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UE5.1 Two Side Foliage材质改进及优化

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本文主要记录下UE5.1引擎升级后对TwoSideFoliage材质的优化

UE5.1 Two Side Foliage材质改进及优化

Lumen Two-Sided Foliage Example 2

本文在于项目升级5.1后做的横向测试对比，写做笔记记录下。

改动

  • Screen probe importance sampling在两侧的树叶上被禁用，因此一半的光线用于收集背面照明
  • Screen traces 过背面光线以避免立即自相交。其他跟踪方法使用背面法线进行偏置。
  • Two Sided Foliage Shading 用 DiffuseLighting * DiffuseColor + BackfaceDiffuseLighting * PreintegratedTwoSidedBxDF * SubsurfaceColor
  • r.Lumen.ScreenProbeGather.TwoSidedFoliageBackfaceDiffuse 关闭后会将次表面的颜色加到DiffuseColor(没有backface lighting)
  • The High GI scalability level disables TwoSidedFoliageBackfaceDiffuse to avoid overhead

代码

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// DiffuseIndirectComposite.usf
if (GBuffer.ShadingModelID == SHADINGMODELID_TWOSIDED_FOLIAGE)
				{
					float3 SubsurfaceColor = ExtractSubsurfaceColor(GBuffer);

					if (bLumenSupportBackfaceDiffuse > 0)
					{
						BackfaceDiffuseIndirectLighting += SubsurfaceColor * DiffuseIndirect_Textures_1.SampleLevel(GlobalPointClampedSampler, SceneBufferUV, 0).rgb;
					}
					else
					{
						// Adding Subsurface energy to the diffuse lobe is a poor approximation when DiffuseColor is small and SubsurfaceColor is large
						// Reduce the error by attenuating SubsurfaceColor, even though diffuse already has the 1/PI for Lambert.
						const float PreintegratedTwoSidedBxDF = 1.0f / PI;
						DiffuseColor += SubsurfaceColor * PreintegratedTwoSidedBxDF;
					}
				}
IndirectLighting.Diffuse = (DiffuseIndirectLighting * DiffuseColor + BackfaceDiffuseIndirectLighting) * Occlusion.DiffuseOcclusion;
IndirectLighting.Transmission = 0;

总结

  • UE 在5.1中对two side foliage类型的材质进行背部次表面散射补偿。通过叠加次表面散射的颜色与间接光的乘积得到背部补偿。但会增加16mb的RT来累积光照
  • 可用点乘的方式在低版本的植被上添加自发光实现补偿trick效果。

参考

https://github.com/EpicGames/UnrealEngine/commit/3ecab7b620fe7c74c0427f5dc24cc32528eba301