Improve Mesh documentation (#9061)

# Objective

This PR continues #8885

It aims to improve the `Mesh` documentation in the following ways:
- Put everything at the "top level" instead of the "impl".
- Explain better what is a Mesh, how it can be created, and that it can
be edited.
- Explain it can be used with a `Material`, and mention
`StandardMaterial`, `PbrBundle`, `ColorMaterial`, and
`ColorMesh2dBundle` since those cover most cases
- Mention the glTF/Bevy vocabulary discrepancy for "Mesh"
- Add an image for the example
- Various nitpicky modifications

## Note

- The image I added is 90.3ko which I think is small enough?
- Since rustdoc doesn't allow cross-reference not in dependencies of a
subcrate [yet](rust-lang/rust#74481), I have a
lot of backtick references that are not links :(
- Since rustdoc doesn't allow linking to code in the crate (?) I put
link to github directly.
- Since rustdoc doesn't allow embed images in doc
[yet](rust-lang/rust#32104), maybe
[soon](rust-lang/rfcs#3397), I had to put only a
link to the image. I don't think it's worth adding
[embed_doc_image](https://docs.rs/embed-doc-image/latest/embed_doc_image/)
as a dependency for this.

crates/bevy_asset/src/loader.rs

@@ -14,8 +14,8 @@ use std::path::Path;
 
 /// A loader for an asset source.
 ///
-/// Types implementing this trait are used by the asset server to load assets into their respective
-/// asset storages.
+/// Types implementing this trait are used by the [`AssetServer`] to load assets
+/// into their respective asset storages.
 pub trait AssetLoader: Send + Sync + 'static {
     /// Processes the asset in an asynchronous closure.
     fn load<'a>(

crates/bevy_gltf/src/lib.rs

@@ -60,7 +60,8 @@ impl Plugin for GltfPlugin {
     }
 }
 
-/// Representation of a loaded glTF file.
+/// Representation of a loaded glTF file
+/// (file loaded via the `AssetServer` with the extension `.glb` or `.gltf`).
 #[derive(Debug, TypeUuid, TypePath)]
 #[uuid = "5c7d5f8a-f7b0-4e45-a09e-406c0372fea2"]
 pub struct Gltf {

crates/bevy_render/src/mesh/mesh/mod.rs

@@ -26,28 +26,24 @@ use wgpu::{
 pub const INDEX_BUFFER_ASSET_INDEX: u64 = 0;
 pub const VERTEX_ATTRIBUTE_BUFFER_ID: u64 = 10;
 
