After a few requests, the MonsterDelta is is now available with open source. As mentioned before, it uses an approximation of Up and Down Volume (because they are not kept by Tradingview/not accessible by Pinescript).
The approximation is:
The "up path" on the "up" candle can be approximated by (High-Low). The "down path" on the "up" candle can be approximated by (High-Close)+(Open-Low).
(and similar for the down candle, like in the 1st post.)
Total path=up path + down path.
Up volume = Tick volume * up path/total path Down volume = Tick volume * down path/total path