# QTangents

Published in the 2011 Siggraph Presentation Spherical Skinning with Dual-Quaternions and QTangents, Crytek proposed a highly efficient way of representing tangent space per vertex. Instead of storing the basis vectors, they store a quaternion representing the tangent space rotation and reconstruct the basis vectors in the shader. Being a simple and straight forward idea, I decided to implement this approach in the anima skinning sample.

As a first step, the tangent space vectors need to be converted into quaternion form. I did so by assembling a rotation matrix that represents the rotation of an arbitrary world space vector into tangent space:

$\mathbf{T} = \left(\begin{array}{l}\mathbf{B}_{binormal} \\ \mathbf{B}_{tangent} \\ \mathbf{B}_{normal} \end{array}\right)$

where $\mathbf{B}_{binormal}$ denotes the tangent space binormal vector (in row form!) etc. This matrix can then be converted into a quaternion by one of the standard algorithms. Note, however, that special care has to be taken during this step: First of all, $\mathbf{T}$ might not be a proper rotation matrix. It is reasonable to assume that orthonormality is guaranteed, i.e.

$\mathbf{T}\mathbf{T}^T = \mathbf{T}^T \mathbf{T} = \mathbf{I}$

for the identity matrix $\mathbf{I}$, but $\mathbf{T}$ might still encode a reflection. If this is the case the matrix to quaternion will fail because unit quaternions cannot encode reflections. Fortunately, the matrix can be converted into a regular rotation by simply picking any basis vector and reflecting it as well. In my case I chose to always reflect the $\mathbf{B}_{normal}$ vector. Have a look at the figure blow which illustrates the case where $\mathbf{B}_{binormal}$ is reflected.

Note that after the reflection of the normal, handedness is restored again. Of course, reconstructing the resulting quaternion yields a tangent space with a flipped normal vector, so we need to un-flip it first before we can use it in any shading algorithms! Thus, we need to store a flag along with our quaternions that indicates if the normal vector need to be flipped after reconstruction. And this is where the smart guys from Crytek scored their points: Realizing that for a given quaternion $\mathbf{q}$ it’s negate $-\mathbf{q}$ represents the exact same rotation (albeit in opposite direction, but that won’t bother us here) we can enforce non negativity of any quaternion element without impact on the reconstructed tangent space. But this also means that we can use the sign of that very component to store our reflection flag! This brings our memory requirements for the whole tangent frame down to 4 floating point values. Not bad! The only thing that can go wrong now is when the chosen quaternion element is zero. Should not be an issue in theory because IEEE 754 makes a distinction between +0 and -0 but GPUs don’t always stick to this rule. In this case we can set the value of this component to a very small bias, say 1e-7. Here’s how I implemented the tangent space matrix to QTangent conversion:

// generate tangent frame rotation matrix
Math::Matrix3x4 tangentFrame(
binormal.x,    binormal.y,    binormal.z,    0,
tangent.x,     tangent.y,     tangent.z,     0,
normal.x,      normal.y,      normal.z,      0
);

// flip y axis in case the tangent frame encodes a reflection
float scale = tangentFrame.Determinant() < 0 ? 1.0f : -1.0f;

tangentFrame.data[2][0] *= scale;
tangentFrame.data[2][1] *= scale;
tangentFrame.data[2][2] *= scale;

// convert to quaternion
Math::Quaternion tangentFrameQuaternion = tangentFrame;

// make sure we don't end up with 0 as w component
{
const float threshold = 0.000001f;
const float renomalization = sqrt( 1.0f - threshold * threshold );

if( abs(tangentFrameQuaternion.data.w)     {
tangentFrameQuaternion.data.w =  tangentFrameQuaternion.data.w > 0
? threshold
: -threshold;
tangentFrameQuaternion.data.x *= renomalization;
tangentFrameQuaternion.data.y *= renomalization;
tangentFrameQuaternion.data.z *= renomalization;
}
}

// encode reflection into quaternion's w element by making sign of w negative
// if y axis needs to be flipped, positive otherwise
float qs = (scale<0 && tangentFrameQuaternion.data.w>0.f) ||
(scale>0 && tangentFrameQuaternion.data.w<0) ? -1.f : 1.f;

tangentFrameQuaternion.data.x *= qs;
tangentFrameQuaternion.data.y *= qs;
tangentFrameQuaternion.data.z *= qs;
tangentFrameQuaternion.data.w *= qs;


On a side note: As implemented in the code above, reflection properties of a matrix can be detected via the matrix’s determinant, i.e. if a matrix $\mathbf{T}$ encodes a reflection then

$det(\mathbf{T}) = -1$

holds. In order to obtain the world space tangent frame vectors we need to rotate the tangent frame quaternion into world space first. This can be done by concatenating it with the vertex’s blended bone transform. In the case of dual quaternion bone transforms we simply need to multiply the real part of the bone dual quaternion $\hat{\mathbf{q}} = \mathbf{q}_r + \epsilon \mathbf{q}_d$ with the tangent space quaternion $\mathbf{t}$

$\mathbf{t'} = \mathbf{q}_r * \mathbf{t}$

The tangent space vectors can then be reconstructed via a standard quaternion to matrix routine. Note that it is advisable to only reconstruct two of the three basis vectors and recompute the third via a cross product in order to guarantee orthonormality.

float2x3 QuaternionToTangentBitangent( float4 q )
{
return float2x3(
1-2*(q.y*q.y + q.z*q.z),  2*(q.x*q.y + q.w*q.z),    2*(q.x*q.z - q.w*q.y),
2*(q.x*q.y - q.w*q.z),    1-2*(q.x*q.x + q.z*q.z),  2*(q.y*q.z + q.w*q.x)
);
}

float3x3 GetTangentFrame( float4 worldTransform, TangentFrame tangentFrame )
{
float4 q = QuaternionMultiply( worldTransform,  tangentFrame.Rotation );
float2x3 tBt = QuaternionToTangentBitangent( q );

return float3x3(
tBt[0],
tBt[1],
cross(tBt[0],tBt[1]) * (tangentFrame.Rotation.w < 0 ? -1 : 1)
);
}