Cycles implements the "Taming the Shadow Terminator" paper by Matt Jen-Yuan
Chiang to solve shadow terminator issues when a bump map is applied, as well
as similar approach for the glossy reflection to ensure ray does not get
reflected to inside of the object.
This correction term is applied unconditionally, which makes it harder to have
full control over shading via normals for stylistic reasons.
This change exposes this corrective term as an option called "Bump Map
Correction" which is available in the shader settings next to the
"Transparent Shadows".
The reason to make it per-shader rather than per-object is to allow flexibility
of a control: it is possible that an object has multiple shaders attached to it,
and only some of them used for bump mapping. Another, and possibly stronger
reason to have it per-shader is ease of assets control: shader brings settings
which are needed for its proper behavior. So if material at some point
decides to take over normals, artists would not need to update settings on
every asset which uses that material.
The option is enabled by default, so there is no changes for existing setups.
Pull Request: https://projects.blender.org/blender/blender/pulls/113480
36e603c430