SimNanite

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Simplified Nanite Implementation in MiniEngine

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# SimNanite[![License](https://img.shields.io/github/license/ShawnTSH1229/SimNanite.svg)](https://github.com/ShawnTSH1229/SimLumen/blob/master/LICENSE)
Simplified Nanite Implementation in MiniEngine

[**SimNanite Development Blog**](https://shawntsh1229.github.io/2024/05/08/Simplified-Nanite-Virtualized-Geometry-In-MiniEngine/)

# Introduction

This is a simplified Nanite implementation **(SimNanite)** based on Unreal's Nanite virtual geometry. We have implemented **most of Unreal Nanite's features**.

In offline, we partition the triangles into **clusters** with the **Metis** graph partition library. Then, SimNanite partitions the clusters into **cluster groups**, builds the **DAG** (Directed Acyclic Graph) and **BVH tree** based on the cluster groups. In order to avoid the **LOD crack** problem, SimNanite simplify the mesh globally rather than simplifying the triangles per cluster.

At runtime, we implement a three-level GPU culling pipeline: **instance culling**, **BVH node culling** and **cluster culling**. In the BVH node culling pass, we use the **MPMC ( Multiple Producers, Single Consumer )** model to transverse the BVH structure and **integrate the cluster culling** into the BVH culling shader for **work balance**.

We generate an **indirect draw** or **indirect dispatch** command for those clusters that pass the culling. If the cluster is small enough, we employ the **software rasterization** with compute shader.

During the rasterization pass, we write the **cluster index and triangle index** to the **visibility buffer**. In the next base pass or GBuffer rendering, we fetch these indices from the visibility buffer, calculate the **pixel attributes** (UV and normal) by **barycentric coordinates** and render the scene with these attributes.

# Getting Started

1.Cloning the repository with `git clone https://github.com/ShawnTSH1229/SimNanite.git`.

2.Compile and run the visual studio solution located in `SimNanite\MiniEngine\SimNanite\SimNanite.sln`

# Source Tree

## SimNaniteBuilder

Offline nanite mesh builder, located at MiniEngine\SimNanite\SimNaniteBuilder

SimNanitePartioner.cpp : Partition the triangle into clusters and partition the clusters into cluster groups

SimNaniteMeshSimplifier.cpp : Simplify the mesh based on the meshoptimizer library

SimNaniteBuildBVH.cpp : Build the cluster group BVH tree

SimNaniteBuildDAG.cpp : Build the cluster group Directed Acyclic Graph

## SimNaniteRuntime

located at MiniEngine\SimNanite\SimNaniteRuntime

SimNaniteInstanceCulling.cpp : instance-level culling

SimNanitePersistentCulling.cpp : MPMC BVH node culling and cluster culling

SimNaniteRasterization.cpp : Hardware rasterization and software rasterization

SimNaniteBasePassRendering.cpp : Rendering the scene based on the visibility buffer

# Visualization

Key 0: Visualize Clusters

Key 1: Update Freezing Data

Key 2 / 5: decrease/increase cluster visualize lod

Key 4: Visualize Visibility Buffer

Key 6: Visualize Node Culling Result( set DEBUG_CLUSTER_GROUP to 1)

Kay 7: Visualize Instance Culling Result

# Screenshot

Clusters Partition

Clusters LOD0:

 <img src="/Resource/vis_clu_0.png" width="60%" height="60%">

Clusters LOD1:

 <img src="/Resource/vis_clu_1.png" width="60%" height="60%">

Clusters LOD2:

 <img src="/Resource/vis_clu_2.png" width="60%" height="60%">

Instance Culling:

Visibility Buffer:

Hardware Rasterization:

Software Rasterization:

Hardware + Software Rasterization Visibility BUffer:

BasePass Rendering + Material Type 0:

BasePass Rendering + Material Type 1 + 0:

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