Add axes_2d gizmo.

[lambertsbennett]

Objective

This PR addresses #12222 (Fixes #12222). Simple addition to add a 2D axes gizmo.

Solution

• Add a new method axes_2d which takes a transform and a case length and then draws two arrows in the XY plane.

The only thing I'm not sure about here is taking a 3D transform as an argument. It says in the transform comments that for 2D the z-axis is used for ordering, so I figured I'd keep it that way?

---

Changelog

• Add method axes_2d.
• Update arrow_2d to also calculate the tip length depending on arrow length as in arrow.
• Add axes_2d to examples 2d_gizmos.

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[lambertsbennett]

I see that the build/CI are failing because the compiler says that From<Vec3> is not implemented for Vec2. This is puzzling to me because I did implement it as:

impl From<Vec3> for Vec2 {
    #[inline]
    fn from(v: Vec3) -> Self {
        (v.x, v.y)
    }
}

I am relatively new to rust though, so perhaps this isn't correct? Any pointers here would be great! The strange thing is that the example works just fine (2d_gizmos). It's just when compiling the crate that there is an error.

[notmd]

You can't implement a trait for a type if both of them are not in your local module. See https://doc.rust-lang.org/reference/items/implementations.html#orphan-rules
With this change, it's work for me

diff --git a/crates/bevy_gizmos/src/arrows.rs b/crates/bevy_gizmos/src/arrows.rs
index 899af3374..e9dad936c 100644
--- a/crates/bevy_gizmos/src/arrows.rs
+++ b/crates/bevy_gizmos/src/arrows.rs
@@ -199,7 +199,7 @@ impl<'w, 's, T: GizmoConfigGroup> Gizmos<'w, 's, T> {
         let end_x = transform.transform_point(base_length * Vec3::X);
         let end_y = transform.transform_point(base_length * Vec3::Y);

-        self.arrow_2d(start.into(), end_x.into(), RED);
-        self.arrow_2d(start.into(), end_y.into(), GREEN);
+        self.arrow_2d(start.truncate(), end_x.truncate(), RED);
+        self.arrow_2d(start.truncate(), end_y.truncate(), GREEN);
     }
 }

[mockersf]

you should avoid changing this

[mockersf]

this example is already quite crowded. this will also put it on the grid gizmo already present, making it hard to read

[mockersf]

Suggested change

	self.arrow_2d(start.into(), end_x.into(), RED);
	self.arrow_2d(start.into(), end_y.into(), GREEN);
	self.arrow_2d(start.xy(), end_x.xy(), RED);
	self.arrow_2d(start.xy(), end_y.xy(), GREEN);

and you'll need to import bevy_math::Vec3Swizzles

[mockersf]

Why did you change this?

[lambertsbennett]

I changed it to be consistent with the 3d arrow drawing method, but I realize now that was not a smart change.

[lambertsbennett]

Ok thanks for the feedback @notmd and @mockersf - I reverted a bunch of things and now it seems to be in a better state.

[pablo-lua]

Good change, thank you. Can you put this feature in the axes example? I think we can place it in the plane on the example? We can make the bevy logo bounce around in the floor I believe.

[pablo-lua]

I think we should adress both methods in the same impl if possible and move this to the impl above this.

[lambertsbennett]

@pablo-lua I moved it into the same impl block as you suggested and added a 2D axis to the axes example. My first try at that caused there to be axes flying around in space and so I pinned them to the plane. Maybe this is not exactly illustrative of the capability of the 2D axes to transform though.

[pablo-lua]

Fair, I was thinking in placing this as a Sprite in reality, like, spawning a Sprite of some kind and just use the transform in it to move it around and so. We can address this in another PR though, so I think we should revert it for now and do this as a follow up.

[lambertsbennett]

@pablo-lua ok reverted the example, but otherwise I hope its all good!