Time of day

Time of day (often shortened to ToD) is a lighting setup in which the sun, sky, fog and ambient light change as the game's clock runs. It touches almost every lighting system, which is why it is one of the first technical decisions on an outdoor game. Ghost of Tsushima's rendering goal states it directly: "Continuous time of day with dynamic weather" (SIGGRAPH 2021: Real-Time Samurai Cinema: Lighting…).

#What changes over the day

When the sun moves, everything that depends on it must follow:

• Direct light and shadows. The sun's angle, colour and intensity. In Unreal, "Any light movement or rotation will invalidate all cached pages for that light" in shadows and virtual shadow maps (Unreal Engine Documentation: Virtual Shadow Maps).
• Sky and clouds. A physical sky reacts automatically, and clouds "need to light correctly according to the time of day" (SIGGRAPH 2015: The Real-time Volumetric Cloudscapes of…). See sky, clouds and volumetrics and fog and atmosphere.
• Ambient and bounce light. The most difficult part, because global illumination is expensive to recompute.
• Exposure. Unity's manual spans about 16 stops from a moonless night (−2 EV) to a sunlit scene (14 EV) (High Definition Render Pipeline manual: Physical light units); see exposure and post-processing and physical light units.
• Colour and mood. The grade, fog colour and emissive lights (street lamps switching on) usually change with the hour.

#Two ways to handle bounce light

Blend baked keyframes. Bake GI for several hours of the day and interpolate between them at runtime. Assassin's Creed IV: Black Flag did this with a "partially baked GI solution" that was "compatible with dynamic time of day and various weather presets" (bartwronski.com: My upcoming GDC 2014 presentation).

Compute it live. Use fully dynamic GI that reacts to any sun angle. Fortnite Chapter 4 moved to Lumen because "the time of day will change" along with building and destruction (Unreal Engine Tech Blog: Lumen brings real-time global illumination to…).

#Costs to plan for

• Shadow caching. A continuously moving sun invalidates cached shadows every frame (Unreal Engine Documentation: Virtual Shadow Maps).
• Sky light updates. Unreal's real-time sky capture is time-sliced over 9 frames, so its cost is set by the most expensive frame (Unreal Engine Documentation: Volumetric Cloud Component).
• Indoor darkness. With accurate GI, "Indoor areas become significantly darker than outdoors", so auto exposure has to work harder (Unreal Engine Tech Blog: Lumen brings real-time global illumination to…).

#Art direction

A time-of-day cycle multiplies every art review. Each area has to read at dawn, noon, dusk and night. Treat it like a colour scripts: choose a small set of key times, light and grade each deliberately, and let the system interpolate between them. Check value and silhouette readability at the darkest and the brightest keys.

Related: real-time lighting, baked vs. dynamic lighting, light probes and reflection probes.

#Further reading and watching