Ambient occlusion

Ambient occlusion (AO) is the darkening of corners, cracks and contact areas where surrounding light cannot easily reach. Epic describes the effect as one that "approximates the attenuation of light due to occlusion" (Unreal Engine documentation: Ambient Occlusion). An AO map stores it per pixel: white where a point sees all of its surroundings, black where it sees none (Unity Manual: Occlusion Map).

#What AO does and does not affect

AO only darkens ambient (indirect) light, the soft light that comes from the sky and from bouncing off other surfaces. glTF's definition: it "indicates areas that would receive less indirect lighting from ambient sources. Direct lighting is not affected" (Khronos Group: glTF 2.0 Specification). Unity and Godot describe their AO inputs the same way (Unity Manual: Occlusion Map; Godot documentation: Standard Material 3D and ORM Material 3D). Direct sunlight and lamps are handled by shadows instead.

Adobe adds that AO "only affects the diffuse contribution and should not occlude the specular contribution" (Adobe Substance 3D: The PBR Guide – Part 2), though engines differ in how they apply it.

A side view of the ground with a narrow slot cut into it. On the open ground, a point sends out 13 orange rays in a half-circle, all reaching the sky. At the bottom of the slot, a point sends the same 13 rays, but only the 3 pointing almost straight up escape; the other 10 are dashed grey and stop at the slot walls. — AO measures how much of its surroundings a point can see. The point in the slot sees only 3 of 13 directions, so its AO value is low and it receives little ambient light.Diagram by the handbook author

#Three sources of AO

Baked AO maps. A baker casts rays from the low poly (or the high poly) and records how much of each point is blocked; Blender's AO bake "ignores all lights in the scene" because it only measures geometry (Blender Manual: Render Baking). Baked AO is detailed and costs nothing at runtime. Godot: "nothing beats the quality of a well-baked AO map. It is recommended to bake ambient occlusion whenever possible" (Godot documentation: Standard Material 3D and ORM Material 3D). Its limitation is that it only knows about the asset itself, not the world around it. Marmoset's floor occlusion option adds downward darkening as if the object stood on a floor, "typically most useful for static objects" (Marmoset: The Toolbag Baking Tutorial).

Screen-space AO (SSAO). Calculated every frame from the depth buffer and normals, "in a deferred pass in screen space" (Unreal Engine documentation: Ambient Occlusion). It handles contact between objects and moving things, but it only sees what the camera sees and "cannot know about the thickness of objects", which can produce dark halos (Unreal Engine documentation: Ambient Occlusion). Material AO and SSAO combine, so the result can get darker than intended (Unreal Engine documentation: Ambient Occlusion).

Real-time global illumination. Systems such as Lumen calculate indirect light properly, which includes occlusion at larger scales. Epic: "Lumen Global Illumination replaces Distance Field Ambient Occlusion (DFAO)" (Unreal Engine documentation: Lumen Global Illumination and Reflections). Lumen still supports Material Ambient Occlusion for small-scale self-occlusion, when "Allow Static Lighting" is disabled in the project settings (Unreal Engine documentation: Lumen Global Illumination and Reflections). See global illumination.

	Baked AO map	SSAO	Lumen / real-time GI
Scale	Small, within one asset	Medium, what is on screen	Large, whole scene
Cost	Texture memory only	Per frame, per pixel	Per frame, higher
Moves with objects	Yes (self-occlusion only)	Yes	Yes
Typical problem	Ignores the environment	Halos, only on-screen data	Misses fine detail, which material AO adds

In practice, current engines use more than one: a baked AO channel for fine detail, plus screen-space or GI-based occlusion for the scene.

#AO in the base colour?

Before PBR, artists routinely multiplied AO into the diffuse texture. Whether you still should is one of the longest-running texturing debates.

Different viewsMultiply AO into base colour, or keep it separate?

Keep it separate. Adobe's PBR Guide: "The AO should not be baked into texture maps, but only supplied as its own channel to the shader" (Adobe Substance 3D: The PBR Guide – Part 2). In a 2015 Polycount thread, s6 argued "If AO lived in the Albedo, there would be no way to accurately light that area", and EarthQuake called baking medium-to-large AO into diffuse physically incorrect, recommending cavity maps for micro detail (Polycount: PBR - AO - Question).

