Calculation of incident_dir

[yyzzyy78]

Thanks for the great job!
When reading the calculation of the incident direction in render_equation.cu, I don't understand the following calculation:

https://github.com/NJU-3DV/Relightable3DGaussian/blob/77827626db514d59b390da2474de5d71be61a2e6/r3dg-rasterization/render_equation.cu#L100C1-L114C11

It seems that z_s = {x, y, z} is a sampled point on a unit sphere, an the rotation matrix is computed based on normal vector.
But I can't understand how to calculate the rotation matrix R, which R is used as:
z_s=matmul(R,z_s).

I'm looking forward to your reply!