GPU Resident Drawer

Information about using Flora with Unity's GPU Resident Drawer.

You can use Flora alongside Unity's GPU Resident Drawer, but there are key technical differences to understand and be aware of.

GameObjects

GPU Resident Drawer requires GameObjects and MeshRenderer components in the scene to work. Flora on the other hand can render prefab instances without requiring GameObjects. This allows Flora to handle terrain trees and details, container instances, and even runtime instances.

Memory Usage

GPU Resident Drawer treats each MeshRenderer as a separate instance, resulting in increased GPU memory usage in complex LOD scenarios. For example, each MeshRenderer instance may include memory overhead for motion vectors, light probe data, and SpeedTree animation parameters:

Parameter	Type	Count	Memory	Always Included
ObjectToWorld	float3x4	1	48 bytes
WorldToObject	float3x4	1	48 bytes
PrevObjectToWorld	float3x4	1	48 bytes
PrevWorldToObject	float3x4	1	48 bytes
SH (Light Probes)	float4	9	144 bytes
World Bounding Sphere	float4	1	16 bytes
SpeedTree Wind	int4	16	256 bytes

The world bounding sphere is added if GPU Occlusion is present, and the wind parameters are added if the MeshRenderer has a Tree component. A complete tree MeshRenderer instance over half of a MB.

For instance, a SpeedTree LODGroup with four MeshRenderers can reach approximately 1GB of GPU memory at around 400,000 instances. However, Unity's current architecture practically limits such high instance counts anyways due to GameObject overhead, though this may evolve in the future.

Flora on the other hand treats each LODGroup as a single instance: only ever allocating space for motion vectors (previous transforms) and light probes on demand. This can save a lot of space, especially when working with large instance counts.

Terrain Rendering

GPU Resident Drawer cannot render Terrain details or trees because it currently depends on MeshRenderers within the scene. Flora, or alternative instancing solutions, are the only method of currently rendering Terrain instances outside of the builtin system.

GPU Occlusion Culling

Both Flora and GPU Resident Drawer use the same GPU occlusion culling technique, but each maintains separate resources. Using both simultaneously will cause double depth sampling overhead. Unity does not expose GPU Resident Drawer's internal occlusion resources or parameters, so there currently is no way for Flora to piggy back on that system.

It's recommended to select either Flora or GPU Resident Drawer's GPU occlusion to avoid performance overhead, particularly on lower-end hardware.

Feature Comparison

Feature	Flora	GPU Resident Drawer
CPU Only Culling	❌	☑️
GPU Indirect Culling	☑️	☑️
GPU Occlusion Culling	☑️	☑️
GameObject Instancing	☑️	☑️
Non-GameObject Instancing	☑️	❌
Terrain Foliage Instancing	☑️	❌