Rename reflect() and bounce() to avoid confusion

[Dark-Horizons]

In mathematics, the term reflect generally means to produce a mirror image with respect to a line or plane. This definition typically implies that the given vector represents the normal to the surface, since mirroring is computed relative to that perpendicular direction.

In most game engines and physics/maths libraries (Unity, Unreal, Box2D, GLM, etc.), reflect() follows this convention: it takes a normal vector and returns the reflection of the input vector as if bouncing off that surface.

Godot’s current implementation differs:
reflect(line) expects the direction of the line (parallel to the surface), not the normal.
bounce(normal) performs the operation that most engines (and maths libraries) call reflect().

This mismatch can be confusing for developers transitioning from other engines or with a background in vector mathematics. It also makes reading code less intuitive when collaborating across different codebases.

Suggested changes:
Rename the current reflect() to mirror() (or a similar name that clearly states it mirrors with respect to a given line direction).
Rename the current bounce() to reflect() to align with established conventions in mathematics and other engines.

If backward compatibility is a concern, both names could coexist for a transitional period, with the old names marked as deprecated.

This change would:

• Reduce onboarding friction for new Godot users.
• Align Godot’s API naming with widespread industry and academic conventions.
• Make vector operation code more self-explanatory.

[AThousandShips]

If backward compatibility is a concern, both names could coexist for a transitional period, with the old names marked as deprecated.

This is not possible unless you suggest to just rename reflect() to mirror() and leave bounce as is, but regardless this breaks compatibility and can't be done until 5.0

[AThousandShips]

I think that if we do rename them they should get new names as it will be far more confusing to have a method suddenly swap names with another method. Especially since the ways these methods work is well documented, in the end method names can be confusing but that can be solved by good documentation, a lot of users won't know what these methods do from their names anyway as a lot of users aren't well versed in linear algebra

The effect of renaming these would IMO be:

• Some entirely new users that are familiar with other terminology will be helped in finding the methods the first time around, they would be able to figure out the correct method by simply reading the documentation though
• All established users coming from Godot 4 will be confused and will have to relearn what method to use, and learn a new method to use in some cases
• New users not familiar with any of this terminology will not be affected either way, except by being confused by reading older tutorials or converting older code

In the end I think the drawbacks far outweigh the benefits, not everything will be descriptively named, and most people won't intuitively know what these methods do in either naming

[Zireael07]

Didn't we already have such a proposal?

[AThousandShips]

There have been issues over clarifying the methods, which has been solved, but can't find any proposals for changing it

[Calinou]

• See Vector2.reflect() result is unexpected godot#11678
  and Vector3.Reflect inconsistent with GLSL godot#89250.