[bevy_camera_controller] Implement customisable rotation system for camera movement

[ExtremTechniker]

Objective

• Add support for different movement coordinate systems in the FreeCamera controller.
• Fix an orientation issue where "forward" and "backward" key bindings were inverted relative to standard conventions.

Solution

• Introduced RotationSystem enum: Added a configuration option to choose between two movement modes:
  • YawPitchRoll (Default): Movement is relative to the camera's full orientation. Looking up and moving forward moves you "up" in world space.
  • Yaw: Movement is constrained to the horizontal plane. Moving forward always keeps you at the same elevation, regardless of where the camera is looking.
• Improved Movement Logic: Refactored run_freecamera_controller to use a rotation-based translation instead of manually calculating forward/right vectors.
• Fixed Inverted Z-Axis: Corrected the forward/back input handling so that key_forward moves the camera along -Z and key_back moves it along +Z, aligning with standard right-handed coordinate systems used in Bevy.

Testing

• Manual Testing: Verified both RotationSystem modes in a local scene.
  • Confirmed that in Yaw mode, looking straight down and pressing "Forward" moves the camera horizontally.
  • Confirmed that in YawPitchRoll mode, looking straight down and pressing "Forward" moves the camera towards the ground.
• Reviewer Testing:
  • Attach a FreeCamera component to a camera or use the free_camera_controller example
  • Toggle rotation_system between RotationSystem::Yaw and RotationSystem::YawPitchRoll in the component initialization or via an inspector.
  • Verify that W moves you forward and S moves you backward.

---

Showcase

This PR adds a new configuration option to the FreeCamera component to control movement behavior.

Click to view usage example

commands.spawn((
    Camera3dBundle::default(),
    FreeCamera {
        // Constrain movement to the floor regardless of look direction
        rotation_system: RotationSystem::Yaw,
        ..default()
    },
));

[github-actions]

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[ExtremTechniker]

Hi, I think I have resolved all my issues but CI is still failing. The error is from another crate. Can I do something on my end or do I have to wait til it's fixed in the main branch?
Best regards

[ExtremTechniker]

Thanks for fixing the main branch. CI is now successful.

What's the normal way now?

[alice-i-cecile]

Now we need to find two contributors to review and approve this PR :) I'll ask around!

[Aceeri]

LGTM!

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Please review the instructions for writing migration guides, then expand or revise the content in the migration guides directory to reflect your changes.

[alice-i-cecile]

I'm generally skeptical of adding significant configurability and complexity to these dev-tool cameras. These tools are intended as very simple dev-mode cameras with a limited scope: they should be easy to replace for users / projects that want different behavior.

That said, fixes like:

Fixed Inverted Z-Axis: Corrected the forward/back input handling so that key_forward moves the camera along -Z and key_back moves it along +Z, aligning with standard right-handed coordinate systems used in Bevy.

Improved Movement Logic: Refactored run_freecamera_controller to use a rotation-based translation instead of manually calculating forward/right vectors.

are welcome improvements though, and I would be very happy to review (and likely merge) them in their own PRs.