Skeletal animation: unexpected quarternion values

Question

I'm following ThinMatrix's skeletal animation tutorial and his custom written Collada parser is deriving a different rotation quaternion from each keyframe matrix than the Assimp importer library does. They are close, except the values are signed inversely and unpredicatbly.

I dug around his importer code but it's a mess. I assume he is manually performing an additional axis calculation somewhere, any ideas what it could be and why?

Original Matrix

1 0 0 0 
0 -0.06466547 -0.997907 0 
0 0.997907 -0.06466556 3.810999
0 0 0 1

Assimp rotation quaternion : (0.7296115 , 0 , 0 , 0.683862)

ThinMatrix's rotation quaternion : -0.72961134 , 0, 0 , 0.68386203)

And for reference, the position vec3: (0, 0, 3.810999)

Update:

With an identity matrix both Assimp and the ThinMatrix code produce (0, 0, 0, 1)

Answer 1

As tkausl suggested, the issue was opposite row/column major matricies. It was problematic because Assimp gives you the position vector and rotation quarternion but does not store the matrix it was derived from, or allow it to be transpose beforehand.

The solution was to manually convert the quarternion back to a matrix, tranpose it, then back to a quarternion, using the following:

public Matrix4 QuarternionToRotationMatrix(OpenTK.Quaternion q)
{
    Matrix4 matrix = new Matrix4();
    float xy = q.X * q.Y;
    float xz = q.X * q.Z;
    float xw = q.X * q.W;
    float yz = q.Y * q.Z;
    float yw = q.Y * q.W;
    float zw = q.Z * q.W;
    float xSquared = q.X * q.X;
    float ySquared = q.Y * q.Y;
    float zSquared = q.Z * q.Z;
    matrix.M11 = 1 - 2 * (ySquared + zSquared);
    matrix.M12 = 2 * (xy - zw);
    matrix.M13 = 2 * (xz + yw);
    matrix.M14 = 0;
    matrix.M21 = 2 * (xy + zw);
    matrix.M22 = 1 - 2 * (xSquared + zSquared);
    matrix.M23 = 2 * (yz - xw);
    matrix.M24 = 0;
    matrix.M31 = 2 * (xz - yw);
    matrix.M32 = 2 * (yz + xw);
    matrix.M33 = 1 - 2 * (xSquared + ySquared);
    matrix.M34 = 0;
    matrix.M41 = 0;
    matrix.M42 = 0;
    matrix.M43 = 0;
    matrix.M44 = 1;
    return matrix;
}

public OpenTK.Quaternion QuaternionFromMatrix(Matrix4 matrix)
{
    float w, x, y, z;
    float diagonal = matrix.M11 + matrix.M22 + matrix.M33;
    if (diagonal > 0)
    {
        float w4 = (float)(Math.Sqrt(diagonal + 1f) * 2f);
        w = w4 / 4f;
        x = (matrix.M32 - matrix.M23) / w4;
        y = (matrix.M13 - matrix.M31) / w4;
        z = (matrix.M21 - matrix.M12) / w4;
    }
    else if ((matrix.M11 > matrix.M22) && (matrix.M11 > matrix.M33))
    {
        float x4 = (float)(Math.Sqrt(1f + matrix.M11 - matrix.M22 - matrix.M33) * 2f);
        w = (matrix.M32 - matrix.M23) / x4;
        x = x4 / 4f;
        y = (matrix.M12 + matrix.M21) / x4;
        z = (matrix.M13 + matrix.M31) / x4;
    }
    else if (matrix.M22 > matrix.M33)
    {
        float y4 = (float)(Math.Sqrt(1f + matrix.M22 - matrix.M11 - matrix.M33) * 2f);
        w = (matrix.M13 - matrix.M31) / y4;
        x = (matrix.M12 + matrix.M21) / y4;
        y = y4 / 4f;
        z = (matrix.M23 + matrix.M32) / y4;
    }
    else
    {
        float z4 = (float)(Math.Sqrt(1f + matrix.M33 - matrix.M11 - matrix.M22) * 2f);
        w = (matrix.M21 - matrix.M12) / z4;
        x = (matrix.M13 + matrix.M31) / z4;
        y = (matrix.M23 + matrix.M32) / z4;
        z = z4 / 4f;
    }
    return new OpenTK.Quaternion(x, y, z, w);
}