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title: Untitled
date: 2024-12-28 21:56:29
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# 前言
相关论文地址：
- Reflective Shadow Maps: https://users.soe.ucsc.edu/~pang/160/s13/proposal/mijallen/proposal/media/p203-dachsbacher.pdf
- Light Propagation Volumes: https://ericpolman.com/
- VXGI:
	- https://on-demand.gputechconf.com/gtc/2014/preseintations/S4552-rt-voxel-based-global-illumination-gpus.pdf
	- https://docs.nvidia.com/gameworks/content/gameworkslibrary/visualfx/vxgi/product.html
	- https://developer.download.nvidia.com/assets/gameworks/downloads/secure/GDC18_Slides/Advances%20in%20Real-Time%20Voxel-Based%20GI%20-%20GDC%202018.pdf?HAwkc19IdhKMFWUs8jSk-If6y4QowlgUUrY8F9SL1-4qnhWWo-u5_gptg4ITWjMJCd1JrFbGmBw4-ztQCJ4M9Bh61am6lFCerKQVApEjN-HQ4chiSb5MLRLY41TyIse6_XW2DpNAe0_4inrYkZqxVb2iGiyZQhxwzfZ40dByP-X8jRYfWhJVsC1aSIqZ8BkooGkjUoUUGw2r7AAxyWVQole9a4je_76g04KtUwlONQ==
- Lumen:
	- [https://youtu.be/2GYXuM10riw](https://youtu.be/2GYXuM10riw)
	- https://www.advances.realtimerendering.com/s2021/Radiance%20Caching%20for%20real-time%20Global%20Illumination%20(SIGGRAPH%202021).pptx
- SSGI: https://www.ea.com/frostbite/news/stochastic-screen-space-reflections
- DDGI: 
	- https://zhuanlan.zhihu.com/p/404520592
	- https://developer.download.nvidia.com/video/gputechconf/gtc/2019/presentation/s9900-irradiance-fields-rtx-diffuse-global-illumination-for-local-and-cloud-graphics.pdf
# HIZ Tracing

![[HierachicalTracing2.png]]

![[HierachicalTracing3.png]]

![[HierachicalTracing1.png]]# Reflective Shadow Maps（RSM，2005）
论文地址：https://users.soe.ucsc.edu/~pang/160/s13/proposal/mijallen/proposal/media/p203-dachsbacher.pdf

Let's inject light in. (Photon Mapping?)
解决如何把“光”注入到场景中。

**Cool Ideas**
- Easy to be implemented
- Photon Injection with RSM
- Cone sampling in mipmap
- Low-res Indirect illumination with error check
**Cons**
- Single bounce
- No visibility check for indirect illumination

# Light Propagation Volumes

**"Freeze" the Radiance in Voxel**
Light Injection
- Pre-subdivide the scene into a 3D grid
- For each grid cell, find enclosed virtual light sources
- Sum up their directional radiance distribution
- Project to first 2 orders of SHs (4 in total)

# Sparse Voxel Octree for Real-time Global Illumination (SVOGI)

## Shading with Cone Tracing in Voxel Tree
Pass 2 from the camera
- Emit some cones based on diffuse+specular BRDF
- Query in octree based on the (growing) size of the cone

# VXGI（Nvidia UE4 Plugins）
**Problems in VXGI**
Incorrect Occlusion(opacity)
- naively combine the opacity with alpha blending.
Light Leaking
- when occlusion wall is much smaller than voxel size

# SSGI
SIGGRAPH2015：Advances in Real-Time Rendering course

**Radiance Sampling in Screen Space**
For each fragment:
- **Step 1**: compute many reflection
rays
- **Step 2**: march along ray direction
**(in depth gbuffer)**
- **Step3**: use color of hit point as
indirect lighting
![[SSGI1.png]]

![[SSGI2.png]]
中间的RayCast使用RayMarching进行。但使用LinearRayMarching相对比较消耗资源，所以采用HierachicalTracing。

