never enters into it. Every route then travels at the same speed with no
 * measurement, which matters because measuring would mean `getTotalLength`, and
 * that exists in a browser but not in jsdom or on a server.
 *
 * Three consequences worth knowing:
 *
 * - The phase is a pure function of time, so panning and zooming cannot disturb
 *   it. A dot driven per frame from a projected position has to be recomputed as
 *   the camera moves, and any disagreement with the path underneath shows up as
 *   drift along a 1px line.
 * - The dots ride `vector-effect: non-scaling-stroke` along with the route, so
 *   their size and spacing stay constant in screen pixels at every zoom.
 * - `stroke-dashoffset` restarts the pattern at each subpath, and d3-geo cuts a
 *   trans-Pacific arc in two at the antimeridian. The dots keep travelling in the
 *   right direction across the seam, but not in step across it. That is the one
 *   thing this approach cannot do and a per-frame dot can.
 *
 * An export catches the beads where they were: the pattern and the offset are both
 * ordinary stroke properties, so `export/Exporter` reads them off the live element
 * along with everything else, and a PNG of a travelling map matches the screen it
 * was taken from.
 *
 * @module series/flow
 */
import type { FlowOptions } from '../types';
import type { FlowSpec } from '../renderers/SvgRenderer';
/**
 * Resolve a series' `flow` option against one route.
 *
 * Returns undefined when flow is off, so a caller can spread the result into a
 * path spec and let the absence mean "no companion path".
 */
export declare function resolveFlow(flow: boolean | FlowOptions | undefined, { key, width, color }: {
    key: string;
    width: number;
    color: string;
}): FlowSpec | undefined;
//# sourceMappingURL=flow.d.ts.map