Unity Standard Shader源码阅读(四) 简化版BasePass

标准实现参考Unity Standard Shader源码阅读(一) BasePass

着色器定义在UnityStandardCoreForward.cginc中：

#include "UnityStandardConfig.cginc"

#if UNITY_STANDARD_SIMPLE
    //简化版实现
    #include "UnityStandardCoreForwardSimple.cginc"
    VertexOutputBaseSimple vertBase (VertexInput v) { return vertForwardBaseSimple(v); }
    VertexOutputForwardAddSimple vertAdd (VertexInput v) { return vertForwardAddSimple(v); }
    half4 fragBase (VertexOutputBaseSimple i) : SV_Target { return fragForwardBaseSimpleInternal(i); }
    half4 fragAdd (VertexOutputForwardAddSimple i) : SV_Target { return fragForwardAddSimpleInternal(i); }
#else
    //标准实现
    #include "UnityStandardCore.cginc"
    VertexOutputForwardBase vertBase (VertexInput v) { return vertForwardBase(v); }
    VertexOutputForwardAdd vertAdd (VertexInput v) { return vertForwardAdd(v); }
    half4 fragBase (VertexOutputForwardBase i) : SV_Target { return fragForwardBaseInternal(i); }
    half4 fragAdd (VertexOutputForwardAdd i) : SV_Target { return fragForwardAddInternal(i); }
#endif

先看顶点着色器：

VertexOutputBaseSimple vertForwardBaseSimple (VertexInput v)
{
    UNITY_SETUP_INSTANCE_ID(v);
    VertexOutputBaseSimple o;
    UNITY_INITIALIZE_OUTPUT(VertexOutputBaseSimple, o);
    UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);

    float4 posWorld = mul(unity_ObjectToWorld, v.vertex);
    o.pos = UnityObjectToClipPos(v.vertex);
    o.tex = TexCoords(v);

    //世界空间眼睛观察方向，指向观察的顶点
    half3 eyeVec = normalize(posWorld.xyz - _WorldSpaceCameraPos);
    half3 normalWorld = UnityObjectToWorldNormal(v.normal);

    o.normalWorld.xyz = normalWorld;
    o.eyeVec.xyz = eyeVec;

    //在切线空间中计算光照
    #ifdef _NORMALMAP
        half3 tangentSpaceEyeVec;
        TangentSpaceLightingInput(normalWorld, v.tangent, _WorldSpaceLightPos0.xyz, eyeVec, o.tangentSpaceLightDir, tangentSpaceEyeVec);
        #if SPECULAR_HIGHLIGHTS
            o.tangentSpaceEyeVec = tangentSpaceEyeVec;
        #endif
    #endif

    //We need this for shadow receiving
    TRANSFER_SHADOW(o);

    //计算顶点球谐光照或用于存储光照贴图uv
    o.ambientOrLightmapUV = VertexGIForward(v, posWorld, normalWorld);

    //视线在顶点反射方向
    o.fogCoord.yzw = reflect(eyeVec, normalWorld);

    //菲涅尔系数
    o.normalWorld.w = Pow4(1 - saturate(dot(normalWorld, -eyeVec))); // fresnel term
    #if !GLOSSMAP
        //射掠角观察顶点时的反射系数
        o.eyeVec.w = saturate(_Glossiness + UNIFORM_REFLECTIVITY()); // grazing term
    #endif

    UNITY_TRANSFER_FOG(o, o.pos);
    return o;
}

该实现在切线空间计算光照：

//获取切线空间下光源方向，观察方向
void TangentSpaceLightingInput(half3 normalWorld, half4 vTangent, half3 lightDirWorld, half3 eyeVecWorld, out half3 tangentSpaceLightDir, out half3 tangentSpaceEyeVec)
{
    half3 tangentWorld = UnityObjectToWorldDir(vTangent.xyz);
    half sign = half(vTangent.w) * half(unity_WorldTransformParams.w);
    half3 binormalWorld = cross(normalWorld, tangentWorld) * sign;
    tangentSpaceLightDir = TransformToTangentSpace(tangentWorld, binormalWorld, normalWorld, lightDirWorld);
    #if SPECULAR_HIGHLIGHTS
        tangentSpaceEyeVec = normalize(TransformToTangentSpace(tangentWorld, binormalWorld, normalWorld, eyeVecWorld));
    #else
        tangentSpaceEyeVec = 0;
    #endif
}

//从世界空间变换到切线空间
half3 TransformToTangentSpace(half3 tangent, half3 binormal, half3 normal, half3 v)
{
    // Mali400 shader compiler prefers explicit dot product over using a half3x3 matrix
    //传入切线空间各坐标轴在世界空间中的表示，对其按行排列构成变换矩阵
    return half3(dot(tangent, v), dot(binormal, v), dot(normal, v));
}

