Light Propagation Volumes

Enabling Light Propagation Volumes

To get the feature enabled, put this into ConsoleVariables.ini and (re)start the engine:

	r.LightPropagationVolume = 1
Copy full snippet

r.LightPropagationVolume = 1

At the moment, this console variable cannot be changed at runtime.

Basic Scene Setup

• Add a Directional Light with its Mobility set to Movable.
• Add/Adjust geometry and the light direction to give you some shadowed parts and some parts that receive the light (ideally bright colored materials).
• Make sure you do not have any Lightmass GI enabled to be confused as dynamic GI (both techniques together would result in too bright indirect lighting).
• Enable Affect Dynamic Indirect Lighting on the Directional Light.
• Enabled Light Propagation Mode viewmode in the showflags to visualize the area that is covered by the Light Propagation Volume (See below).
• Add/Change a post process volume (should be unbound) to tweak the "Size" in the LightPropagationVolume settings to cover the area with the required detail and distance (Tradeoff).
• Use the Global Illumination showflags to enable/disable GI.

Light Propagation Volume Settings

Some global settings can be found in the post process settings (add a post process volume, make it unbound and make sure it has a proper priority):

The tooltips (hover over with the mouse) give you more details on the settings.

Having those settings in a volume allows them to be soft blendable depending on the camera position (e.g. less GI bounce in an indoor environment or under some trees).

Secondary occlusion should be enabled, and since secondary bounces are very cheap, they should also be enabled.

Adjusting the Look and Performance

The technique stores data in a low resolution volume grid. In order to avoid leaking through geometry smaller than the volume, we bias the injection. This is adjustable in the post process settings:

The bias can be adjusted per mesh with the "LPV Bias Multiplier".

For better performance, (less draw call for no CPU cost, some GPU cost) you can disable the feature in the material:

Directional Light Settings

For performance reasons, Light Propagation Volumes only use Directional Lights as incoming light. The method computes a so called Reflective Shadow Map nearby the camera. This is like a normal shadow map (2d texture of opaque depth from light perspective) but with additional material properties (material color, normal). Doing it for each point light would be very costly. Make sure you have the right settings on the directional light:

You want to have "Affect Dynamic Indirect Lighting" and "Cast Shadows" enabled and an indirect lighting intensity of around 1.0.

Viewing Global Illumination

You can disable LPV in the show flags (note that this also disables baked global illumination which was computed by Lightmass):

Visualizing Light Propagation Volumes

The actual Light Propagation volume can be visualized through show flags and is displayed in the Viewport:

GI Replace Material Switch

Notice the two red boxes - one is bouncing off green instead of red.

This was achieved by using a new material expression.

Normally you do not want to have a completely different color but some darkening, brightening, or a minor color adjustment can be useful.

Other Notes

Properties of the current LPV implementation:

• Computed each frame which allows dynamic material/light/geometry.
• Diffuse and approximated specular material interaction.
• Limited distance (larger volume e.g. 2x would be easy but memory requirements and some computations would grow by 8x).
• Constant detail in world space (it would be better to have more detail nearby and less detail in distance).
• Not affecting translucency.
• Light bounce is not affected by decals.
• Decal shading just works (unlike with baked lighting where the lighting is applied in the base pass before decals get applied).
• Requires compute shader (DirectX 11) support.
• Supports one or more Directional light for light bounce.
• Changing the size at runtime has minor artifacts that fade away over time.
• Emissive lighting for cheap area lights (not currently implemented).
• Occlusion (not currently implemented).
• Point light shadows approximated by occlusion (not currently implemented).
• Secondary occlusion through geometry voxelization which has some performance cost but quite better quality (not currently implemented).