Not sure about CloD's megatexture - but John Carmack first coined it for the idTech5 engine. Basically it's a texture virtualization system that allow the use of up to 128000×128000 textures.
Crazy stuff. The game rage had 20gigs texture and it's absolutely beautiful and it has almost no repeating terrain textures. The whole world felt hand crafted.
It's also why Rage was plagued with initial technical difficulties during release. Graphics manufactures had to put their act together for the new methods of retrieving textures.
I assume CloD dropped this direction because they simply didn't have the resources (and that's why CloD had some crazy huge textures, I assume they were planning to go the mega textures route)
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