Real-time volumetric clouds in games usually pay for fast performance with a reduction in quality. The most successful approaches are limited to low altitude fluffy and translucent stratus-type clouds. For Horizon Zero Dawn, Guerrilla need a solution that can fill a sky with evolving and realistic results that closely match highly detailed reference images which represent high altitude cirrus clouds and all of the major low level cloud types, including thick billowy cumulus clouds. These clouds need to light correctly according to the time of day and other cloud-specific lighting effects. Additionally, we are targeting GPU performance of 2ms. Our solution is a volumetric cloud shader which handles the aspects of modeling, animation and lighting logically without sacrificing quality or draw time. Special emphasis will be placed on our solutions for direct-ability of cloud shapes and formations as well as on our lighting model and optimizations.