![[SSGI3.png]]
最低层级
![[HierachicalTracing1.png]]
层级+1，相当于RayMarching2个像素。
![[HierachicalTracing2.png]]
层级+2，相当于RayMarching4个像素。此时RayHit。
![[HierachicalTracing3.png]]
回退当前HiZ像素的上一层级。
![[HierachicalTracing4.png]]
回退当前HiZ像素的上上一层级。
![[HierachicalTracing5.png]]
找到RayHit位置。
![[HierachicalTracing6.png]]

## Ray Reuse among Neighbor Pixels
- Store **hitpoint data**
- Assume visibility is the same between neighbors
- Regard **ray to neighbor's hitpoint** as valid

![[ConeTracingWithMipmapFiltering1.png]]

# Lumen
## Phase1: Fast Ray Track in Any Hardward
Signed Distance Field(SDF)
1. 它是均匀的。
2. 在空间上是连续的。

### Cone Tracing with SDF(ie. Soft Shadow)

## Phase2:Radiance Injection and Cacheing
![[MeshCard1.png]]
MeshCard的目的是为了将直接光照存储在模型上（Surface Cache）

![[GenerateSurfaceCache1.png]]
![[GenerateSurfaceCache2.png]]

最终目的是通过SurfaceCache这4张图渲染出SurfaceCache FinalLighting
![[LightingCachePipeline1.png]]
1. 计算SurfaceCache DirectLighting
2. 通过1计算体素光照。
3. 通过体素光照来计算间接照明。
4. 最终计算Surface Cache FinalLighting。

以此进行循环。![[DirectLighting1.png]]

针对多个光源会渲染对应数量的cache，之后累加在一起。
![[MultiLightSurfaceCache.png]]

### Voxel Lighting to Sample
![[VoxelLightingToSample.png]]
对于近处的物体可以准确拿到Hit到物体的上一点的Radiance；对于远处的物体，会以相机坐标轴生成一个Voxel形式的表达，之后通过Global SDF拿到对应的Radiance。
***PS. 该Voxel存储的数据为：每个面对应方向上被其他直接照明照亮的亮度。***

![[VoxelClipmap.png]]![[BuildVoxelFaces.png]]
其Voxel的计算是基于SDF的。

![[InjectLightIntoClipmap.png]]

![[IndirectLighting.png]]
在SurfaceCache中 8x8的tile中（行与列间隔4个像素，放置2个探针），进行4次空间Voxel采样。
之后进行球谐插值：
![[IndirectLighting_SHLerp.png]]![[Per-PixelIndirectLighting.png]]![[CombineLighting.png]]

## Phase3:Build a lot of probes with Different Kinds
![[ScreenProbeStructure.png]]

每隔 16 * 16 个像素采样一个ScreenSpaceProbe。采样的内容是Radiance与HitDistance，以8面体（Octahedron Mapping）的方式进行存储。

![[ScreenProbePlacement.png]]![[PlaneDistanceWeightingOfProbeInterpolation.png]]![[DetectNon-InterpolatableCases.png]]![[ScreenProbeAtlas.png]]
将重采样的结果（部分区域的屏幕空间探针因为实际空间距离太远，进行插值没有意义，所以需要额外填充探针进行重采样）存在Atlas下面的空出来的区域。

![[ScreenProbeJitter.png]]

### 重要性采样
![[ApproximateRadianceImportanceFromLastFrameProbes.png]]![[AccumulateNormalDistributionNearby.png]]![[NearbyNormalAccumulation.png]]![[StructuredImportanceSampling.png]]![[FixBudgetImportanceSampling.png]]

### Denoise
![[Denoise_SpatialFilteringForProbe.png]]![[Denoise_GatherRadianceFromNeightbors.png]]![[ClampDistanceMismatching.png]]

### WorldSpace Probes and Ray Connecting 
![[WorldSapceRadianceCache.png]]![[WorldSpaceRadianceCache.png]]![[ConnectingRays.png]]![[ConnectingRays2.png]]![[ConnectingRay3.png]]![[PlacementAndCacheing.png]]

## Phase4:Shading Full Pixels with Screen Space Probes
在ScreenSpaceProbes将场景Radiance都收集好。
![[ConvertProbeRadianceTo3rdOrderSphericalHarmonic.png]]

![[FinalIntegarationWithSH.png]]

## 性能问题
![[LumenTrackMethod.png]]