203 lines
11 KiB
Markdown
203 lines
11 KiB
Markdown
## UE4 渲染功能探究
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New: Planar Reflections
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New: High Quality Reflections
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## UE4.26 SingleLayerWater笔记
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官方论坛讨论
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https://forums.unrealengine.com/development-discussion/rendering/1746626-actually-realistic-water-shader#post1789028
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### SingleLayerCommon.ush
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计算光照强度、透明度。
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struct WaterVolumeLightingOutput
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{
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float3 Luminance;
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float3 WaterToSceneTransmittance;
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float3 WaterToSceneToLightTransmittance;
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};
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Output.Luminance = WaterVisibility * (ScatteredLuminance + Transmittance * BehindWaterSceneLuminance);
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Output.WaterToSceneTransmittance = Transmittance;
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Output.WaterToSceneToLightTransmittance;
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目前没有开启RayMarching,所以核心代码为：
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```
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const float3 OpticalDepth = ExtinctionCoeff * BehindWaterDeltaDepth;
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float3 Transmittance = exp(-OpticalDepth);
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float3 ScatteredLuminance = ScatteringCoeff * (AmbScattLuminance + SunScattLuminance * DirectionalLightShadow);
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ScatteredLuminance = (ScatteredLuminance - ScatteredLuminance * Transmittance) / ExtinctionCoeffSafe;
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// Apply Fresnel effect to out-scattering towards the view
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ScatteredLuminance *= CameraIsUnderWater ? 1.0 : (1.0 - EnvBrdf); // Under water is less visible due to Fresnel effect
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Transmittance *= CameraIsUnderWater ? (1.0 - EnvBrdf) : 1.0; // Above " " " " "
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// Add single in-scattering apply colored transmittance to scene color
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Output.Luminance = WaterVisibility * (ScatteredLuminance + Transmittance * (BehindWaterSceneLuminance* ColorScaleBehindWater));
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Output.WaterToSceneTransmittance = Transmittance;
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Output.WaterToSceneToLightTransmittance = Transmittance * MeanTransmittanceToLightSources;
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```
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```c++
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const float BehindWaterDeltaDepth = CameraIsUnderWater ? WaterDepth : max(0.0f, SceneDepth - WaterDepth);
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const float3 ScatteringCoeff = max(0.0f, GetSingleLayerWaterMaterialOutput0(MaterialParameters));
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const float3 AbsorptionCoeff = max(0.0f, GetSingleLayerWaterMaterialOutput1(MaterialParameters));
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const float PhaseG = clamp(GetSingleLayerWaterMaterialOutput2(MaterialParameters), -1.0f, 1.0f);
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//Sample the optional Material Input ColorScaleBehindWater and fade it out at shorelines to avoid hard edge intersections
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float3 ColorScaleBehindWater = lerp(1.0f, max(0.0f, GetSingleLayerWaterMaterialOutput3(MaterialParameters)), saturate(BehindWaterDeltaDepth * 0.02f));
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const float3 ExtinctionCoeff = ScatteringCoeff + AbsorptionCoeff;
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// Max to avoid division by 0 with the analytical integral below.
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// 1e-5 is high enough to avoid denorms on mobile
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const float3 ExtinctionCoeffSafe = max(ScatteringCoeff + AbsorptionCoeff, 1e-5);
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float DirLightPhaseValue = 0.0f; // Default when Total Internal Reflection happens.
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{
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#if SIMPLE_SINGLE_LAYER_WATER
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DirLightPhaseValue = IsotropicPhase();
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#else
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float IorFrom = 1.0f; // assumes we come from air
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float IorTo = DielectricF0ToIor(DielectricSpecularToF0(Specular)); // Wrong if metal is set to >1. But we still keep refraction on the water surface nonetheless.
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const float relativeIOR = IorFrom / IorTo;
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float3 UnderWaterRayDir = 0.0f;
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if (WaterRefract(MaterialParameters.CameraVector, MaterialParameters.WorldNormal, relativeIOR, UnderWaterRayDir))
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{
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DirLightPhaseValue = SchlickPhase(PhaseG, dot(-ResolvedView.DirectionalLightDirection.xyz, UnderWaterRayDir));
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}
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#endif
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}
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// We also apply transmittance from light to under water surface. However, the scene has been lit by many sources already.
