Screen Space Ambient Occlusion (SSAO)
The Screen Space Ambient Occlusion override is a real-time, full-screen lighting effect available in the High Definition Render Pipeline (HDRP). This effect approximates ambient occlusion in the current field of view. It approximates the intensity and position of ambient light on a GameObject’s surface, based on the light in the Scene and the environment around the GameObject. To achieve this, it darkens creases, holes, intersections, and surfaces that are close to one another. In real life, these areas tend to block out, or occlude, ambient light, and so appear darker.
For information on how to use a Texture to specify ambient occlusion caused by details present in a GameObject's Material but not on it's surface geometry, see Ambient Occlusion.
HDRP implements ray-traced ambient occlusion on top of this override. This means that the properties visible in the Inspector change depending on whether you enable ray tracing.
Enabling Screen Space Ambient Occlusion
To use this feature in your Scene, you must first enable it for your project and then enable it for your Cameras. To enable features in your project, you use the HDRP Asset and to enable features for your Cameras, you use Frame Settings. You can enable features either for all Cameras, using the Default Frame Settings, or for specific Cameras, by overriding each Camera's individual Frame Settings.
- To enable SSAO in your HDRP Asset go to Lighting > Screen Space Ambient Occlusion.
- To enable SSAO in your Frame Settings go to Edit > Project Settings > Graphics > HDRP Global Settings > Frame Settings (Default Values) > Camera > Lighting > Screen Space Ambient Occlusion.
Using Screen Space Ambient Occlusion
Screen Space Ambient Occlusion uses the Volume framework, so to enable and modify Screen Space Ambient Occlusion properties, you must add an Screen Space Ambient Occlusion override to a Volume in your Scene. To add Ambient Occlusion to a Volume:
- In the Scene or Hierarchy view, select a GameObject that contains a Volume component to view it in the Inspector.
- In the Inspector, navigate to Add Override > Lighting and click on Ambient Occlusion. HDRP now applies Ambient Occlusion to any Camera this Volume affects.
API
To access and control this override at runtime, use the Volume scripting API. Because of how the Volume system works, you edit properties in a different way to standard Unity components. There are also other nuances to be aware of too, such as each property has an overrideState. This indicates to the Volume system whether to use the property value you set, or use the default value stored in the Volume Profile. For information on how to use the API correctly, see Volume scripting API.
Properties
To edit properties in any Volume component override, enable the checkbox to the left of the property. This also tells HDRP to use the property value you specify for the Volume component rather than the default value. If you disable the checkbox, HDRP ignores the property you set and uses the Volume’s default value for that property instead.
The properties visible in the Inspector change depending on whether you enable ray tracing for this effect:
- To not use ambient occlusion without ray tracing, disable Ray Tracing in the Inspector and see Screen-space for the list of properties.
- To use ambient occlusion with ray tracing, enable Ray Tracing in the Inspector and see Ray-traced for the list of properties.
