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Use the distribution of visible normals for runtime IBL prefiltering

/stochastic_alpha_test
Evgenii Golubev 7 年前
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cf57d1d5
共有 5 个文件被更改，包括 121 次插入 和 78 次删除
  1. 8
      ScriptableRenderPipeline/Core/ShaderLibrary/Fibonacci.hlsl
  2. 141
      ScriptableRenderPipeline/Core/ShaderLibrary/ImageBasedLighting.hlsl
  3. 47
      ScriptableRenderPipeline/HDRenderPipeline/Sky/ComputeGgxIblSampleData.compute
  4. 2
      ScriptableRenderPipeline/HDRenderPipeline/Sky/GGXConvolve.shader
  5. 1
      ScriptableRenderPipeline/HDRenderPipeline/Sky/RuntimeFilterIBL.cs

8
ScriptableRenderPipeline/Core/ShaderLibrary/Fibonacci.hlsl
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return float2(i / fibN1 + (0.5f / fibN1), frac(i * (fibN2 / fibN1)));
}
#define GOLDEN_RATIO 1.6180339887498948482
// Replaces the Fibonacci sequence in Fibonacci2dSeq() with the Golden ratio.
float2 Golden2dSeq(int i, float n)
{
return float2(i / n + (0.5f / n), frac(i * rcp(GOLDEN_RATIO)));
}
static const int k_FibonacciSeq[] = {
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181
};

141
ScriptableRenderPipeline/Core/ShaderLibrary/ImageBasedLighting.hlsl
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L = mul(localL, localToWorld);
}
// Ref: "A Simpler and Exact Sampling Routine for the GGX Distribution of Visible Normals".
void SampleVisibleAnisoGGXDir(float2 u, float3 V, float3x3 localToWorld,
float roughnessT, float roughnessB,
out float3 L,
out float NdotL,
out float NdotH,
out float VdotH,
bool VeqN = false)
{
float3 localV = mul(V, transpose(localToWorld));
// Construct an orthonormal basis around the stretched view direction.
float3x3 viewToLocal;
if (VeqN)
{
viewToLocal = k_identity3x3;
}
else
{
viewToLocal[2] = normalize(float3(roughnessT * localV.x, roughnessB * localV.y, localV.z));
viewToLocal[0] = (viewToLocal[2].z < 0.9999) ? normalize(cross(viewToLocal[2], float3(0, 0, 1))) : float3(1, 0, 0);
viewToLocal[1] = cross(viewToLocal[0], viewToLocal[2]);
}
// Compute a sample point with polar coordinates (r, phi).
float r = sqrt(u.x);
float b = viewToLocal[2].z + 1;
float a = rcp(b);
float c = (u.y < a) ? u.y * b : 1 + (u.y * b - 1) / viewToLocal[2].z;
float phi = PI * c;
float p1 = r * cos(phi);
float p2 = r * sin(phi) * ((u.y < a) ? 1 : viewToLocal[2].z);
// Unstretch.
float3 viewH = normalize(float3(roughnessT * p1, roughnessB * p2, sqrt(1 - p1 * p1 - p2 * p2)));
VdotH = viewH.z;
float3 localH = mul(viewH, viewToLocal);
NdotH = localH.z;
// Compute { localL = reflect(-localV, localH) }
float3 localL = -localV + 2 * VdotH * localH;
NdotL = localL.z;
#if 0
H = mul(localH, localToWorld);
#endif
L = mul(localL, localToWorld);
}
// ref: http://blog.selfshadow.com/publications/s2012-shading-course/burley/s2012_pbs_disney_brdf_notes_v3.pdf p26
void SampleAnisoGGXDir(float2 u,
float3 V,

float3 N,
float roughness,
float index, // Current MIP level minus one
float lastMipLevel,
float invOmegaP,
uint sampleCount, // Must be a Fibonacci number
bool prefilter,

float NdotV = 1; // N == V
float preLambdaV = GetSmithJointGGXPreLambdaV(1, roughness);
#endif
// Bias samples towards the mirror direction to reduce variance.
// This will have a side effect of making the reflection sharper.
// Ref: Stochastic Screen-Space Reflections, p. 67.
const float bias = 0.5 * roughness;
float3 lightInt = float3(0.0, 0.0, 0.0);
float cbsdfInt = 0.0;

float3 L;
float NdotL, NdotH, VdotH;
bool isValid;
float NdotL, NdotH, LdotH;
L = mul(localL, localToWorld);
NdotL = localL.z;
isValid = true;
L = mul(localL, localToWorld);
NdotL = localL.z;
LdotH = sqrt(0.5 + 0.5 * NdotL);
u.x = lerp(u.x, 0.0, bias);
SampleGGXDir(u, V, localToWorld, roughness, L, NdotL, NdotH, VdotH, true);
SampleVisibleAnisoGGXDir(u, V, localToWorld, roughness, roughness, L, NdotL, NdotH, LdotH, true);
isValid = NdotL > 0.0;
if (NdotL <= 0) continue; // Note that some samples will have 0 contribution
}
float mipLevel;

