/**
 * Lightweight PNG analysis for capture QA (brightness / "all black" detection).
 * Used by tests and optional runtime checks — keep dependency-free (no pngjs).
 */
/** ITU-R BT.709 luma from RGB */
export declare function rgbToLuma(r: number, g: number, b: number): number;
/**
 * Sample mean luma of PNG bytes by drawing to a small canvas (browser / happy-dom).
 * Returns 0 if decode fails. For very large images, samples a grid (max ~4096 samples).
 */
export declare function meanLuminanceFromPngBytes(png: Uint8Array): Promise<number>;
/**
 * True if the image is uniformly dark (likely failed render / empty GL buffer).
 * Threshold is mean luma on 0–255 scale (default 5 ≈ pure black).
 */
export declare function isPngMostlyBlack(png: Uint8Array, threshold?: number): Promise<boolean>;