The Witcher 3: Wild Hunt engine programmer Przemysław Czatrowski held a panel session at GDC last month that looked at how the team used Umbra 3 middleware to construct the game's sprawling open world.
The panel, was titled, 'Solving Visibility and Streaming in the The Witcher 3: Wild Hunt with Umbra 3,' and explained that by using Umbra Software's tech, CD Projekt RED reduced graphic card load by rendering only visible objects in the world. It sounds a bit like what id Software did in Rage.
The process sees the world divided into tiles, each with its own 'tome' of data that includes the occlusion data necessary to make it work. Then, the engine assesses which of those tomes are needed to properly display the world depending on where the camera is pointing, and an asynchronous mechanic kicks in once the parameters change, and the old tome is discarded to free up your system's memory.
Here's a video of the tech at work:
For example, here's an in-game shot of Geralt in Novigrad, and a look at what is being rendered from above:
DualShockers has many more details on the intricate process in its report. Hit it up.