再看片元着色器：

half4 fragForwardBaseSimpleInternal (VertexOutputBaseSimple i)
{
    UNITY_APPLY_DITHER_CROSSFADE(i.pos.xy);

    FragmentCommonData s = FragmentSetupSimple(i);

    UnityLight mainLight = MainLightSimple(i, s);

    //法线方向上的光
    #if !defined(LIGHTMAP_ON) && defined(_NORMALMAP)
    half ndotl = saturate(dot(s.tangentSpaceNormal, i.tangentSpaceLightDir));
    #else
    half ndotl = saturate(dot(s.normalWorld, mainLight.dir));
    #endif

    //we can't have worldpos here (not enough interpolator on SM 2.0) so no shadow fade in that case.
    half shadowMaskAttenuation = UnitySampleBakedOcclusion(i.ambientOrLightmapUV, 0);
    half realtimeShadowAttenuation = SHADOW_ATTENUATION(i);
    half atten = UnityMixRealtimeAndBakedShadows(realtimeShadowAttenuation, shadowMaskAttenuation, 0);

    half occlusion = Occlusion(i.tex.xy);
    //视角与光线直接的夹角，用于高光反射计算
    half rl = dot(REFLECTVEC_FOR_SPECULAR(i, s), LightDirForSpecular(i, mainLight));

    //计算直接光源和间接光源
    UnityGI gi = FragmentGI (s, occlusion, i.ambientOrLightmapUV, atten, mainLight);
    half3 attenuatedLightColor = gi.light.color * ndotl;

    half3 c = BRDF3_Indirect(s.diffColor, s.specColor, gi.indirect, PerVertexGrazingTerm(i, s), PerVertexFresnelTerm(i));
    c += BRDF3DirectSimple(s.diffColor, s.specColor, s.smoothness, rl) * attenuatedLightColor;
    c += Emission(i.tex.xy);

    UNITY_APPLY_FOG(i.fogCoord, c);

    return OutputForward (half4(c, 1), s.alpha);
}

展开FragmentSetupSimple：

FragmentCommonData FragmentSetupSimple(VertexOutputBaseSimple i)
{
    half alpha = Alpha(i.tex.xy);
    #if defined(_ALPHATEST_ON)
        clip (alpha - _Cutoff);
    #endif

    FragmentCommonData s = UNITY_SETUP_BRDF_INPUT (i.tex);

    // NOTE: shader relies on pre-multiply alpha-blend (_SrcBlend = One, _DstBlend = OneMinusSrcAlpha)
    s.diffColor = PreMultiplyAlpha (s.diffColor, alpha, s.oneMinusReflectivity, /*out*/ s.alpha);

    s.normalWorld = i.normalWorld.xyz;
    s.eyeVec = i.eyeVec.xyz;
    s.posWorld = IN_WORLDPOS(i);
    s.reflUVW = i.fogCoord.yzw;

    #ifdef _NORMALMAP
        //切线空间中法线
        s.tangentSpaceNormal =  NormalInTangentSpace(i.tex);
    #else
        s.tangentSpaceNormal =  0;
    #endif

    return s;
}

对于UNITY_SETUP_BRDF_INPUT，设置BRDF输入，包含漫反射颜色diffColor，高光反射颜色specColor，漫反射部分系数oneMinusReflectivity，光滑度smoothness，按工作流分为MetallicSetup，SpecularSetup，RoughnessSetup；

Standard shader使用金属工作流 MetallicSetup：

inline FragmentCommonData MetallicSetup (float4 i_tex)
{
    half2 metallicGloss = MetallicGloss(i_tex.xy);
    half metallic = metallicGloss.x;
    half smoothness = metallicGloss.y; // this is 1 minus the square root of real roughness m.

    half oneMinusReflectivity;
    half3 specColor;
    half3 diffColor = DiffuseAndSpecularFromMetallic (Albedo(i_tex), metallic, /*out*/ specColor, /*out*/ oneMinusReflectivity);

    FragmentCommonData o = (FragmentCommonData)0;
    o.diffColor = diffColor;
    o.specColor = specColor;
    o.oneMinusReflectivity = oneMinusReflectivity;
    o.smoothness = smoothness;
    return o;
}

inline half3 DiffuseAndSpecularFromMetallic (half3 albedo, half metallic, out half3 specColor, out half oneMinusReflectivity)
{
    specColor = lerp (unity_ColorSpaceDielectricSpec.rgb, albedo, metallic);
    oneMinusReflectivity = OneMinusReflectivityFromMetallic(metallic);
    return albedo * oneMinusReflectivity;
}

inline half OneMinusReflectivityFromMetallic(half metallic)
{
    // We'll need oneMinusReflectivity, so
    //   1-reflectivity = 1-lerp(dielectricSpec, 1, metallic) = lerp(1-dielectricSpec, 0, metallic)
    // store (1-dielectricSpec) in unity_ColorSpaceDielectricSpec.a, then
    //   1-reflectivity = lerp(alpha, 0, metallic) = alpha + metallic*(0 - alpha) =
    //                  = alpha - metallic * alpha
    half oneMinusDielectricSpec = unity_ColorSpaceDielectricSpec.a;
    //金属度越高，漫反射部分越弱
    return oneMinusDielectricSpec - metallic * oneMinusDielectricSpec;
}