// in order to reduce the variance.
// Ref: http://http.developer.nvidia.com/GPUGems3/gpugems3_ch20.html
//
// pdf = D * NdotH * jacobian, where jacobian = 1.0 / (4* LdotH).
//
// Since L and V are symmetric around H, LdotH == VdotH.
// Since we pre-integrate the result for the normal direction,
// N == V and then NdotH == LdotH. Therefore, the BRDF's pdf
// can be simplified:
// pdf = D * NdotH / (4 * LdotH) = D * 0.25;
//
// - OmegaS : Solid angle associated with the sample
// - OmegaP : Solid angle associated with the texel of the cubemap

}
else
{
float pdf = D_GGX(NdotH, roughness) * 0.25;
// float pdf = D_v / (4 * LdotH).
omegaS = rcp(sampleCount) / pdf;
float rcpPdf = (4 * LdotH) / D_GGX_Visible(NdotH, NdotV, LdotH, roughness);
omegaS = rcp(sampleCount) * rcpPdf;
mipLevel = 0.5 * log2(omegaS * invOmegaP);
mipLevel = 0.5 * log2(omegaS * invOmegaP);
if (isValid)
{
// Bias the MIP map level to compensate for the importance sampling bias.
// This will blur the reflection.
// TODO: find a more accurate MIP bias function.
mipLevel = lerp(mipLevel, lastMipLevel, bias);
// TODO: use a Gaussian-like filter to generate the MIP pyramid.
float3 val = SAMPLE_TEXTURECUBE_LOD(tex, sampl, L, mipLevel).rgb;
// TODO: use a Gaussian-like filter to generate the MIP pyramid.
float3 val = SAMPLE_TEXTURECUBE_LOD(tex, sampl, L, mipLevel).rgb;
// The goal of this function is to use Monte-Carlo integration to find
// X = Integral{Radiance(L) * CBSDF(L, N, V) dL} / Integral{CBSDF(L, N, V) dL}.
// Note: Integral{CBSDF(L, N, V) dL} is given by the FDG texture.
// using the formulation with the distribution of visible normals D_v,
// CBSDF = F * D_v * G * NdotL / (4 * G_v * NdotL * VdotH) = F * D_v * G / (4 * G_v * NdotV).
// PDF = D_v / (4 * LdotH).
// Weight = CBSDF / PDF = F * G / G_v.
// Since we perform filtering with the assumption that (V == N), G_v is constant.
// Therefore, after the Monte Carlo expansion of the integrals,
// X = Sum(Radiance(L) * Weight) / Sum(Weight) = Sum(Radiance(L) * F * G) / Sum(F * G).
// The goal of this function is to use Monte-Carlo integration to find
// X = Integral{Radiance(L) * CBSDF(L, N, V) dL} / Integral{CBSDF(L, N, V) dL}.
// Note: Integral{CBSDF(L, N, V) dL} is given by the FDG texture.
// CBSDF = F * D * G * NdotL / (4 * NdotL * NdotV) = F * D * G / (4 * NdotV).
// PDF = D * NdotH / (4 * LdotH).
// Weight = CBSDF / PDF = F * G * LdotH / (NdotV * NdotH).
// Since we perform filtering with the assumption that (V == N),
// (LdotH == NdotH) && (NdotV == 1) && (Weight == F * G).
// Therefore, after the Monte Carlo expansion of the integrals,
// X = Sum(Radiance(L) * Weight) / Sum(Weight) = Sum(Radiance(L) * F * G) / Sum(F * G).
#ifndef USE_KARIS_APPROXIMATION
float F = 1; // The choice of the Fresnel factor does not appear to affect the result
float V = V_SmithJointGGX(NdotL, NdotV, roughness, preLambdaV);
float G = V * /* 4 * */ NdotL * NdotV; // 4 cancels out
lightInt += F * G * val;
cbsdfInt += F * G;
#else
// Use the approximation of Brian Karis from "Real Shading in Unreal Engine 4":
// Weight ≈ NdotL.
lightInt += NdotL * val;
cbsdfInt += NdotL;
#endif
}
#ifndef USE_KARIS_APPROXIMATION
float F0 = 1; // The choice of the Fresnel factor does not appear to affect the result
float F = F_Schlick(F0, LdotH);
float V = V_SmithJointGGX(NdotL, NdotV, roughness, preLambdaV);
float G2 = V * NdotL * NdotV; // 4 cancels out
lightInt += F * G2 * val;
cbsdfInt += F * G2;
#else
// Use the approximation from "Real Shading in Unreal Engine 4": Weight ≈ NdotL.
lightInt += NdotL * val;
cbsdfInt += NdotL;
#endif
}
return float4(lightInt / cbsdfInt, 1.0);

