问题
I need a faster quaternion-vector multiplication routine for my math library. Right now I'm using the canonical v' = qv(q^-1), which produces the same result as multiplying the vector by a matrix made from the quaternion, so I'm confident in it's correctness.
So far I've implemented 3 alternative "faster" methods:
#1, I have no idea where I got this one from:
v' = (q.xyz * 2 * dot(q.xyz, v)) + (v * (q.w*q.w - dot(q.xyz, q.zyx))) + (cross(q.xyz, v) * q.w * w)
Implemented as:
vec3 rotateVector(const quat& q, const vec3& v)
{
vec3 u(q.x, q.y, q.z);
float s = q.w;
return vec3(u * 2.0f * vec3::dot(u, v))
+ (v * (s*s - vec3::dot(u, u)))
+ (vec3::cross(u, v) * s * 2.0f);
}
#2, courtesy of this fine blog
t = 2 * cross(q.xyz, v);
v' = v + q.w * t + cross(q.xyz, t);
Implemented as:
__m128 rotateVector(__m128 q, __m128 v)
{
__m128 temp = _mm_mul_ps(vec4::cross(q, v), _mm_set1_ps(2.0f));
return _mm_add_ps(
_mm_add_ps(v, _mm_mul_ps(_mm_shuffle_ps(q, q, _MM_SHUFFLE(3, 3, 3, 3)), temp)),
vec4::cross(q, temp));
}
And #3, from numerous sources,
v' = v + 2.0 * cross(cross(v, q.xyz) + q.w * v, q.xyz);
implemented as:
__m128 rotateVector(__m128 q, __m128 v)
{
//return v + 2.0 * cross(cross(v, q.xyz) + q.w * v, q.xyz);
return _mm_add_ps(v,
_mm_mul_ps(_mm_set1_ps(2.0f),
vec4::cross(
_mm_add_ps(
_mm_mul_ps(_mm_shuffle_ps(q, q, _MM_SHUFFLE(3, 3, 3, 3)), v),
vec4::cross(v, q)),
q)));
}
All 3 of these produce incorrect results. I have, however, noticed some interesting patterns. First of all, #1 and #2 produce the same result. #3 produces the same result that I get from multiplying the vector by a derived matrix if said matrix is transposed (I discovered this by accident, previously my quat-to-matrix code assumed row-major matrices, which was incorrect).
The data storage of my quaternions are defined as:
union
{
__m128 data;
struct { float x, y, z, w; };
float f[4];
};
Are my implementations flawed, or am I missing something here?
回答1:
Main issue, if you want to rotate the 3d vector by quaternion, you require to calculate all 9 scalars of rotation matrix. In your examples, calculation of rotation matrix is IMPLICIT. The order of calculation can be not optimal.
If you generate 3x3 matrix from quaternion and multiply vector, you should have same number of arithmetic operations (@see code at bottom).
What i recommend.
Try to generate matrix 3x3 and multiply your vector, measure the speed and compare with previous.
Analyze the explicit solution, and try to optimize for custom architecture.
try to implement alternative quaternion multiplication, and derived multiplication from equation q*v*q'.
//============ alternative multiplication pseudocode
/**
alternative way of quaternion multiplication,
can speedup multiplication for some systems (PDA for example)
http://mathforum.org/library/drmath/view/51464.html
http://www.lboro.ac.uk/departments/ma/gallery/quat/src/quat.ps
in provided code by url's many bugs, have to be rewriten.
*/
inline xxquaternion mul_alt( const xxquaternion& q) const {
float t0 = (x-y)*(q.y-q.x);
float t1 = (w+z)*(q.w+q.z);
float t2 = (w-z)*(q.y+q.x);
float t3 = (x+y)*(q.w-q.z);
float t4 = (x-z)*(q.z-q.y);
float t5 = (x+z)*(q.z+q.y);
float t6 = (w+y)*(q.w-q.x);
float t7 = (w-y)*(q.w+q.x);
float t8 = t5 + t6 + t7;
float t9 = (t4 + t8)*0.5;
return xxquaternion ( t3+t9-t6,
t2+t9-t7,
t1+t9-t8,
t0+t9-t5 );
// 9 multiplications 27 addidtions 8 variables
// but of couse we can clean 4 variables
/*
float r = w, i = z, j = y, k =x;
float br = q.w, bi = q.z, bj = q.y, bk =q.x;
float t0 = (k-j)*(bj-bk);
float t1 = (r+i)*(br+bi);
float t2 = (r-i)*(bj+bk);
float t3 = (k+j)*(br-bi);
float t4 = (k-i)*(bi-bj);
float t5 = (k+i)*(bi+bj);
float t6 = (r+j)*(br-bk);
float t7 = (r-j)*(br+bk);
float t8 = t5 + t6 + t7;
float t9 = (t4 + t8)*0.5;
float rr = t0+t9-t5;
float ri = t1+t9-t8;
float rj = t2+t9-t7;
float rk = t3+t9-t6;
return xxquaternion ( rk, rj, ri, rr );
*/
}
//============ explicit vector rotation variants
/**
rotate vector by quaternion
*/
inline vector3 rotate(const vector3& v)const{
xxquaternion q(v.x * w + v.z * y - v.y * z,
v.y * w + v.x * z - v.z * x,
v.z * w + v.y * x - v.x * y,
v.x * x + v.y * y + v.z * z);
return vector3(w * q.x + x * q.w + y * q.z - z * q.y,
w * q.y + y * q.w + z * q.x - x * q.z,
w * q.z + z * q.w + x * q.y - y * q.x)*( 1.0f/norm() );
// 29 multiplications, 20 addidtions, 4 variables
// 5
/*
// refrence implementation
xxquaternion r = (*this)*xxquaternion(v.x, v.y, v.z, 0)*this->inverted();
return vector3( r.x, r.y, r.z );
*/
/*
// alternative implementation
float wx, wy, wz, xx, yy, yz, xy, xz, zz, x2, y2, z2;
x2 = q.x + q.x; y2 = q.y + q.y; z2 = q.z + q.z;
xx = q.x * x2; xy = q.x * y2; xz = q.x * z2;
yy = q.y * y2; yz = q.y * z2; zz = q.z * z2;
wx = q.w * x2; wy = q.w * y2; wz = q.w * z2;
return vector3( v.x - v.x * (yy + zz) + v.y * (xy - wz) + v.z * (xz + wy),
v.y + v.x * (xy + wz) - v.y * (xx + zz) + v.z * (yz - wx),
v.z + v.x * (xz - wy) + v.y * (yz + wx) - v.z * (xx + yy) )*( 1.0f/norm() );
// 18 multiplications, 21 addidtions, 12 variables
*/
};
Good luck.
来源：https://stackoverflow.com/questions/22497093/faster-quaternion-vector-multiplication-doesnt-work