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// So the transmittance toabove surface is simply approximated using the travel distance from the scene pixel to the water top, assuming a flat water surface.
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// We cannot combine this transmittance with the transmittance from view because this would change the behavior of the analytical integration of light scattering integration.
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const float3 BehindWaterSceneWorldPos = SvPositionToWorld(float4(MaterialParameters.SvPosition.xy, SceneDeviceZ, 1.0));
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const float DistanceFromScenePixelToWaterTop = max(0.0, MaterialParameters.AbsoluteWorldPosition.z - BehindWaterSceneWorldPos.z);
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const float3 MeanTransmittanceToLightSources = exp(-DistanceFromScenePixelToWaterTop * ExtinctionCoeff);
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#if SIMPLE_SINGLE_LAYER_WATER
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const float3 BehindWaterSceneLuminance = 0.0f; // Cannot read back the scene color in this case
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#else
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// We use the pixel SvPosition instead of the scene one pre refraction/distortion to avoid those extra ALUs.
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float3 BehindWaterSceneLuminance = SceneColorWithoutSingleLayerWaterTexture.SampleLevel(SceneColorWithoutSingleLayerWaterSampler, ViewportUV, 0).rgb;
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BehindWaterSceneLuminance = MeanTransmittanceToLightSources * (USE_PREEXPOSURE ? ResolvedView.OneOverPreExposure : 1.0f) * BehindWaterSceneLuminance;
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#endif
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float3 SunScattLuminance = DirLightPhaseValue * SunIlluminance;
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float3 AmbScattLuminance = IsotropicPhase() * AmbiantIlluminance;
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#define VOLUMETRICSHADOW 0
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#if !VOLUMETRICSHADOW || SIMPLE_SINGLE_LAYER_WATER
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const float3 OpticalDepth = ExtinctionCoeff * BehindWaterDeltaDepth;
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float3 Transmittance = exp(-OpticalDepth);
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float3 ScatteredLuminance = ScatteringCoeff * (AmbScattLuminance + SunScattLuminance * DirectionalLightShadow);
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ScatteredLuminance = (ScatteredLuminance - ScatteredLuminance * Transmittance) / ExtinctionCoeffSafe;
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#else
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// TODO Make the volumetric shadow part work again
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float3 Transmittance = 1.0f;
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float3 ScatteredLuminance = 0.0f;
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const float RayMarchMaxDistance = min(BehindWaterDeltaDepth, 200.0f); // 20 meters
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const float RayMarchStepSize = RayMarchMaxDistance / 10.0f; // Less samples wil lresult in a bit brighter look due to TransmittanceToLightThroughWater being 1 on a longer first sample. Would need it part of analiytical integration
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const float ShadowDither = RayMarchStepSize * GBufferDither;
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for (float s = 0.0f; s < RayMarchMaxDistance; s += RayMarchStepSize)
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{
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// Only jitter shadow map sampling to not lose energy on first sample
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float Shadow = ComputeDirectionalLightDynamicShadowing(MaterialParameters.AbsoluteWorldPosition - (s + ShadowDither)*MaterialParameters.CameraVector, GBuffer.Depth);
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float3 WP = MaterialParameters.AbsoluteWorldPosition - s * MaterialParameters.CameraVector;
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float WaterHeightAboveSample = max(0.0, MaterialParameters.AbsoluteWorldPosition.z - WP.z);
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float3 TransmittanceToLightThroughWater = 1.0; // no self shadow, same energy as above analytical solution
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//float3 TransmittanceToLightThroughWater = exp(-ExtinctionCoeff * WaterHeightAboveSample); // self shadow as transmittance to water level, close to reference, depends a bit on sample count due to first sample being critical for dense medium
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float3 SampleTransmittance = exp(-ExtinctionCoeff * RayMarchStepSize); // Constant
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float3 SS = (ScatteringCoeff * TransmittanceToLightThroughWater * (SunScattLuminance * Shadow + AmbScattLuminance));
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ScatteredLuminance += Transmittance * (SS - SS * SampleTransmittance) / ExtinctionCoeffSafe;
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Transmittance *= SampleTransmittance;
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}
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// The rest of the medium
