Implement skewing

[ziriax]

Like in the old days, we might use:

• https://github.com/erich666/GraphicsGems/blob/master/gemsiv/polar_decomp/Decompose.c
• http://research.cs.wisc.edu/graphics/Courses/838-s2002/Papers/polar-decomp.pdf

[BigRoy]

Would it be required to decompose this manaully, shouldn't the use of MTransformationMatrix.getShear suffice? Or what use case are you after?

[ziriax]

GLTF doesn't support shearing, so it would need to be emulated by the exporter by adding multiple GLTF nodes.

But since this plugin samples world space matrices (to allow various Maya constraints), it will be very difficult to support this I guess, since the decomposition can be ambiguous, and must be continuous under animation.

In the past we used https://research.cs.wisc.edu/graphics/Courses/838-s2002/Papers/polar-decomp.pdf, the code is already in the develop branch, but I didn't find a way to get this working with GLTF.

So for now, artists need to manually add multiple nodes in Maya to support shearing.

[BigRoy]

GLTF doesn't support shearing, so it would need to be emulated by the exporter by adding multiple GLTF nodes.

Ah, so it's the other way around. Now this issue makes more sense, great!

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To be complete, it's stated in the GLTF 2.0 Transformation specifications that GLTF does not contain shearing/skewing:

When matrix is defined, it must be decomposable to TRS. This implies that transformation matrices cannot skew or shear.

Similarly it has no mention of any dedicated shear or skew attribute.

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So for now, artists need to manually add multiple nodes in Maya to support shearing.

Having this stated in this issue will hopefully help others to find a potential workflow for it if they need to. Great!