rotation_quat fails in some cases about linalg HOT 8 CLOSED

check the matrix first, since the above is not just non-orthonormal, its singular

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Typo, I meant {{0,-1,0},{-1,0,0},{0,0,-1}} (row-ordered)

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off topic: note that if the input has a negative determinant. its is a reflection. thus cant be expressed as just a rotation.

from linalg.

testing now I get the right answer:

	float3x3 m({ 0,-1,0 }, { -1,0,0 }, { 0,0,-1 });
	float4 q = quatfrommat(m);
	std::cout << q << std::endl; 

gives

-0.707107 0.707107 0 0

I'll test your copy of linalg.h to make sure its the same as mine.

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OOps, just realized my test was using my quatfrommat() code instead. , looks like linalg's rotation_quat might be broken. in the meantime, here's some code that works.

inline float4 quatfrommat(const float3x3 &m)
{
	float magw = m[0][0] + m[1][1] + m[2][2];
	float magxy;
	float magzw;
	float3 pre;
	float3 prexy;
	float3 prezw;

	bool wvsz = magw  > m[2][2];
	magzw = wvsz ? magw : m[2][2];
	prezw = wvsz ? float3(1, 1, 1) : float3(-1, -1, 1);
	auto postzw = wvsz ? float4(0, 0, 0, 1) : float4(0, 0, 1, 0);

	bool xvsy = m[0][0] > m[1][1];
	magxy = xvsy ? m[0][0] : m[1][1];
	prexy = xvsy ? float3(1, -1, -1) : float3(-1, 1, -1);
	auto postxy = xvsy ? float4(1, 0, 0, 0) : float4(0, 1, 0, 0);

	bool zwvsxy = magzw > magxy;
	pre = zwvsxy ? prezw : prexy;
	auto post = zwvsxy ? postzw : postxy;

	float t = pre.x * m[0][0] + pre.y * m[1][1] + pre.z * m[2][2] + 1;
	float s = 1 / sqrt(t) / 2;
	float4 qp
	{
		(pre.y * m[1][2] - pre.z * m[2][1]) * s,
		(pre.z * m[2][0] - pre.x * m[0][2]) * s,
		(pre.x * m[0][1] - pre.y * m[1][0]) * s,
		t * s
	};
	return qmul(qp, post);
}

from linalg.

Good catch. This is a numerical stability issue in my implementation related to exact 90 degree rotations.

I've added a failing test case for now while I investigate my options. Stan's function works perfectly, to the best of my knowledge. I'd like to see if I can come up with something a little terser, but definitely make use of his function if you have an immediate need.

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And it looks like we've got it! Do let me know if you encounter any other corner cases, and I apologize for the inconvenience.

from linalg.

wow that implementation is definitely clean and compact. very nice.
thx

from linalg.