Real-time lighting

Real-time lighting is lighting that the engine calculates while the game runs, every frame, instead of storing it in textures ahead of time. Unity's manual describes the trade-off: real-time lights are "not baked into lightmaps", and "their runtime cost can be high, depending on the complexity of the scene" (Unity Manual: Light Modes).

The same trade-off applies throughout this chapter. Real-time light can move, change colour and react to a player who knocks down a wall. Baked light is cheap to draw but frozen. Most games mix the two; see baked vs. dynamic lighting. This page covers the parts every engine shares: light types, units, falloff, light budgets and when to depart from physical accuracy. For the art side of light, read light and shadow first.

#Direct and indirect light

Every lit pixel gets light in two ways.

• Direct light travels straight from a light to the surface. Shadows are places where direct light is blocked.
• Indirect light has bounced off other surfaces first. Godot's documentation explains that indirect diffuse light comes from light bouncing off surfaces and from emissive materials (Godot Engine documentation: Introduction to global illumination).

Direct light is cheap to compute in real time. Indirect light is harder, and the techniques that handle it are called global illumination. When people say a game has "fully dynamic lighting" in 2026, they usually mean the indirect light is real-time too, as with Unreal's Lumen.

#The four light types

Engines offer the same small set of light types under different names. Godot 4 has DirectionalLight3D, OmniLight3D, SpotLight3D and AreaLight3D (Godot Engine documentation: 3D lights and shadows). Unreal's Lumen supports Directional, Sky, Point, Spot and Rect lights (Unreal Engine Documentation: Lumen Global Illumination and Reflections).

• Directional: parallel rays from an infinitely distant source. Use it for the sun and moon. Position does not matter, only angle.
• Point (Godot: omni): light in all directions from one spot. Bulbs, candles, fires.
• Spot: a cone. Torches, street lamps, stage lights.
• Rect or area: light from a surface. Windows, light panels, TV screens. Softer and more expensive.

#Units and falloff

Older engines used "unitless" brightness numbers that only meant something relative to each other. Modern engines can use physical units, which makes it easier to match real places and to share values between scenes.

Unreal's documentation lists the units (Unreal Engine Documentation: Using Physical Lighting Units):

Epic also gives conversions for old scenes: "1 cd = 625 unitless" (Unreal Engine Documentation: Using Physical Lighting Units).

Falloff. Light from a point source spreads over a growing sphere, so it weakens with the square of the distance. Unreal calls its option "Inverse Squared Falloff", described as "physically based distance falloff" (Unreal Engine Documentation: Using Physical Lighting Units).

Physical units only work with sensible exposure. Real light levels differ enormously: full sun is about EV 15, a home interior EV 5 to 7 (Wikipedia: Exposure value). If you place a physically bright light and the image goes white, Epic's advice is that "the auto exposure range may be too limited" (Unreal Engine Documentation: Using Physical Lighting Units). See exposure and post-processing.

For the shape of a light's beam (the scallops of a wall washer, a window pattern on the floor), see IES profiles and light functions.

#How many lights can you afford?

Every engine has hard and soft limits. Knowing them early avoids a clean-up pass later.

Shadows usually cost more than the lights themselves. A point light without shadows is cheap; the same light casting shadows renders the scene again into a shadow map. Godot's default shadow map size is 4096, which it suggests keeping "in most cases" (Godot Engine documentation: 3D lights and shadows). For how shadows are stored and why Unreal moved to virtual shadow maps, see shadows and virtual shadow maps.

#Departing from physical accuracy

Physically based lighting gives a correct starting point. Games also have to be readable: players need to see enemies, doors and paths.

#In different engines

#Common mistakes

• Lighting with exposure locked wrong. If you raise lights to fight an exposure setting, every later scene inherits the problem. Fix exposure first (Unreal Engine Documentation: Using Physical Lighting Units).
• Shadows on every small light. Keep shadow casting for lights that need it; count shadowed lights against your engine's budget (Godot Engine documentation: 3D lights and shadows).
• Thin walls under Lumen. Epic says walls "should be no thinner than 10 centimeters (cm) to avoid light leaking" (Unreal Engine Documentation: Lumen Technical Details).
• More than four overlapping Stationary lights in Unreal. The editor marks the extra light with a red X (Unreal Engine Documentation: Stationary Light Mobility).
• Fill lights hiding a missing bounce. Check your global illumination settings before adding lights.

This page pairs with exercise: lighting one scene four ways, which asks you to light one scene four ways.

#Further reading and watching