sample_baked_with_rotation in Curve3D gives wrong rotation near first and last points

[AstroStucky]

Tested versions

Reproducible in

• 4.4.dev.custom_build.fafc07335
• 4.3.stable.official.77dcf97d8
• 4.2.2.stable.official.15073afe3

System information

Godot v4.3.stable - Ubuntu 22.04.5 LTS 22.04 - X11 - GLES3 (Compatibility) - NVIDIA GeForce RTX 3080 (nvidia; 550.120) - Intel(R) Core(TM) i5-8600K CPU @ 3.60GHz (6 Threads)

Issue description

Curve3D's sample_baked_with_rotation returns strange rotation values near the first point and the last point of a curve.

Steps to reproduce

1. Construct a Curve3D with at least 3 points.
2. Give the middle point a non-zero in and out vector.
3. Sample the curve with rotation using sample_baked_with_rotation at regular intervals to see that the rotation value veers off in a weird direction as the first and the final points are approached.

Minimal reproduction project (MRP)

mrp_curve3d-edge-rotation.zip

The attached MRP is a 3D scene with a Camera3D, a Path3D, and a MeshInstance3D. The Path3D has a script attached to it that draws basis provided by sample_baked_with_rotation along the entire length of the Path3D. This is not a tool script, so the project must be ran to see the basis drawn. The Path3D script has some exported parameters to help the user investigate this issue: count is the number of samples of the curve taken and drawn in the 3D viewport as basis gizmos, size is the length of the vector that represents a basis in a gizmo, and cubic changes whether sample_baked_with_rotation is called with cubic = true or cubic = false (the issue behavior does not appear to change based on this).

Examples from MRP

The following is the 3D scene with only 50 samples taken so the basis gizmo can be more easily seen.

As the number of samples increases so does the number of problematic rotation samples near the first and last points.

When the middle point has zero in and out vectors, there are only straight lines and this issue does not occur.

[AstroStucky]

sample_baked_with_rotation unit tests are only ever performed with a straight line. My 3rd MRP example implies this issue cannot be caught by the existing tests.

godot/tests/scene/test_curve_3d.h

Line 151 in 77dcf97

// Sampling over a simple straight line to make assertions simpler

[AThousandShips]

Sounds like:

• [BUG] sample_baked_with_rotation return wrong rotation when offset=0 #82658

[AstroStucky]

They appear to be the same issue, only that one is reporting on Curve2D. Perhaps the underline cause is the same?

[AThousandShips]

Probably, let's keep this open until we know for sure