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const float3 OpticalDepth2 = ExtinctionCoeff * max(0.0, BehindWaterDeltaDepth - RayMarchMaxDistance);
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if (any(OpticalDepth2 > 0.0f))
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{
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float3 Transmittance2 = exp(-OpticalDepth2);
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float3 ScatteredLuminance2 = ScatteringCoeff * (SunScattLuminance + AmbScattLuminance);
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ScatteredLuminance += Transmittance * (ScatteredLuminance2 - ScatteredLuminance2 * Transmittance2) / ExtinctionCoeffSafe;
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Transmittance *= Transmittance2;
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}
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#endif
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// Apply Fresnel effect to out-scattering towards the view
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ScatteredLuminance *= CameraIsUnderWater ? 1.0 : (1.0 - EnvBrdf); // Under water is less visible due to Fresnel effect
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Transmittance *= CameraIsUnderWater ? (1.0 - EnvBrdf) : 1.0; // Above " " " " "
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// Add single in-scattering apply colored transmittance to scene color
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Output.Luminance = WaterVisibility * (ScatteredLuminance + Transmittance * (BehindWaterSceneLuminance* ColorScaleBehindWater));
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Output.WaterToSceneTransmittance = Transmittance;
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Output.WaterToSceneToLightTransmittance = Transmittance * MeanTransmittanceToLightSources;
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}
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```
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海洋是不透明的，使用SceneColor缓存合成出的透明效果。
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## GDC2012 神秘海域3演讲
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### 渲染方案
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### FlowShader
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没看懂，为什么需要用2张贴图叠加，是因为要过度么？
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4.5.1.1 Flow Map变体：《神秘海域3》Flow Map + Displacement
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另外，Flow Map可以和其他渲染技术结合使用，比如《神秘海域3》中的Flow Map + Displacement：
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4.5.1.2 Flow Map变体：《堡垒之夜》Flow Map + Distance Fields + Normal Maps
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以及《堡垒之夜》中的Flow Map + Distance Fields + Normal Maps [GDC 2019, Technical Artist Bootcamp Distance Fields and Shader Simulation Tricks]
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4.5.1.3 Flow Map变体：《神秘海域4》Flow Map + Wave Particles
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或者《神秘海域4》中的Flow Map + Wave Particles[SIGGRAPH 2016, Rendering Rapids in Uncharted 4]，都是进阶模拟水体表面流动与起伏效果的不错选择。
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### Wave System
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如果我们能找到一个好的模型，程序化的几何和动画是不错的。
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仿真计算成本太高（即使在SPU中），设计者也很难控制。
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Perlin噪音效果在视觉上不是很好，往往看起来很人工化
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FFT技术很好，但是参数很难被艺术家控制和调整。也是很难搞好的
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**Gerstner waves**
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简单易于控制效果，但高频细节不够多，只能叠加几组大浪，否则太消耗资源。
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**FFT Waves**
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真实，细节丰富。但是美术难以控制效果。
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神秘海域3采用4组Gerstner waves+4组波动粒子的方式来实现Wave Vector Displacement。
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#### 大浪
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大浪采用贝塞尔曲线建模完成
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**之后再叠加大浪**。
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这是整个波系的局部公式。
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bspline是一个均匀的、非理性的bspline。我们本可以使用贝塞尔，但它需要更多的代码。
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grid(u,v)函数返回一个给定坐标u,v的标量值。在这种情况下，我们有一个波标的乘数
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## Sea of Thieves approach [Ang, 2018]
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## Crest Siggraph2019
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### Light Scattering
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使用了类似盗贼之海的光线散射算法，光线散射项是基于海面置换项的水平长度。这里补充一下：使它在我们的框架中更好地工作--我们通过将置换项除以波长来做一种特殊的归一化，并将这个归一化版本用于光散射项。
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基于海平面高度的散射在海洋参数发生改变时容易出问题。
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如果将置换项除以波长，就可以针对大波与小波进行缩放。
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### Shadering
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Cascade scale used to scale shading inputs
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Normals
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Foam
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Underwater bubbles
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Works for range of viewpoints
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Breaks up patterns
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Combats mipmapping
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Increase visible range of detail
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Doubles texture samples
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