code: function isMasked(x, y) {
      var bit;
  
      if (x > y) {
        bit = x;
        x   = y;
        y   = bit;
      }
  
      bit   = y;
      bit  += y * y;
      bit >>= 1;
      bit  += x;
  
      return frameMask[bit] === 1;
    }
docs: Check mask since symmetricals use half.