Some AO in colour is acceptable. Adobe allows micro-occlusion in the base colour where the shader cannot represent that detail with an AO channel alone (Adobe Substance 3D: The PBR Guide – Part 2). In the same Polycount thread, Joopson argued that where light never reaches, "you can totally multiply the AO over the diffuse input", and gnoop felt PBR was "too honest" for cracked surfaces with low-resolution normal maps (Polycount: PBR - AO - Question). Epic's docs describe multiplying a cavity map into Base Color and Specular for micro-shadowing (Unreal Engine documentation: Physically Based Materials in Unreal Engine).

In practice: keep large-scale AO in its own channel, packed into the ORM or mask texture (see channel packing). Fine cavity detail (cracks, pores, stitching) can go lightly into the base colour, inside the value ranges on base colour (albedo) map. A project whose shaders have no AO channel has to bake more into colour or go without; either way it is a deliberate choice, so write it in the art bibles and style guides.

#Example

#Common mistakes

Strong AO multiplied into base colour, which shows up as dirty shadows under direct light (Adobe Substance 3D: The PBR Guide – Part 2).
AO from a floating part baked onto the surface below it. Marmoset notes that an AO setting that captures object intersections "will cause shadows around floating geometry" (Marmoset: The Toolbag Baking Tutorial). See floaters.
Double darkening from stacking material AO, SSAO and baked colour AO (Unreal Engine documentation: Ambient Occlusion).
Imported as sRGB. AO is linear data (Khronos Group: glTF 2.0 Specification).
Using AO as a dirt mask without thinking. AO is a good starting mask for grime, but grime follows gravity and use, not only occlusion.

#In different tools

Unreal Engine: Ambient Occlusion input, usually the red channel of an ORM texture; Lumen replaces DFAO (Unreal Engine documentation: Lumen Global Illumination and Reflections).
Unity: Occlusion Map (URP); green channel of the HDRP mask map (High Definition Render Pipeline manual 17.0: Mask map and detail map).
Godot: Ambient Occlusion; red channel of the ORM texture in ORMMaterial3D (Godot documentation: Standard Material 3D and ORM Material 3D).
Marmoset Toolbag, Substance, Blender: bake AO from the high poly; see baking.

baking · texture maps explained · channel packing · global illumination · real-time lighting · base colour (albedo) map

#Further reading and watching

Adobe: The PBR Guide, part 2

the rule against baking AO into colour, and its exception. Beginner, free.

WebBeginnerFree

Polycount: PBR - AO - Question

a debate between working artists. Intermediate, free.

ForumIntermediateFree

Epic: Lumen Global Illumination and Reflections

how Lumen handles occlusion and material AO. Intermediate, free.

DocsIntermediateFree

Godot: Standard Material 3D

short, practical notes on baked AO versus SSAO. Beginner, free.

WebBeginnerFree

References

Epic Games · Ambient Occlusion (Unreal Engine 4.27) · Unreal Engine documentation, 2021 · docs · accessed 2026-10-21

Unity Technologies · Occlusion Map (Standard Shader) · Unity Manual · docs · accessed 2026-10-21

Khronos Group · glTF 2.0 Specification · Khronos Group (GitHub) · docs · accessed 2026-10-21

Godot Engine contributors · Standard Material 3D and ORM Material 3D (documentation source) · Godot documentation (GitHub) · docs · accessed 2026-10-21

Allegorithmic · The PBR Guide – Part 2 · Adobe Substance 3D (Adobe Learn) · article · accessed 2026-10-21

Blender Foundation · Render Baking (Cycles) · Blender Manual (2.93), 2021 · docs · accessed 2026-10-21

Joe Wilson · The Toolbag Baking Tutorial · Marmoset, 2017 · article · accessed 2026-10-21

Epic Games · Lumen Global Illumination and Reflections · Unreal Engine documentation · docs · accessed 2026-10-21

Epic Games · Material Inputs (Unreal Engine 4.27) · Unreal Engine documentation, 2021 · docs · accessed 2026-10-21

Polycount forum (Maximum-Dev, EarthQuake, s6, Joopson, gnoop, teaandcigarettes and others) · PBR - AO - Question · Polycount, 2015 · forum · accessed 2026-10-21

Epic Games · Physically Based Materials in Unreal Engine · Unreal Engine documentation · docs · accessed 2026-10-21

Unity Technologies · Mask map and detail map (HDRP) · High Definition Render Pipeline manual 17.0 · docs · accessed 2026-10-21