core/masking.ts

/**
 * Perceptual masking — adapt watermark strength based on local image content
 *
 * High-energy (textured) areas can tolerate stronger watermarks,
 * while smooth areas need weaker embedding to remain imperceptible.
 */

/**
 * Compute AC energy of an 8x8 DCT block (sum of squared AC coefficients)
 * Assumes the block is already in DCT domain.
 */
export function blockAcEnergy(dctBlock: Float64Array): number {
  let energy = 0;
  for (let i = 1; i < 64; i++) { // Skip DC (index 0)
    energy += dctBlock[i] * dctBlock[i];
  }
  return energy;
}

/**
 * Compute perceptual masking factors for a set of DCT blocks
 *
 * Returns per-block multiplier for delta:
 *   Δ_effective = Δ_base × masking_factor
 *
 * Factors are in [0.1, 2.0]:
 *   - Smooth/flat blocks → factor near 0.1 (barely embed — changes would be visible)
 *   - Textured/noisy blocks → factor up to 2.0 (can embed aggressively)
 *
 * Uses a square-root curve so that low-energy blocks are suppressed
 * more aggressively than a linear mapping would.
 *
 * @param blockEnergies - AC energy for each block
 * @returns Array of masking factors, same length as input
 */
export function computeMaskingFactors(blockEnergies: Float64Array): Float64Array {
  const n = blockEnergies.length;
  if (n === 0) return new Float64Array(0);

  // Compute median energy
  const sorted = new Float64Array(blockEnergies).sort();
  const median = n % 2 === 0
    ? (sorted[n / 2 - 1] + sorted[n / 2]) / 2
    : sorted[Math.floor(n / 2)];

  // Avoid division by zero
  const safeMedian = Math.max(median, 1e-6);

  const factors = new Float64Array(n);
  for (let i = 0; i < n; i++) {
    const ratio = blockEnergies[i] / safeMedian;
    // sqrt curve: suppresses low-energy blocks more aggressively
    // ratio=0 → 0, ratio=0.25 → 0.5, ratio=1 → 1, ratio=4 → 2
    const curved = Math.sqrt(ratio);
    // Clamp to [0.1, 2.0] — flat areas barely embed
    factors[i] = Math.max(0.1, Math.min(2.0, curved));
  }

  return factors;
}