Merge pull request #2101 from hannobraun/vertex

Improve documentation of `Vertex`

crates/fj-core/src/objects/kinds/vertex.rs

@@ -1,21 +1,71 @@
-/// A vertex, defined in global (3D) coordinates
+/// # A vertex that identifies a point in space
 ///
-/// This struct exists to distinguish between vertices and points at the type
-/// level. This is a relevant distinction, as vertices are part of a shape that
-/// help define its topology.
+/// ## Purpose
 ///
-/// Points, on the other hand, might be used to approximate a shape for various
-/// purposes, without presenting any deeper truth about the shape's structure.
+/// Vertices are referenced by [`HalfEdge`]s. They are topological objects,
+/// which means that their purpose is to define how parts of a shape relate to
+/// each other. They *identify* a point in space, but they do not define its
+/// position.
 ///
-/// # Validation
+/// In fact, this struct does not contain any data at all which could define
+/// anything. As such, [`Vertex`] is solely intended to be used through a
+/// [`Handle`], which provides the vertex with a unique identity, allowing it
+/// to do its job.
 ///
-/// Vertices must be unique within a shape, meaning an identical vertex must not
-/// exist in the same shape. In the context of vertex uniqueness, points that
-/// are close to each other are considered identical. The minimum distance
-/// between distinct vertices can be configured using the respective field in
-/// [`ValidationConfig`].
+/// Having a unique identity for a point in space is very valuable, as we can't
+/// just rely on the value of points to compare them. If two half-edges connect,
+/// we expect them to connect at a single point, not two points that are very
+/// close to each other.
 ///
-/// # Equality
+/// Due to the realities of computing (and specifically, the realities of
+/// computing floating-point numbers), we might very well end up *thinking* that
+/// our code computed a single point, when in fact it does not, which only shows
+/// up outside of our testing environment. This can be a pervasive source of
+/// bugs, if left unchecked.
+///
+/// With [`Vertex`], we can provide a unique identity to each point where it is
+/// computed. This allows [validation code](crate::validate) to exist, which can
+/// identify where our code generates multiple distinct points that might end up
+/// in slightly different positions in a real-life scenario.
+///
+///
+/// ## Positions
+///
+/// (Warning: If the following information is relevant to you, please double-
+/// check it by looking at the code that this section references. Everything
+/// here is true at the time of writing, but there are planned changes which
+/// could make this section obsolete.)
+///
+/// A vertex can exist in multiple distinct spaces, and can thus have multiple
+/// positions. Those positions are defined by the objects that reference the
+/// vertex. How exactly that happens depends on the overall shape.
+///
+/// If the shape is defined by a [`Sketch`], it is 2D-only. The (2D) position of
+/// the vertex will then be defined by the half-edges that reference it.
+/// Validation code can make sure that those redundant definitions don't result
+/// in wildly different values.
+///
+/// If the shape is defined by a [`Solid`], then it it 3-dimensional. The
+/// referencing half-edges still define the surface-local 2D positions of the
+/// vertex. Since such half-edges could meet in the same surface, or exist where
+/// multiple surfaces meet, these positions could end up being in one or more
+/// surfaces.
+///
+/// The corresponding [`Surface`] objects can then be used to convert those 2D
+/// positions into global 3D positions.
+///
+/// As you might have noted, in each case, we still have redundant definitions
+/// of the vertex position, and might end up with multiple values for the
+/// position that are not exactly equal. However, since we know these positions
+/// belong to the same vertex, this is not a problem.
+///
+/// Validation code can make sure that the actual values are very close
+/// together, and where we lose this identity information(when generating a
+/// triangle mesh for a file export, for example), we can choose exactly one of
+/// those values.
+///
+///
+/// ## Equality
 ///
 /// `Vertex` contains no data and exists purely to be used within a `Handle`,
 /// where `Handle::id` can be used to compare different instances of `Vertex`.
@@ -28,12 +78,16 @@
 /// `Eq`/`Ord`/..., you can use `HandleWrapper<Vertex>` to do that. It will
 /// use `Handle::id` to provide those `Eq`/`Ord`/... implementations.
 ///
-/// [`ValidationConfig`]: crate::validate::ValidationConfig
+/// [`HalfEdge`]: crate::objects::HalfEdge
+/// [`Handle`]: crate::storage::Handle
+/// [`Sketch`]: crate::objects::Sketch
+/// [`Solid`]: crate::objects::Solid
+/// [`Surface`]: crate::objects::Surface
 #[derive(Clone, Copy, Debug, Default, Hash)]
 pub struct Vertex {}
 
 impl Vertex {
-    /// Construct a `Vertex` from a position
+    /// Construct a `Vertex`
     pub fn new() -> Self {
         Self::default()
     }