Add a "dumb vector" storage option for custom normals, with the
"custom_normal" attribute. Adjust the mesh normals caching to
provide this attribute if it's available, and add a geometry node to
store custom normals.
## Free Normals
They're called "free" in the sense that they're just direction vectors
in the object's local space, rather than the existing "smooth corner
fan space" storage. They're also "free" in that they make further
normals calculation very inexpensive, since we just use the custom
normals instead. That's a big improvement from the existing custom
normals storage, which usually significantly decreases
viewport performance. For example, in a simple test file just storing
the vertex normals on a UV sphere, using free normals gives 25 times
better playback performance and 10% lower memory usage.
Free normals are adjusted when applying a transformation to the entire
mesh or when realizing instances, but in general they're not updated for
vertex deformations.
## Set Mesh Normal Node
The new geometry node allows storing free custom normals as well as
the existing corner fan space normals. When free normals are chosen,
free normals can be stored on vertices, faces, or face corners. Using
the face corner domain is necessary to bake existing mixed sharp and
smooth edges into the custom normal vectors.
The node also has a mode for storing edge and mesh sharpness, meant
as a "soft" replacement to the "Set Shade Smooth" node that's a bit
more convenient.
## Normal Input Node
The normal node outputs free custom normals mixed to whatever domain is
requested. A "true normal" output that ignores custom normals and
sharpness is added as well.
Across Blender, custom normals are generally accessed via face and
vertex normals, when "true normals" are not requested explicitly.
In many cases that means they are mixed from the face corner domain.
## Future Work
1. There are many places where propagation of free normals could be
improved. They should probably be normalized after mixing, and it
may be useful to not just use 0 vectors for new elements. To keep
the scope of this change smaller, that sort of thing generally isn't
handled here. Searching `CD_NORMAL` gives a hint of where better
propagation could be useful.
2. Free normals are displayed properly in edit mode, but the existing
custom normal editing operators don't work with free normals yet.
This will hopefully be fairly straightforward since custom normals
are usually converted to `float3` for editing anyway. Edit mode
changes aren't included here because they're unnecessary for the
procedural custom normals use cases.
3. Most importers can probably switch to using free normals instead,
or at least provide an option for it. That will give a significant
import performance improvement, and an improvement of Blender's
FPS for imported scenes too.
Pull Request: https://projects.blender.org/blender/blender/pulls/132583
d3f84449ad