-// TODO: allow values to be unloaded after been submitting to the GPU to conserve memory
-#[derive(Debug, TypeUuid, TypePath, Clone)]
-#[uuid = "8ecbac0f-f545-4473-ad43-e1f4243af51e"]
-pub struct Mesh {
-    primitive_topology: PrimitiveTopology,
-    /// `std::collections::BTreeMap` with all defined vertex attributes (Positions, Normals, ...)
-    /// for this mesh. Attribute ids to attribute values.
-    /// Uses a BTreeMap because, unlike HashMap, it has a defined iteration order,
-    /// which allows easy stable VertexBuffers (i.e. same buffer order)
-    attributes: BTreeMap<MeshVertexAttributeId, MeshAttributeData>,
-    indices: Option<Indices>,
-    morph_targets: Option<Handle<Image>>,
-    morph_target_names: Option<Vec<String>>,
-}
-
-/// Contains geometry in the form of a mesh.
+/// A 3D object made out of vertices representing triangles, lines, or points,
+/// with "attribute" values for each vertex.
+///
+/// Meshes can be automatically generated by a bevy `AssetLoader` (generally by loading a `Gltf` file),
+/// or by converting a primitive [`shape`](crate::mesh::shape) using [`into`](std::convert::Into).
+/// It is also possible to create one manually.
+/// They can be edited after creation.
 ///
-/// Often meshes are automatically generated by bevy's asset loaders or primitives, such as
-/// [`shape::Cube`](crate::mesh::shape::Cube) or [`shape::Box`](crate::mesh::shape::Box), but you can also construct
-/// one yourself.
+/// Meshes can be rendered with a `Material`, like `StandardMaterial` in `PbrBundle`
+/// or `ColorMaterial` in `ColorMesh2dBundle`.
 ///
-/// Example of constructing a mesh (to be rendered with a `StandardMaterial`):
+/// A [`Mesh`] in Bevy is equivalent to a "primitive" in the glTF format, for a
+/// glTF Mesh representation, see `GltfMesh`.
+///
+/// ## Manual creation
+///
+/// The following function will construct a flat mesh, to be rendered with a
+/// `StandardMaterial` or `ColorMaterial`:
 /// ```
 /// # use bevy_render::mesh::{Mesh, Indices};
 /// # use bevy_render::render_resource::PrimitiveTopology;
@@ -78,50 +74,90 @@ pub struct Mesh {
 ///         1, 3, 2
 ///     ])));
 ///     mesh
-///     // For further visualization, explanation, and examples see the built-in Bevy examples
-///     // and the implementation of the built-in shapes.
 /// }
 /// ```
-/// Common points of confusion:
-/// - UV maps in Bevy are "flipped", (0.0, 0.0) = Top-Left (not Bot-Left like `OpenGL`)
-/// - It is normal for multiple vertices to have the same position
-///   attribute - it's a common technique in 3D modelling for complex UV mapping or other calculations.
 ///
-/// To render correctly with `StandardMaterial` a mesh needs to have properly defined:
-/// - [`UVs`](Mesh::ATTRIBUTE_UV_0): Bevy needs to know how to map a texture onto the mesh.
-/// - [`Normals`](Mesh::ATTRIBUTE_NORMAL): Bevy needs to know how light interacts with your mesh. ([0.0, 0.0, 1.0] is very
-///              common for simple meshes because simple meshes are smooth, and they don't require complex light calculations.)
-/// - Vertex winding order -
-///              the default behavior is with `StandardMaterial.cull_mode` = Some([`Face::Front`](crate::render_resource::Face::Front)) which means
-///              that by default Bevy would *only* render the front of each triangle, and the front
-///              is the side of the triangle in which the vertices appear in a *counter-clockwise* order.
+/// You can see how it looks like [here](https://github.com/bevyengine/bevy/blob/main/assets/dovs/Mesh.png),
+/// used in a `PbrBundle` with a square bevy logo texture, with added axis, points,
+/// lines and text for clarity.
+///
+/// ## Other examples
+///
+/// For further visualization, explanation, and examples, see the built-in Bevy examples,
+/// and the [implementation of the built-in shapes](https://github.com/bevyengine/bevy/tree/main/crates/bevy_render/src/mesh/shape).
+/// In particular, [generate_custom_mesh](https://github.com/bevyengine/bevy/blob/main/examples/3d/generate_custom_mesh.rs)
+/// teaches you to access modify a Mesh's attributes after creating it.
+///
+/// ## Common points of confusion
 ///
+/// - UV maps in Bevy start at the top-left, see [`ATTRIBUTE_UV_0`](Mesh::ATTRIBUTE_UV_0),
+/// other APIs can have other conventions, `OpenGL` starts at bottom-left.
+/// - It is possible and sometimes useful for multiple vertices to have the same
+/// [position attribute](Mesh::ATTRIBUTE_POSITION) value,
+/// it's a common technique in 3D modelling for complex UV mapping or other calculations.
+///
+/// ## Use with `StandardMaterial`
+///
+/// To render correctly with `StandardMaterial`, a mesh needs to have properly defined:
+/// - [`UVs`](Mesh::ATTRIBUTE_UV_0): Bevy needs to know how to map a texture onto the mesh
+/// (also true for `ColorMaterial`).
+/// - [`Normals`](Mesh::ATTRIBUTE_NORMAL): Bevy needs to know how light interacts with your mesh.
+/// [0.0, 0.0, 1.0] is very common for simple flat meshes on the XY plane,
+/// because simple meshes are smooth and they don't require complex light calculations.
+/// - Vertex winding order: by default, `StandardMaterial.cull_mode` is [`Some(Face::Back)`](crate::render_resource::Face),
+/// which means that Bevy would *only* render the "front" of each triangle, which
+/// is the side of the triangle from where the vertices appear in a *counter-clockwise* order.
+///
+// TODO: allow values to be unloaded after been submitting to the GPU to conserve memory
+#[derive(Debug, TypeUuid, TypePath, Clone)]
+#[uuid = "8ecbac0f-f545-4473-ad43-e1f4243af51e"]
+pub struct Mesh {
+    primitive_topology: PrimitiveTopology,
+    /// `std::collections::BTreeMap` with all defined vertex attributes (Positions, Normals, ...)
+    /// for this mesh. Attribute ids to attribute values.
+    /// Uses a BTreeMap because, unlike HashMap, it has a defined iteration order,
+    /// which allows easy stable VertexBuffers (i.e. same buffer order)
+    attributes: BTreeMap<MeshVertexAttributeId, MeshAttributeData>,
+    indices: Option<Indices>,
+    morph_targets: Option<Handle<Image>>,
+    morph_target_names: Option<Vec<String>>,
+}
+
 impl Mesh {
-    /// Where the vertex is located in space. Use in conjunction with [`Mesh::insert_attribute`]
+    /// Where the vertex is located in space. Use in conjunction with [`Mesh::insert_attribute`].
     pub const ATTRIBUTE_POSITION: MeshVertexAttribute =
         MeshVertexAttribute::new("Vertex_Position", 0, VertexFormat::Float32x3);
 
