/** Peak moonlight as a fraction of the authored sunlight, at the moon's highest point.
 * Real moonlight is ~2.5e-6 of daylight; every game exaggerates it to stay readable, and this is
 * the exaggeration, not a measurement. Games override it through `SkyEnvironmentConfig.moon`. */
export declare const MOONLIGHT_PEAK = 0.12;
/** Moonlight's colour: scattered sunlight, blue-shifted because the eye's scotopic response peaks there. */
export declare const MOONLIGHT_COLOR = "#a9c3e8";
/** Fog color once the sun is gone: the night sky's own haze, lifted enough to keep silhouettes readable. */
export declare const NIGHT_FOG_COLOR = "#0a0e16";
/** Elevation of a world-space direction in degrees; a zero vector reads as overhead. */
export declare function elevationOf(direction: readonly [number, number, number]): number;
/**
 * Relative air mass along a line of sight at `elevationDeg` - Kasten & Young (1989). 1 at the
 * zenith, ~38 at the horizon, and clamped below the horizon so a set sun does not run away.
 * @internal
 */
export declare function airMass(elevationDeg: number): number;
/**
 * Fraction of each RGB channel of sunlight that survives the atmosphere at `elevationDeg`:
 * `exp(-τ · air mass)`. High sun keeps nearly all of it; a low sun loses the short wavelengths
 * first, which is the red sunset.
 * @internal
 */
export declare function sunTransmission(elevationDeg: number): [number, number, number];
/**
 * The sun's colour at `elevationDeg`, normalised so its brightest channel is full: white overhead,
 * orange at the horizon, moon-blue once it has set. Intensity stays the authored cycle's job.
 */
export declare function sunlightColor(elevationDeg: number): string;
/** Perceived brightness (Rec. 709 luma) of a `#rrggbb` color, 0–1.
 * @internal
 */
export declare function luminanceOf(color: string): number;
/**
 * Moonlight at `sunElevationDeg`'s opposite point: full at the top of the moon's arc, zero while
 * the moon is below the horizon, scaled by `peak` (a fraction of the authored sun intensity).
 */
export declare function moonIllumination(moonElevationDeg: number, peak?: number): number;
/** How visible the stars are: none in daylight, half at the first hint of a set sun, full once it is 6° under. */
export declare function starOpacity(sunElevationDeg: number): number;