# Objective
Related to #10572
Allow the `Annulus` primitive to be meshed.
## Solution
We introduce a `Meshable` structure, `AnnulusMeshBuilder`, which allows
the `Annulus` primitive to be meshed, leaving optional configuration of
the number of angular sudivisions to the user. Here is a picture of the
annulus's UV-mapping:
<img width="1440" alt="Screenshot 2024-03-26 at 10 39 48 AM"
src="https://github.com/bevyengine/bevy/assets/2975848/b170291d-cba7-441b-90ee-2ad6841eaedb">
Other features are essentially identical to the implementations for
`Circle`/`Ellipse`.
---
## Changelog
- Introduced `AnnulusMeshBuilder`
- Implemented `Meshable` for `Annulus` with `Output =
AnnulusMeshBuilder`
- Implemented `From<Annulus>` and `From<AnnulusMeshBuilder>` for `Mesh`
- Added `impl_reflect!` declaration for `Annulus` and `Triangle3d` in
`bevy_reflect`
---
## Discussion
### Design considerations
The only interesting wrinkle here is that the existing UV-mapping of
`Ellipse` (and hence of `Circle` and `RegularPolygon`) is non-radial
(it's skew-free, created by situating the mesh in a bounding rectangle),
so the UV-mapping of `Annulus` doesn't limit to that of `Circle` as its
inner radius tends to zero, for instance. I don't see this as a real
issue for `Annulus`, which should almost certainly have this kind of
UV-mapping, but I think we ought to at least consider allowing mesh
configuration for `Circle`/`Ellipse` that performs radial UV-mapping
instead. (In these cases in particular, it would be especially easy,
since we wouldn't need a different parameter set in the builder.)
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
c8aa3ac7d1