     /// The direction the vertex normal is facing in.
-    /// Use in conjunction with [`Mesh::insert_attribute`]
+    /// Use in conjunction with [`Mesh::insert_attribute`].
     pub const ATTRIBUTE_NORMAL: MeshVertexAttribute =
         MeshVertexAttribute::new("Vertex_Normal", 1, VertexFormat::Float32x3);
 
-    /// Texture coordinates for the vertex. Use in conjunction with [`Mesh::insert_attribute`]
+    /// Texture coordinates for the vertex. Use in conjunction with [`Mesh::insert_attribute`].
+    ///
+    /// Values are generally between 0. and 1., with `StandardMaterial` and `ColorMaterial`
+    /// `[0.,0.]` is the top left of the texture, and [1.,1.] the bottom-right.
+    /// You usually want to only use values in that range, values outside will be
+    /// clamped per pixel not for the vertex, "stretching" the borders of the texture.
+    /// This behavior can be useful in some cases, usually when the borders have only
+    /// one color, for example a logo, and you want to "extend" those borders.
     pub const ATTRIBUTE_UV_0: MeshVertexAttribute =
         MeshVertexAttribute::new("Vertex_Uv", 2, VertexFormat::Float32x2);
 
-    /// The direction of the vertex tangent. Used for normal mapping
+    /// The direction of the vertex tangent. Used for normal mapping.
+    /// Usually generated with [`generate_tangents`](Mesh::generate_tangents).
     pub const ATTRIBUTE_TANGENT: MeshVertexAttribute =
         MeshVertexAttribute::new("Vertex_Tangent", 3, VertexFormat::Float32x4);
 
-    /// Per vertex coloring. Use in conjunction with [`Mesh::insert_attribute`]
+    /// Per vertex coloring. Use in conjunction with [`Mesh::insert_attribute`].
     pub const ATTRIBUTE_COLOR: MeshVertexAttribute =
         MeshVertexAttribute::new("Vertex_Color", 4, VertexFormat::Float32x4);
 
-    /// Per vertex joint transform matrix weight. Use in conjunction with [`Mesh::insert_attribute`]
+    /// Per vertex joint transform matrix weight. Use in conjunction with [`Mesh::insert_attribute`].
     pub const ATTRIBUTE_JOINT_WEIGHT: MeshVertexAttribute =
         MeshVertexAttribute::new("Vertex_JointWeight", 5, VertexFormat::Float32x4);
-    /// Per vertex joint transform matrix index. Use in conjunction with [`Mesh::insert_attribute`]
+
+    /// Per vertex joint transform matrix index. Use in conjunction with [`Mesh::insert_attribute`].
     pub const ATTRIBUTE_JOINT_INDEX: MeshVertexAttribute =
         MeshVertexAttribute::new("Vertex_JointIndex", 6, VertexFormat::Uint16x4);