Simplygon 10.3.2100.0

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Geometry clipping ​

It is quite common that objects in a level have geometries that can never be visible from a player's perspective; for example assets that have been placed on the ground, but has parts that pierces through the ground. These parts may have a negative impact on performance, in terms of geometry, memory and texture utilization.

Figure 1: A collection of intersecting Static Mesh Actors.
Figure 1: A collection of intersecting Static Mesh Actors.

The Simplygon plugin exposes two features that can remedy these issues:

Both these methods have the ability to remove geometry in the clipping plane's / geometry's negative normal direction, and if an asset's volume is air-tight, Simplygon will go ahead and remove the inside to reduce both geometry complexity and texture space.

FeatureSupports clipping planeSupports clipping geometry
Stand-InYesYes
HLODNoYes*
LOD RecipeNoNo
AsteriskDescription
*HLOD supports geometry clipping in form of Landscape Culling.

Clipping planes ​

Axis-aligned clipping planes allow users to clip objects along the axes. The user can add clipping planes to a level from inside the UE5 editor and specify that they get taken into account by supported Pipelines. Clipping planes are helpful when dealing with openings that are simply too large. In the following image sequence we can see how clipping planes can affect open geometries and textures (when material baking is enabled).

Original asset:

Figure 2: Original asset without clipping planes. There are several intersecting parts inside the asset and against the ground.
Figure 2: Original asset without clipping planes. There are several intersecting parts inside the asset and against the ground.

Optimized asset:

Figure 3: Aggregation with a clipping plane at the bottom. The geometries have been merged into one, and the geometry underneath the ground has been removed.
Figure 3: Aggregation with a clipping plane at the bottom. The geometries have been merged into one, and the geometry underneath the ground has been removed.
Figure 4: A front shot of the optimized asset, here we can see that most of the triangles intersecting with the ground have been removed. (Remeshing will clip each individual triangle along the plane)
Figure 4: A front shot of the optimized asset, here we can see that most of the triangles intersecting with the ground have been removed. (Remeshing will clip each individual triangle along the plane)
Figure 5: As this geometry (with the cutting plane included) is air-tight the triangles on the inside has been removed as well, resulting in a "hollow shell" with less overdraw.
Figure 5: As this geometry (with the cutting plane included) is air-tight the triangles on the inside has been removed as well, resulting in a "hollow shell" with less overdraw.

A clipping plane can be added to a level by clicking Simplygon → Add Cutting Plane in the UE5 editor, this action adds a SimplygonClippingPlanesVisualizer which position and orientation can be adjusted by the user. To enable usage of cutting planes in supported Pipelines; enable "Use Clipping Planes".

Clipping geometries ​

Clipping geometries allow users to clip objects more freely compared to clipping planes. A common use case would be if there is a landscape with various heights (bumps and hills) with assets such as buildings or other props that intersect the ground. The aim is to remove geometry that intersects and pierces through the ground. Using the landscape as a clipping geometry and removing everything underneath is exactly what we want.

The user can add clipping geometries to a level from inside the UE5 editor, then tag the geometries from the Stand-In outliner. To enable clipping geometries in supported Pipelines; enable "Use Clipping Geometry".

It is also possible to use Landscape Actors as clipping geometries. While these Actors does not need to be tagged, there is a setting that has to be set. In the Landscape Actor, navigate to the HLOD section and enable the "Use Landscape for Culling Invisible HLOD Vertices"-setting.

Figure 6: Required setting for Landscape culling.
Figure 6: Required setting for Landscape culling.