UE5.8 Substrate Toon Shading

uint PackToonCustomData(float2 UVs, float ProfileId, float Dither) {
    uint U12bits = saturate(UVs.x) * 4095.0f;
    uint V12bits = saturate(UVs.y) * 4095.0f;
    uint Packed  = U12bits;
    Packed |= V12bits << 12;
    Packed |= (uint(ProfileId) & 0xFF) << 24;
    return Packed;
}

void UnpackToonCustomData(uint Packed, inout float2 UVs, inout float ProfileId) {
    uint U12bits = BitFieldExtractU32(Packed, 12, 0);
    uint V12bits = BitFieldExtractU32(Packed, 12, 12);
    UVs = float2(U12bits, V12bits) * (1.0f / 4095.0f);
    ProfileId = float(BitFieldExtractU32(Packed, 8, 24));
}

Dither 参数被声明但未写入 Packed 字段——仅局部使用于抖动偏移计算。

FSubstrateEvaluateToonResult SubstrateToonEvaluateCommon(
    float3 DiffuseColor, float3 SpecularColor,
    float  Roughness,     float  ToonProfileId,
    float2 ToonPatternUVs, float  Anisotropy,
    float3 Emissive,      BxDFContext Context,
    bool   bHasAnisotropy, bool   bIsRectLight,
    bool   bThisPassAppliesDiffuseRamp,
    half3  N, half3 V,   FAreaLight AreaLight,
    float  ShadowTermsSurfaceShadow,     // ShadowTerms.SurfaceShadow
    float  ShadowTermsTransmittance);    // TransmittanceOrOpticalThickness

Diffuse Ramp 采样

float3 Coord = float3(
    NoL,                                              // X: 漫反射角 [-1,1]
    (RAMP_Y + 0.5) * InvSize.w,                        // Y: 行中心
    ProfileId);                                       // Z: 切片索引
return ToonProfileTexture.SampleLevel(Sampler, Coord, 0).xyz;

Dither 图案采样

PatternUVs = frac(PatternUVs * OffsetSize);          // 平铺
BakeCoords = BakeOrigin + PatternUVs * DitherSize;     // 像素坐标
Coord = float3(BakeCoords.xy * InvSize.zw, ProfileId);
Dither = OffsetStrength * (Tex.SampleLevel(..., Coord).x * 2 - 1);

Dither 值在 [-Strength, +Strength] 范围内，加到 Ramp 采样位置以消除色带。

struct FToonProfileEvaluateSettings {
    float DiffuseRampOffsetStrength;   // Data.x
    float DiffuseRampOffsetSize;       // Data.y
    float SpecularRampOffsetStrength;  // Data.z
    float SpecularRampOffsetSize;      // Data.w
    float bDiffuseRampIncludeShadow;   // Data2.x
};

需要两次 SampleLevel：一次读取 xyzw（EVAL_PARAMS_X），一次读取 x（EVAL_PARAMS2_X）。

#if SUBSTRATE_FASTPATH
    #define UnpackSubstrateBSDF  UnpackFastPathSubstrateBSDFIn
#else
    #define UnpackSubstrateBSDF  UnpackSubstrateBSDFIn
#endif

// 循环展开：单 Closure 直接用 if，多 Closure 用静态展开 for
#if SUBSTRATE_CLAMPED_CLOSURE_COUNT == 1
    #define Substrate_for_unroll(X,Y,Z) X; if(Y)
#else
    #define Substrate_for_unroll(X,Y,Z) \
        SUBSTRATE_UNROLL_N(CLOSURE_COUNT) for(X;Y;Z)
#endif

静态展开避免真实 GPU 循环引起的寄存器膨胀。包含 Toon BSDF 的 SubstrateToonBSDF.ush 被 Substrate.ush 的 include 链直接引入。