Overview

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Occlusion Modes

RTAO (Ray Traced Ambient Occlusion) - hardware accelerated, real-time ray traced ambient occlusion. Provides the most physically accurate results.

• Pros: doesn't suffer from screen-space limitations, the most accurate of all.

• Cons: requires ray-tracing capable hardware and API, requires denoising, high performance cost.

GTAO (Ground Truth Ambient Occlusion) - the cutting-edge screen-space algorithm based on XeGTAO and augmented by the most recent research on Visibility Bitmasks.

• Pros: very accurate, captures small details well, basically it's as good as it gets in screen-space.

• Cons: may require denoising to gather enough samples, though it can work without it.

SSAO (Screen Space Ambient Occlusion) - the fastest of the three modes, both in terms of performance and responsiveness, and the easiest to set up. Heavily inspired by the Mini Engine AO.

• Pros: the fastest ambient occlusion, doesn't require denoising or motion vectors, zero temporal lag.

• Cons: doesn't capture small details well, very approximate, not physically correct.

RTAO

GTAO

SSAO

Feature	RTAO	GTAO	SSAO
Motion Vectors	Required	Required if temporal denoiser is active	Not Required
Normal Buffer	Required	Required	Not Required
Temporal Denoiser	Required	Optional	Not Required
Spatial Denoiser	Required	Optional	Not Required

Pipeline Differences

• Integration - describes the level of HTrace integration into the given rendering pipeline. Full means a complete replacement of the native AO and, as a result, full compatibility with all other rendering effects. Partial means a more traditional, post-processing approach, where the final buffer is overlayed on top of the frame at a certain injection point.

• RTAO Alpha Cutout Mode - describes the way ray tracing treats alpha cutout materials. Refer to this section for additional information.

• RTAO Motion Rejection - the ability of RTAO's denoiser to detect pixels whose rays hit moving objects and reject temporal history accordingly.

• RTAO Specular Occlusion - the ability of a rendering pipeline to use ambient occlusion results to construct specular occlusion.

• VR Support - whether the single pass VR rendering is supported or not.

• Dir. Light Occlusion - indicates the availability of directional light occlusion control. No - means no control (occlusion will always be overlayed across directional lighting).

Feature	HDRP	URP	BIRP
Integration	Full	Full	Partial
RTAO Support	Unity 2023.2 +	Unity 2023.2 +	Unity 2023.2 +
RTAO Alpha Cutout Mode	Depth Test, Evaluate	Depth Test	Depth Test
RTAO Motion Rejection	Yes	Yes	No
RTAO Specular Occlusion	Yes	No	No
Directional Light Occlusion	Yes	Yes	No
VR Support	Yes	No	No

Other Differences and Requirements

• RTAO mode requires ray-tracing compatible API (e.g. DX12) and hardware. For more information, please, refer to this page. This is relevant for all 3 rendering pipelines.

• RTAO in URP and BiRP as well the Evaluate alpha cutout option in HDRP utilize Inline Ray Tracing and require Unity 2023.2 as the minimum version.

• To use the Checkerboarding option alongside the Evaluate alpha cutout option in HDRP, Unity 2023.3 or higher is required due to the DispatchIndirect function being utilized for this.

• In URP HTrace requires Depth Priming Mode to be enabled if used with Forward rendering.

• HTrace requires Compatability Mode enabled in URP if used with Unity 6.0+.

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