/**
 * 4×4 IDCT. `coeffs` is 16 quantised-then-dequantised DCT coefficients
 * in raster order. `dst` is a 4×4 patch (16 entries with stride
 * `dstStride`, starting at `dstOffset`) that the residual is added to
 * and clipped to [0, 255]. The patch is read+written so the caller can
 * pre-fill it with the prediction.
 */
export declare function idct4x4Add(coeffs: Int16Array, dst: Uint8Array, dstStride: number, dstOffset: number): void;
/**
 * Walsh-Hadamard transform for the Y2 block — port of libwebp's
 * `TransformWHT_C`. Decoded coefficients (16 entries, raster order)
 * are transformed in-place; the result holds the 16 DCs that the
 * per-block IDCT will inject as `coeffs[0]` of each of the 16 luma
 * sub-blocks.
 */
export declare function iwht4x4(io: Int16Array): void;
/**
 * libwebp also has `TransformDC` and `TransformDCUV` shortcuts that
 * skip the full IDCT when only the DC coefficient is non-zero — they
 * compute `dc = (coef[0] + 4) >> 3` and add `dc` to every pixel.
 * We provide it here for use by the orchestrator.
 */
export declare function transformDC(coef: Int16Array, dst: Uint8Array, dstStride: number, dstOffset: number): void;
/** Clip a value to [0, 255]. */
export declare function clip255(v: number): number;