Bendy Bones scale badly when using cm scene scale
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Description

System Information
macOS Sierra 10.12.1, GeForce GTX 680MX 2048 MB

Blender Version
Broken: 2.78 (release)
Worked: n/a

Creating Bendy Bones when using centimetre scene scale causes very odd scaling.

Exact steps for others to reproduce the error
Open Blender, set scene units to centimetres

Create an armature (single bone), edit mode, subdivide twice.

Object Mode, got to Armature Tab and change to B-Bone.

The bones will get very long and thin.

If you do this without changing the units/scene scale, the b-bone is correct.

Details

Type
Bug

Related Objects

There is a related problem with cloth physics (maybe with other physics too). Basically the scene scale has no impact on cloth physics at all, meaning the dimensions of the scene remain interpreted as if measured in the default units. The same seems to be happening here.

Can be easily tested by setting up a cloth sim and playing around with the scene scale. Observe it having no impact at all (unless entering big numbers -> probably precision issues). This means when keeping the object size constant with respect to changing scene scale the physics behave differently.

Change Scene Scale to 0.1 -> Scale the models up by 10 -> Cloth sim behaves as if the cloth were gigantic

The same happens in the case of the bendy bones. Observe that changing the scene scale in the provided file has no impact on the bone whatsoever. Since the units are interpreted as meters regardless, a 100cm long bone is displayed like a 100m long bone.

Doing this, I noticed that you have to zoom out the view at lot more to see the entire bone (when it is created using cm units). It looks like the bone isn't especially thin; it's more that the thickness doesn't automatically scale to match a 100 BU (i.e. 100 BU = 100 "cm", assuming 1 BU = 1 cm) high bone. In other words, BBone sizes are in absolute units (which are not adjusted to deal with the unit scaling stuff).

EDIT:
I guess the main question we need to address here is:

Does the BBone size here have any actual effect on quality of deformations, or is this purely a cosmetic thing?