/**
 * SubsurfaceScattering.ts
 *
 * Subsurface scattering (SSS) implementation:
 *   - Burley/Christensen normalized diffusion profile
 *   - Multi-channel RGB scatter radii (skin: R large, G medium, B small)
 *   - Multi-layer SSS (epidermis / dermis)
 *   - Thin-slab transmission approximation
 *   - Thickness map sampling (Float32Array or constant)
 *   - Separable SSS blur kernel (screen-space blur weights via Gaussian sum)
 *   - Pre-built material profiles (skin, wax, jade, marble, leaves)
 *
 * No WebGPU — fully CPU-testable in Vitest.
 *
 * @module rendering
 */
export type RGB = [number, number, number];
export type SSSModel = 'burley' | 'christensen' | 'separable_sss';
export interface SSSLayer {
    /** Weight of this layer (0–1, sum of all layers should ≤ 1) */
    weight: number;
    /** Per-channel (R, G, B) scattering radius in world units */
    scatterRadius: RGB;
    /** Base scatter color (tint) */
    color: RGB;
}
export interface SSSConfig {
    model: SSSModel;
    /** Diffusion radius falloff shape (1 = linear, 2 = quadratic like Burley) */
    falloffPower: number;
    /** Transmission (0 = opaque, 1 = fully transparent thin slab) */
    transmission: number;
    /** Constant thickness if no thickness map provided (0–1) */
    thickness: number;
    layers: SSSLayer[];
}
/**
 * Burley normalized diffusion profile: R(r) = A/r * (e^(-r/d) + e^(-r/(3d)))
 * Returns relative diffusion weight at distance r for a given mean free path d.
 *
 * Reference: Burley 2015, "Extending the Disney BRDF to a BSDF with Integrated Subsurface Scattering"
 */
export declare function burleyProfile(r: number, d: number, A?: number): number;
/**
 * Sample Burley profile across R, G, B channels.
 * scatterRadius = mean free path per channel.
 */
export declare function burleyProfileRGB(r: number, scatterRadius: RGB): RGB;
/**
 * Christensen–Burley profile (fitted polynomial, cheaper to evaluate).
 * From "An Approximate Reflectance Profile for Efficient Subsurface Scattering"
 *
 * R(r) = (e^(-A*r/d) + e^(-A*r/(3d))) * A / (8πd)
 */
export declare function christensenProfile(r: number, d: number, A?: number): number;
/**
 * Gaussian sum blur weights (Jimenez 2012 Separable SSS).
 * Returns N weight-variance pairs [{ weight, variance }] per channel.
 *
 * The kernel width is controlled by scatterRadius.
 */
export declare function buildSeparableSSSKernel(scatterRadius: RGB, numGaussians?: number): Array<{
    weight: number;
    stddev: RGB;
}>;
/**
 * Evaluate a separable Gaussian blur at offset x (signed, in world units).
 * Returns RGB weight.
 */
export declare function evalSeparableKernel(kernel: Array<{
    weight: number;
    stddev: RGB;
}>, x: number): RGB;
/**
 * Thin-slab transmission amount using Beer-Lambert approximation.
 * thickness: 0 = thin (high transmission), 1 = thick (low transmission).
 * Returns transmission factor [0, 1].
 */
export declare function thinSlabTransmission(thickness: number, config: Pick<SSSConfig, 'transmission' | 'layers'>): RGB;
export declare class SSSMaterial {
    private config;
    constructor(config?: Partial<SSSConfig>);
    getConfig(): Readonly<SSSConfig>;
    setModel(model: SSSModel): void;
    setTransmission(t: number): void;
    setThickness(t: number): void;
    addLayer(layer: SSSLayer): void;
    getLayerCount(): number;
    /**
     * Evaluate SSS diffusion at a surface point for a given light exit radius.
     * r: distance between light entry and exit point on the surface.
     */
    evaluate(r: number): RGB;
    /** Evaluate thin-slab transmission at the material's configured thickness */
    getTransmission(thicknessOverride?: number): RGB;
    /** Get Separable SSS kernel for this material (first layer) */
    getKernel(numGaussians?: number): ReturnType<typeof buildSeparableSSSKernel>;
}
export declare const SSS_PRESETS: {
    /** Human skin: epidermis (yellow tint) + dermis (red) */
    humanSkin: () => SSSMaterial;
    /** Wax candle (high SSS, warm light transmission) */
    wax: () => SSSMaterial;
    /** Jade gemstone (cool, dense SSS) */
    jade: () => SSSMaterial;
    /** Marble (dense, RGB-balanced) */
    marble: () => SSSMaterial;
    /** Leaf / plant (very thin, high transmission) */
    leaf: () => SSSMaterial;
};
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