Deferred lighting and Deferred shading
Deferred lighting is an alternate lighting technique and modification of deferred shading, a screen-space shading technique that allows many lights to be rendered on the scene without a significant performance hit.
It is much different from the default (usually static) radiosity lighting that comes with 
Source. It uses many different lighting passes to generate higher quality lighting, with real-time shadows.
Source 2 games generally use forward rendering, like 
Half-Life: Alyx, but games like 
Dota 2 and 
Deadlock use deferred rendering.
Differences from vanilla radiosity
When compiling a map in any of 
Valve's games, each map is run through VRAD (which can take a long time depending on hardware and optimizations), which takes data from all the light sources and compiles them into a lightmap texture. While this does provide high-quality and variable lighting, it is completely static, save for the limited ability to toggle on some lights in a scene. The difference between radiosity and deferred lighting is that deferred lighting computes all its lights in real-time, with a low performance cost (cheap). The game takes data from the normals of the object, its geometry, its albedo, and its specular lighting to create real-time accurate lighting.
Dynamic lights in forward rendering
Performance can be significantly reduced if there are too many dynamic light entities like env_projectedtexture (also used on player's flashlight since 
) in forward rendering. Other dynamic lights such as light_dynamic also reduce performance if the lightmap has a small luxel scale and/or the light has custom appearance (lightstyles). Additionally, env_projectedtexture is also limited by the Source engine itself (by default), meaning that it only works properly with one env_projectedtexture entity without engine modifications. Deferred lighting (and deferred shading) does not have these issues or limitations and almost an infinite number of dynamic lights can be active with faster performance than with forward rendering.
Features
- Quite cheap real-time lighting and shadows.
- Almost an infinite number of dynamic lights that can be active (stock Source only allows for one env_projectedtexture at a time).
- High-fidelity godrays.
- Volumetric lighting.
- Light cookies (premade shadow maps).
Drawbacks
- Traditional anti-aliasing methods like MSAA might not work with deferred lighting, like in
Black Mesa (which replaced it with FXAA because MSAA caused certain models to have a glowing outline),[1] but it depends how it's implemented in the game, as Alien Swarm Deferred, and Lambda Wars still support it, though it's recommended that you use post-processing anti-aliasing methods instead as they work best with deferred lighting, and have a smaller performance hit. MSAA is also more demanding with deferred lighting/shading compared to forward rendering. - All lightmaps are disabled and only deferred lighting lights the world. (only in
) - Can be more taxing on older systems.
- DX9 SM3 or later only, older DirectX compatibility versions are not supported.
Media
Availability
Source
Few Source Engine games utilize this lighting technique, as it's complicated to implement, the games that do use this are:
- Alien Swarm Deferred
- Black Mesa
Lambda Wars
Portal 2: Desolation
Titanfall branch (including 
Apex Legends) — using deferred shading
Hammer (Lighting Preview in stock HL2 and 
Strata Source)
Source 2
Tutorials
See also
- Alien Swarm Deferred
- Light editor
- Volumetric lighting
- newLight_Point
- newLight_Spot
- light_deferred
- light_deferred_global
Deferred lighting (Wikipedia)
External links
- Deferred Shading on LearnOpenGL (basics)
References
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