对于高光工作流：

inline FragmentCommonData SpecularSetup (float4 i_tex)
{
    half4 specGloss = SpecularGloss(i_tex.xy);
    half3 specColor = specGloss.rgb;
    half smoothness = specGloss.a;

    half oneMinusReflectivity;
    half3 diffColor = EnergyConservationBetweenDiffuseAndSpecular (Albedo(i_tex), specColor, /*out*/ oneMinusReflectivity);

    FragmentCommonData o = (FragmentCommonData)0;
    o.diffColor = diffColor;
    o.specColor = specColor;
    o.oneMinusReflectivity = oneMinusReflectivity;
    o.smoothness = smoothness;
    return o;
}

// Diffuse/Spec Energy conservation
inline half3 EnergyConservationBetweenDiffuseAndSpecular (half3 albedo, half3 specColor, out half oneMinusReflectivity)
{
    oneMinusReflectivity = 1 - SpecularStrength(specColor);
    #if !UNITY_CONSERVE_ENERGY
        return albedo;
    #elif UNITY_CONSERVE_ENERGY_MONOCHROME
        return albedo * oneMinusReflectivity;
    #else
        return albedo * (half3(1,1,1) - specColor);
    #endif
}

half SpecularStrength(half3 specular)
{
    #if (SHADER_TARGET < 30)
        // SM2.0: instruction count limitation
        // SM2.0: simplified SpecularStrength
        return specular.r; // Red channel - because most metals are either monocrhome or with redish/yellowish tint
    #else
        return max (max (specular.r, specular.g), specular.b);
    #endif
}

对于PreMultiplyAlpha，diffColor 预乘alpha值，以及考虑高光反射会移除部分透明：

inline half3 PreMultiplyAlpha (half3 diffColor, half alpha, half oneMinusReflectivity, out half outModifiedAlpha)
{
    #if defined(_ALPHAPREMULTIPLY_ON)
        // NOTE: shader relies on pre-multiply alpha-blend (_SrcBlend = One, _DstBlend = OneMinusSrcAlpha)

        // Transparency 'removes' from Diffuse component
        diffColor *= alpha;

        #if (SHADER_TARGET < 30)
            // SM2.0: instruction count limitation
            // Instead will sacrifice part of physically based transparency where amount Reflectivity is affecting Transparency
            // SM2.0: uses unmodified alpha
            outModifiedAlpha = alpha;
        #else
            // Reflectivity 'removes' from the rest of components, including Transparency
            //显示部分 = 1 - 透明部分 * （1 - 高光反射）
            // outAlpha = 1-(1-alpha)*(1-reflectivity) = 1-(oneMinusReflectivity - alpha*oneMinusReflectivity) =
            //          = 1-oneMinusReflectivity + alpha*oneMinusReflectivity
            //最终显示 = 高光占比 + alpha * 漫反射占比
            outModifiedAlpha = 1-oneMinusReflectivity + alpha*oneMinusReflectivity;
        #endif
    #else
        outModifiedAlpha = alpha;
    #endif
    return diffColor;
}

最后看BRDF实现：

扫描二维码关注公众号，回复： 17416012 查看本文章

half3 BRDF3_Indirect(half3 diffColor, half3 specColor, UnityIndirect indirect, half grazingTerm, half fresnelTerm)
{
    half3 c = indirect.diffuse * diffColor;
    c += indirect.specular * lerp (specColor, grazingTerm, fresnelTerm);
    return c;
}

half3 BRDF3DirectSimple(half3 diffColor, half3 specColor, half smoothness, half rl)
{
    #if SPECULAR_HIGHLIGHTS
        return BRDF3_Direct(diffColor, specColor, Pow4(rl), smoothness);
    #else
        return diffColor;
    #endif
}

sampler2D_float unity_NHxRoughness;
half3 BRDF3_Direct(half3 diffColor, half3 specColor, half rlPow4, half smoothness)
{
    half LUT_RANGE = 16.0; // must match range in NHxRoughness() function in GeneratedTextures.cpp
    // Lookup texture to save instructions
    half specular = tex2D(unity_NHxRoughness, half2(rlPow4, SmoothnessToPerceptualRoughness(smoothness))).r * LUT_RANGE;
#if defined(_SPECULARHIGHLIGHTS_OFF)
    specular = 0.0;
#endif

    return diffColor + specular * specColor;
}