A modder has revealed how Rockstar Games fit Grand Theft Auto III's open world into the PlayStation 2's 32 MB of RDRAM. The game's Liberty City is 4 kilometers wide, but its assets total around 130 MB. That is four times the console's memory capacity.
The streaming system explained
The trick is a streaming system. As the player moves, the game loads models from the DVD into memory and unloads assets behind them. Liberty City is split into three islands, each divided into sectors. Only the necessary sector's assets are kept in memory at any time.
Former GTA III programmer Obbe Vermeij said streaming was the hardest technical challenge during development. He credited programmer Adam Fowler for coding the system. Game Maker's Toolkit modded the executable to confirm how the asset loading works.
Mark Brown of Game Maker's Toolkit explained that the game secretly builds the world in front of the player and silently deletes it behind them. Instead of fitting a whole city into memory, Rockstar created a moving window that shows just enough to make the illusion work.
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