Incorrect naming in UE4 skylight source code.

Recently, I am reading UE4 skylight code. and found that they call projected SH coefficients as "IrradianceEnvironmentMap".

Actually, this naming is wrong.
They make this Irradiance SH Function by producting cosine lobe in the end. But until then it is projected "Radiance SH" not "Irradiance SH".

Let's recall basic rendering process using spherical harmonics.

1. Project SH coefficients from radiance cube map.
2. Convert Radiance SH to Irradiance SH by dot product with cosine lobe
(Ravi Ramamoorthi's paper)
3. Evaluate Irradiance env map with normal vector

Let's see two different shader code for SH shading.

Below code is hlsl from MJP in gamedev.net
This is easy to understand.
There is radiance SH, cosine lobe.

static const float Pi = 3.141592654f; static const float CosineA0 = Pi; static const float CosineA1 = (2.0f * Pi) / 3.0f; static const float CosineA2 = Pi * 0.25f; struct SH9 { float c[9]; }; struct SH9Color { float3 c[9]; }; SH9 SHCosineLobe(in float3 dir) { SH9 sh; // Band 0 sh.c[0] = 0.282095f * CosineA0; // Band 1 sh.c[1] = 0.488603f * dir.y * CosineA1; sh.c[2] = 0.488603f * dir.z * CosineA1; sh.c[3] = 0.488603f * dir.x * CosineA1; // Band 2 sh.c[4] = 1.092548f * dir.x * dir.y * CosineA2; sh.c[5] = 1.092548f * dir.y * dir.z * CosineA2; sh.c[6] = 0.315392f * (3.0f * dir.z * dir.z - 1.0f) * CosineA2; sh.c[7] = 1.092548f * dir.x * dir.z * CosineA2; sh.c[8] = 0.546274f * (dir.x * dir.x - dir.y * dir.y) * CosineA2; return sh; } float3 ComputeSHIrradiance(in float3 normal, in SH9Color radiance) { // Compute the cosine lobe in SH, oriented about the normal direction SH9 shCosine = SHCosineLobe(normal); // Compute the SH dot product to get irradiance float3 irradiance = 0.0f; for(uint i = 0; i < 9; ++i) irradiance += radiance.c[i] * shCosine.c[i]; return irradiance; } float3 ComputeSHDiffuse(in float3 normal, in SH9Color radiance, in float3 diffuseAlbedo) { // Diffuse BRDF is albedo / Pi return ComputeSHIrradiance(normal, radiance) * diffuseAlbedo * (1.0f / Pi); }

And below is code from Peter-Pike Sloan
This code seems to be just a sh evaluation code.
The secret is coefficient for cosine lobe is already producted to radiance SH(and it becomes Irradiance finally!)
// constants containing irradiance environmne map float4 cAr; float4 cAg; float4 cAb; float4 cBr; float4 cBg; float4 cBb; float4 cC; float3 ShadeIrad(float4 vNormal) { float3 x1, x2, x3; // Linear + constant polynomial terms x1.r = dot(cAr,vNormal); x1.g = dot(cAg,vNormal); x1.b = dot(cAb,vNormal); // 4 of the quadratic polynomials float4 vB = vNormal.xyzz * vNormal.yzzx; x2.r = dot(cBr,vB); x2.g = dot(cBg,vB); x2.b = dot(cBb,vB); // Final quadratic polynomial float vC = vNormal.x*vNormal.x - vNormal.y*vNormal.y; x3 = cC.rgb * vC; return x1+x2+x3; }

Later code may be a lot faster(because use fewer shader instructions).

UE4 call value after 1 as IrradianceEnvironmentMap(it becomes irradiance in the end, but not at first).

Although final rendering is OK, But Incorrect naming confuse stupid reader like me.