How to blend World Space Normals

Question

I am trying to blend two world space normals inside a shader. One comes from a tangent space normal map converted into world space using a classic TBN matrix and the other one is a mesh normal map in world space.

I found some interesting resources here :

• Blending in Detail
• Shadertoy - Normal Map Blending

But those blending technics seem to be only available for tangent space normals, especially the Reoriented Normal Mapping (RNM). I tried to apply the RNM technic with unpack already done.

n1 += vec3(0, 0, 1);
n2 *= vec3(-1, -1, 1);

return n1 * dot(n1, n2) / n1.z - n2;

But this doesn't give expected results and I don't get why. Is there a way to apply the RNM blending on world space normals ?

Thanks a lot.

Edit: Here are some output and the result I am having with RNM. The A normal is not disturbed by the B normal, it gives some strange massively reoriented results.

float3 nv = dot(normal, viewDir);
color.rgb = nv;

Answer 1

Huge thanks to @MJP who answered this.
The aim is to avoid the simplification made when using tangent space normals. Here is the paper : Blending in detail

But only implement equation (4) which gives you this.

float3 ReorientNormal(in float3 u, in float3 t, in float3 s)
{
    // Build the shortest-arc quaternion
    float4 q = float4(cross(s, t), dot(s, t) + 1) / sqrt(2 * (dot(s, t) + 1));
    // Rotate the normal
    return u * (q.w * q.w - dot(q.xyz, q.xyz)) + 2 * q.xyz * dot(q.xyz, u) + 2 * q.w * cross(q.xyz, u);
}

Original answer is here.