/**
 * Keyframe sampling + easing curve helpers.
 *
 * Used by file-only ffmpeg helpers (Ken-Burns zoompan, speed-ramp curves)
 * and by tests that want to verify FCPXML emission lines up with the
 * mathematical sampled trajectory.
 *
 * Convention: t is normalized 0..1. Curves are pure functions of t so they
 * compose cleanly. easeIn / easeOut use simple quadratic curves; smooth is
 * smoothstep (3t² − 2t³). Linear is the identity.
 */
import type { Keyframe } from "./fcpxml.js";
/** Linear interpolation. f(t) = t. */
export declare function linear(t: number): number;
/** Quadratic ease-in. f(t) = t². Slow start, fast finish. */
export declare function easeIn(t: number): number;
/** Quadratic ease-out. f(t) = 1 − (1−t)². Fast start, slow finish. */
export declare function easeOut(t: number): number;
/** Smoothstep. f(t) = 3t² − 2t³. Symmetric ease-in-out. */
export declare function smooth(t: number): number;
/** Resolve an "interp" string to its curve function. */
export declare function curveFor(interp: "linear" | "easeIn" | "easeOut" | "smooth" | undefined): (t: number) => number;
/**
 * Sample a scalar keyframe sequence at a given clip-relative frame.
 *
 * Behaviour:
 *   - Empty list → throws (caller bug).
 *   - Single keyframe → returns its value (constant).
 *   - Frame before first kf → first value (clamp).
 *   - Frame after last kf → last value (clamp).
 *   - Otherwise → interpolate between the two surrounding kfs using the
 *     LEFT keyframe's `interp` setting.
 */
export declare function sampleCurve(kfs: Keyframe<number>[], frame: number, _fps: number): number;
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