Screen-space
| Property | Description |
|---|---|
| Ray Tracing | Indicates whether HDRP uses ray tracing to calculate ambient occlusion. If you enable this property, it completely changes the implementation for this Volume override and exposes a new set of properties to control the ray-traced ambient occlusion. For information on ray-traced ambient occlusion, see ray-traced ambient occlusion. For information on the properties that control the ray-traced ambient occlusion, see the Ray-traced properties section below. |
| Intensity | Use the slider to guide the intensity of the ambient occlusion. Higher values lead to darker results. HDRP uses this value as an exponent to evaluate a pixel's final value for ambient occlusion. |
| Direct Lighting Strength | Use this slider to change how much the occlusion affects direct diffuse lighting. |
| Radius | Use the slider to set the distance that HDRP searches around a point for occluders. Set a higher value to make ambient occlusion cover larger scale features. Be aware that a higher distance value often produces a lower quality result. Note: HDRP clamps the radius in screen space to the value you set in Maximum Radius in Pixels. |
| Quality | Specifies the preset HDRP uses to populate the values of the following nested properties. The options are: • Low: A preset that emphasizes performance over quality. • Medium: A preset that balances performance and quality. • High: A preset that emphasizes quality over performance. • Custom: Allows you to override each property individually. |
| - Maximum Radius In Pixels | Use the slider to set an upper limit, in pixels, for the area that HDRP searches for occluders. The numerical value assumes that you are using a resolution of 1920 x 1080. HDRP scales this value accordingly when you use a different resolution. Keep this value as low as possible to achieve good performance. |
| - Full Resolution | Enable the checkbox to process the ambient occlusion algorithm in full resolution. This improves quality significantly but is a resource-intensive operation and has an impact on performance. Disable the checkbox to process the ambient occlusion algorithm at half the resolution your application runs at. This setting is disabled by default. |
| - Step Count | The number of steps HDRP takes to search for occluders. Increase this value to produce more precise results. This often produces darker results because HDRP finds more occluders. |
| Temporal Accumulation | Indicates whether HDRP accumulates the result of ambient occlusion over time. This leads to better quality, but might result in artifacts like non-instant convergence and ghosting. Note: This mode doesn't produce good results if motion vectors aren't available. |
| - Direction Count | Determines how many directions are searched for occlusion, increasing this will impact performance considerably. This property only appears if you disable Temporal Accumulation. |
| - Blur sharpness | Determines the sharpness of the non-temporal blur. Higher values preserve sharp features better (with higher risk of noise), lower values have a softer look. This property only appears if you disable Temporal Accumulation. |
| - Bilateral Aggressiveness | Higher this value, the less lenient with depth differences the spatial filter is and so more likely is to reject samples that are at different depth values. Increasing this for could reduce white halos where AO should be around objects. This property only appears if you enable Temporal Accumulation. |
| - Ghosting Reduction | Moving this factor closer to 0 will increase the amount of accepted samples during temporal accumulation, increasing the ghosting, but reducing the temporal noise. Moving the value closer to 1 will reduce the ghosting, at expense of more visible temporal noise. This property only appears if you enable Temporal Accumulation. |
| - Bilateral Upsample | Enable the checkbox to upsample the low resolution AO through bilateral upsampling. This preserves sharp edges better, however it's slightly more resource intensive and might result is more visible aliasing. This property only appears if you enable Temporal Accumulation and disable Full Resolution. |
Ray-traced
| Property | Description |
|---|---|
| Intensity | See Intensity in Screen-space. |
| Direct Lighting Strength | See Direct Lighting Strenght in Screen-space. |
| LayerMask | Defines the layers that HDRP processes this ray-traced effect for. |
| Quality | Specifies the preset HDRP uses to populate the values of the following nested properties. The options are: • Low: A preset that emphasizes performance over quality. • Medium: A preset that balances performance and quality. • High: A preset that emphasizes quality over performance. • Custom: Allows you to override each property individually. |
| - Max Ray Length | Controls the maximal ray length of the rays that HDRP uses for ray traced ambient occlusion. This allows you to have smaller scale, local, ambient occlusion. |
| - Sample Count | Controls the number of rays that HDRP uses per pixel, per frame. Increasing this value increases execution time linearly. |
| - Denoise | Enables the spatio-temporal filter that HDRP uses to remove noise from the ambient occlusion. |
| - - Denoiser Radius | Controls the radius of the spatio-temporal filter. A higher value reduces noise further. |
| Occluder Motion Rejection | When enabled, the occluder's movement should be considered a valid rejection condition. |
| Receiver Motion Rejection | When enabled, the receiver's movement should be considered a valid rejection condition. |
Limitations
Screen-space ambient occlusion
A screen-space effect only processes what's on the screen at a given point. This means that objects outside of the field of view can't visually occlude objects in the view. You can sometimes see this on the edges of the screen. When rendering Reflection Probes screen space ambient occlusion isn't supported.