add keep_projection, gpu_instancing rotation

[bertt]

No description provided.

[Copilot]

Pull request overview

Adds Cartesian (--keep_projection=true) rotation support for GPU instancing so yaw/pitch/roll are applied in a local XYZ (East/North/Up) frame while still outputting glTF in Y-up space.

Changes:

• Thread keepProjection through GPU instancing tile generation and saving APIs.
• Add Cartesian-basis rotation computation (EnuCalculator.GetLocalCartesianBasis) and apply it in GPUTileHandler when keepProjection=true.
• Add new NUnit coverage for Cartesian rotation behavior and update documentation/README to reflect the new support.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/CartesianRotationTests.cs	Adds new unit tests for Cartesian-basis rotation and GPU tile output validation.
src/ImplicitTiling.cs	Passes keepProjection through to GPU tile creation/saving paths.
src/GPUTileHandler.cs	Implements Cartesian-mode rotation/translation handling for GPU instancing; adds keepProjection plumbing.
src/EnuCalculator.cs	Introduces GetLocalCartesianBasis to apply yaw/pitch/roll in a local XYZ frame.
docs/technical-transforms.md	Updates transform documentation to include GPU instancing Cartesian rotation behavior.
README.md	Updates user-facing limitations section to state Cartesian rotation is supported in GPU instancing mode.

Comments suppressed due to low confidence (3)

src/GPUTileHandler.cs:219

• northYUp is computed but never used. This creates unnecessary work and may introduce compiler warnings; consider removing it or using it as part of the basis construction to avoid dead code.

            var northYUp = Vector3.Normalize(ToYUp(north));
            var upYUp = Vector3.Normalize(ToYUp(up));

docs/technical-transforms.md:71

• In Cartesian mode, the doc says translation is a direct XYZ offset from RTC center "(no Y-up swizzle)", but GPUTileHandler.GetInstanceTransform still applies ToYUp before subtracting the RTC translation. Update the docs to reflect the actual behavior (Cartesian coordinates are still swizzled to glTF Y-up for output; the key difference is skipping ECEF/ENU conversion).

**Cartesian mode** (`--keep_projection=true`):
1. **Translation**: Direct XYZ offset from RTC center (no Y-up swizzle)
2. **Rotation**: Apply yaw/pitch/roll to Cartesian basis (East=X, North=Y, Up=Z) → map to glTF space → re-orthonormalize → quaternion
3. **Scale**: Uniform or non-uniform (`--use_scale_non_uniform`)

tests/CartesianRotationTests.cs:119

• These tests parse the output GLB and look for EXT_mesh_gpu_instancing via GetExtension<MeshGpuInstancing>(), but Tiles3DExtensions.RegisterExtensions() is not called in this test class. Other tests register extensions explicitly; without it, results can be null or test outcomes can become order-dependent. Add a [SetUp] (or call in each test) to register the 3D Tiles extensions before parsing GLBs.

        var model = ModelRoot.ParseGLB(tile);
        var instancingNode = model.LogicalNodes.FirstOrDefault(n => n.GetExtension<MeshGpuInstancing>() != null);
        Assert.That(instancingNode, Is.Not.Null);

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