Okay, stick with me here because Horizon: Zero Dawn is doing amazing things with its sky that's verging on impressively Matrix-levels of simulation.
You see, skyboxes - the technical term for what you see when you look up in a game - are usually just a pretty picture. Something that's really obvious in older things. Horizon's doing something different. Not only is it modelling three dimensional clouds but also weather systems and how light passes through them.
Here's what that looks like in the game:
Because Horizon takes place in an open, simulated world rather than anything predefined, the studio wants clouds that can change and evolve, as well as behave like the real thing. They need performance, not pictures, and a number of approaches were tried from full polygon clouds modelled like fluids, through to voxel systems.
In the end Guerrilla settled on something that tries to recreate the actual rules clouds follow. Things like density changing with temperature, varying wind speeds, even how they look as they move over the horizon. That's all tried into a weather simulation so that when it rains in game it creates the right clouds and wind for the conditions. Impressive tech for a cosmetic flourish a lot of people might not even register as anything more than 'Ooooh, pretty'.
You can read the rest over at Guerrilla's blog but it's hella technical.
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