Legacy: Features Script

The calculateFeatures() function was an earlier approach to defining token features on Art Blocks. It is documented here for artists with existing projects that use this pattern.

How It Worked

Artists defined a calculateFeatures(tokenData) function in their script. Art Blocks' infrastructure called this function after the script executed, passing in tokenData, and used the return value to index token traits.

// Legacy approach — for reference only
function calculateFeatures(tokenData) {
  // Re-initialize the PRNG from the hash to get consistent values
  const rand = new Random(tokenData.hash); // using the Random class below

  const palette = rand.random_choice(["Midnight", "Flame", "Ocean"]);
  const density = rand.random_int(10, 90);

  return {
    Palette: palette,
    Density: density,
  };
}

Key differences from window.$features:

• The function was called by Art Blocks' server-side infrastructure, not the browser
• It received tokenData as an argument, requiring the PRNG to be re-initialized from the hash
• The function signature was required to be exactly calculateFeatures(tokenData)

The Random Class (Legacy)

The following Random class was commonly used with calculateFeatures(). It uses the sfc32 PRNG seeded from the token hash:

class Random {
  constructor(hash) {
    const hex = hash.slice(2);
    this.prng = this._sfc32(
      parseInt(hex.slice(0, 8), 16),
      parseInt(hex.slice(8, 16), 16),
      parseInt(hex.slice(16, 24), 16),
      parseInt(hex.slice(24, 32), 16)
    );
  }

  _sfc32(a, b, c, d) {
    return function () {
      a |= 0; b |= 0; c |= 0; d |= 0;
      let t = (((a + b) | 0) + d) | 0;
      d = (d + 1) | 0;
      a = b ^ (b >>> 9);
      b = (c + (c << 3)) | 0;
      c = (c << 21) | (c >>> 11);
      c = (c + t) | 0;
      return (t >>> 0) / 4294967296;
    };
  }

  random_dec()       { return this.prng(); }
  random_num(a, b)   { return a + this.prng() * (b - a); }
  random_int(a, b)   { return Math.floor(a + this.prng() * (b - a + 1)); }
  random_bool(p)     { return this.prng() < p; }
  random_choice(arr) { return arr[Math.floor(this.prng() * arr.length)]; }
}

Feature Field Configuration

In the old system, artists also configured "Feature Fields" in the Creator Dashboard — metadata describing each feature key (display name, type, options). This configuration was separate from the script.

Migration to window.$features

If you're migrating an existing project from calculateFeatures() to window.$features:

1. Move your feature computation into the main script body (not a separate function)
2. Assign the result to window.$features synchronously at the top level
3. Remove the calculateFeatures function
4. Ensure the PRNG call order is consistent with your existing script

The window.$features assignment should produce the same trait values as your calculateFeatures() function for any given hash, to preserve trait consistency for already-minted tokens.

Contact Art Blocks in #artist-tech on Discord if you need assistance with migration.