Gta'nın içine portallarla gta 3 ve vice city eklemişler.

some technical details:
- all three games run inside the same process, sharing one Direct3D device
- each portal is a live GPU texture, the other game is actually running behind it, it's not a video or a captured window
- walking through swaps which engine has control and the others keep running
- bullets can travel through portals and affect NPCs and vehicles in another game
- each game keeps its own saves, missions and settings
- each world continues running its own AI, traffic and physics in real time