Meshable extrusions

[lynn-lumen]

Objective

• Implement Meshable for Extrusion<T>

Solution

• Meshable requires Meshable::Output: MeshBuilder now. This means that all some_primitive.mesh() calls now return a MeshBuilder. These were added for primitives that did not have one prior to this.
• A new trait Extrudable: MeshBuilder has been added. This trait allows you to specify the indices of the perimeter of the mesh created by this MeshBuilder and whether they are to be shaded smooth or flat.
• Extrusion<P: Primitive2d + Meshable> is now Meshable aswell. The associated MeshBuilder is ExtrusionMeshBuilder which is generic over P and uses the MeshBuilder of its baseshape internally.
• ExtrusionMeshBuilder exposes the configuration functions of its base-shapes builder.
• Updated the 3d_shapes example to include Extrusions

Migration Guide

• Depending on the context, you may need to explicitly call .mesh().build() on primitives where you have previously called .mesh()
• The Output type of custom Meshable implementations must now derive MeshBuilder.

Additional information

• The extrusions UVs are done so that
  • the front face (+Z) is in the area between (0, 0) and (0.5, 0.5),
  • the back face (-Z) is in the area between (0.5, 0) and (1, 0.5)
  • the mantle is in the area between (0, 0.5) and (1, 1). Each PerimeterSegment you specified in the Extrudable implementation will be allocated an equal portion of this area.
• The UVs of the base shape are scaled to be in the front/back area so whatever method of filling the full UV-space the base shape used is how these areas will be filled.

Here is an example of what that looks like on a capsule:

Extrusion.UVs.mp4

This is the texture used:

The 3d_shapes example now looks like this:

[mweatherley]

I like this quite a lot! Mostly I just want more documentation (both user-facing and internal) about how the process is expected to work.

Specifically:

• I think the MeshBuilder constraint is reasonable, and adding it to everything should be fine (if anything, I think making these uniform is good).
• I think that encoding the perimeter of two-dimensional meshes in a trait on the builder seems like the right approach.
• I think that the way that flat and interpolated normals are computed seems quite reasonable.

The only things I'm kind of iffy about are the way that UVs are constructed (not a big deal anyhow) and the reimplementations of resolution on extruded builders.

[mweatherley]

Just a couple minor remaining things but I'm pretty happy with this now. :)

The migration guide should also probably mention the obligatory MeshBuilder bound on Meshable::Output, since Meshable is a public trait.

[alice-i-cecile]

I'm happy with this API, and this functionality seems like a reasonable way to use extrusions.

Like always, docs and code quality are excellent.

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1363 if you'd like to help out.