47
ScriptableRenderPipeline/HDRenderPipeline/Sky/ComputeGgxIblSampleData.compute
查看文件


[numthreads(MAX_IBL_SAMPLE_CNT, UNITY_SPECCUBE_LOD_STEPS, 1)]
void ComputeGgxIblSampleData(uint3 groupThreadId : SV_GroupThreadID)
{
uint sampleIndex = groupThreadId.x;
uint mipLevel = groupThreadId.y + 1;
uint sampleCount = GetIBLRuntimeFilterSampleCount(mipLevel);
uint sampleIndex = groupThreadId.x;
uint mipLevel = groupThreadId.y + 1;
float roughness = PerceptualRoughnessToRoughness(MipmapLevelToPerceptualRoughness(mipLevel));
uint sampleCount = GetIBLRuntimeFilterSampleCount(mipLevel);
if (sampleIndex >= sampleCount)
{

float perceptualRoughness = MipmapLevelToPerceptualRoughness(mipLevel);
float roughness = PerceptualRoughnessToRoughness(perceptualRoughness);
float3 V = float3(0, 0, 1);
float NdotV = 1;
float bias = 0.5 * roughness;
float3 localL;
float NdotL, NdotH, LdotH;
float2 u = Fibonacci2d(sampleIndex, sampleCount);
u.x = lerp(u.x, 0, bias);
const int validSampleCount = sampleCount;
float3 localL;
float NdotL, NdotH, VdotH;
while (true)
{
int wastedSampleCount = 0;
for (int i = 0; i < sampleCount; i++)
{
// We switch to the Golden sequence instead of the Fibonacci sequence
// since the sample count is not guaranteed to be a Fibonacci number.
float2 u = Golden2dSeq(i, sampleCount);
SampleVisibleAnisoGGXDir(u, V, k_identity3x3, roughness, roughness, localL, NdotL, NdotH, LdotH, true);
SampleGGXDir(u, float3(0, 0, 1), k_identity3x3, roughness, localL, NdotL, NdotH, VdotH, true);
if (NdotL <= 0) wastedSampleCount++;
}
if (NdotL <= 0)
{
// We are not supposed to generate wasteful samples.
output[groupThreadId.xy] = float4(0, 0, 0, 0);
return;
if (validSampleCount == sampleCount - wastedSampleCount) break;
sampleCount++;
float pdf = D_GGX(NdotH, roughness) * 0.25;
float omegaS = rcp(sampleCount * pdf);
float2 u = Golden2dSeq(sampleIndex, sampleCount);
SampleVisibleAnisoGGXDir(u, V, k_identity3x3, roughness, roughness, localL, NdotL, NdotH, LdotH, true);
float rcpPdf = (4 * LdotH) / D_GGX_Visible(NdotH, NdotV, LdotH, roughness);
float omegaS = rcp(sampleCount) * rcpPdf;
output[groupThreadId.xy] = float4(localL, omegaS);
}

2
ScriptableRenderPipeline/HDRenderPipeline/Sky/GGXConvolve.shader
查看文件


#endif
float _Level;
float _LastLevel;
float _InvOmegaP;
float4x4 _PixelCoordToViewDirWS; // Actually just 3x3, but Unity can only set 4x4

V, N,
roughness,
_Level - 1,
_LastLevel,
_InvOmegaP,
sampleCount, // Must be a Fibonacci number
true,

1
ScriptableRenderPipeline/HDRenderPipeline/Sky/RuntimeFilterIBL.cs
查看文件


m_GgxConvolveMaterial.SetTexture("_MainTex", source);
m_GgxConvolveMaterial.SetTexture("_GgxIblSamples", m_GgxIblSampleData);
m_GgxConvolveMaterial.SetFloat("_LastLevel", mipCount - 1);
m_GgxConvolveMaterial.SetFloat("_InvOmegaP", invOmegaP);
for (int mip = 1; mip < ((int)EnvConstants.SpecCubeLodStep + 1); ++mip)

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