Clarify GlobalTransform::transform_point (#14292)

The existing doc comment for GlobalTransform::transform_point is
unclear, or, arguably, incorrect.
https://github.com/bevyengine/bevy/discussions/8501 also mentions this.

Additionally, a user reading the doc for transform_point might be
looking for one of the three other transforms that I mentioned in this
doc comment.

---------

Co-authored-by: Mason Kramer <mason@masonkramer.net>
Co-authored-by: Pascal Hertleif <killercup@gmail.com>

crates/bevy_transform/src/components/global_transform.rs

@@ -7,23 +7,22 @@ use bevy_math::{Affine3A, Dir3, Mat4, Quat, Vec3, Vec3A};
 #[cfg(feature = "bevy-support")]
 use bevy_reflect::{std_traits::ReflectDefault, Reflect};
 
-/// Describe the position of an entity relative to the reference frame.
+/// [`GlobalTransform`] is an affine transformation from entity-local coordinates to worldspace coordinates.
+///
+/// You cannot directly mutate [`GlobalTransform`]; instead, you change an entity's transform by manipulating
+/// its [`Transform`], which indirectly causes Bevy to update its [`GlobalTransform`].
 ///
-/// * To place or move an entity, you should set its [`Transform`].
-/// * [`GlobalTransform`] is fully managed by bevy, you cannot mutate it, use
-///   [`Transform`] instead.
 /// * To get the global transform of an entity, you should get its [`GlobalTransform`].
 /// * For transform hierarchies to work correctly, you must have both a [`Transform`] and a [`GlobalTransform`].
 ///   * You may use the [`TransformBundle`](crate::bundles::TransformBundle) to guarantee this.
 ///
 /// ## [`Transform`] and [`GlobalTransform`]
 ///
-/// [`Transform`] is the position of an entity relative to its parent position, or the reference
-/// frame if it doesn't have a [`Parent`](bevy_hierarchy::Parent).
+/// [`Transform`] transforms an entity relative to its parent's reference frame, or relative to world space coordinates,
+/// if it doesn't have a [`Parent`](bevy_hierarchy::Parent).
 ///
-/// [`GlobalTransform`] is the position of an entity relative to the reference frame.
-///
-/// [`GlobalTransform`] is updated from [`Transform`] by systems in the system set
+/// [`GlobalTransform`] is managed by Bevy; it is computed by successively applying the [`Transform`] of each ancestor
+/// entity which has a Transform. This is done automatically by Bevy-internal systems in the system set
 /// [`TransformPropagate`](crate::TransformSystem::TransformPropagate).
 ///
 /// This system runs during [`PostUpdate`](bevy_app::PostUpdate). If you
@@ -189,9 +188,39 @@ impl GlobalTransform {
         (self.0.matrix3 * extents).length()
     }
 
-    /// Transforms the given `point`, applying shear, scale, rotation and translation.
+    /// Transforms the given point from local space to global space, applying shear, scale, rotation and translation.
     ///
-    /// This moves `point` into the local space of this [`GlobalTransform`].
+    /// It can be used like this:
+    ///
+    /// ```
+    /// # use bevy_transform::prelude::{GlobalTransform};
+    /// # use bevy_math::prelude::Vec3;
+    /// let global_transform = GlobalTransform::from_xyz(1., 2., 3.);
+    /// let local_point = Vec3::new(1., 2., 3.);
+    /// let global_point = global_transform.transform_point(local_point);
+    /// assert_eq!(global_point, Vec3::new(2., 4., 6.));
+    /// ```
+    ///
+    /// ```
+    /// # use bevy_transform::prelude::{GlobalTransform};
+    /// # use bevy_math::Vec3;
+    /// let global_point = Vec3::new(2., 4., 6.);
+    /// let global_transform = GlobalTransform::from_xyz(1., 2., 3.);
+    /// let local_point = global_transform.affine().inverse().transform_point3(global_point);
+    /// assert_eq!(local_point, Vec3::new(1., 2., 3.))
+    /// ```
+    ///
+    /// To apply shear, scale, and rotation *without* applying translation, different functions are available:
+    /// ```
+    /// # use bevy_transform::prelude::{GlobalTransform};
+    /// # use bevy_math::prelude::Vec3;
+    /// let global_transform = GlobalTransform::from_xyz(1., 2., 3.);
+    /// let local_direction = Vec3::new(1., 2., 3.);
+    /// let global_direction = global_transform.affine().transform_vector3(local_direction);
+    /// assert_eq!(global_direction, Vec3::new(1., 2., 3.));
+    /// let roundtripped_local_direction = global_transform.affine().inverse().transform_vector3(global_direction);
+    /// assert_eq!(roundtripped_local_direction, local_direction);
+    /// ```
     #[inline]
     pub fn transform_point(&self, point: Vec3) -> Vec3 {
         self.0.transform_point3(point)

crates/bevy_transform/src/components/transform.rs

@@ -500,14 +500,15 @@ impl Transform {
 
     /// Transforms the given `point`, applying scale, rotation and translation.
     ///
-    /// If this [`Transform`] has a parent, this will transform a `point` that is
-    /// relative to the parent's [`Transform`] into one relative to this [`Transform`].
+    /// If this [`Transform`] has an ancestor entity with a [`Transform`] component,
+    /// [`Transform::transform_point`] will transform a point in local space into its
+    /// parent transform's space.
     ///
-    /// If this [`Transform`] does not have a parent, this will transform a `point`
-    /// that is in global space into one relative to this [`Transform`].
+    /// If this [`Transform`] does not have a parent, [`Transform::transform_point`] will
+    /// transform a point in local space into worldspace coordinates.
     ///
-    /// If you want to transform a `point` in global space to the local space of this [`Transform`],
-    /// consider using [`GlobalTransform::transform_point()`] instead.
+    /// If you always want to transform a point in local space to worldspace, or if you need
+    /// the inverse transformations, see [`GlobalTransform::transform_point()`].
     #[inline]
     pub fn transform_point(&self, mut point: Vec3) -> Vec3 {
         point = self.scale * point;