我正在尝试在我的OpenGL程序中将骨架动画的矩阵转换为四元数,但是我遇到了一个问题:
给定一些单位四元数,我需要获得一个四元数,用它来变换一个向量,使得变换后的向量是每个四元数分别变换后向量的平均值。(对于矩阵,我只需要将矩阵相加并除以矩阵数量即可)
我正在尝试在我的OpenGL程序中将骨架动画的矩阵转换为四元数,但是我遇到了一个问题:
给定一些单位四元数,我需要获得一个四元数,用它来变换一个向量,使得变换后的向量是每个四元数分别变换后向量的平均值。(对于矩阵,我只需要将矩阵相加并除以矩阵数量即可)
很不幸,这并不是非常简单的事情,但是它是可能实现的。这里有一篇白皮书解释了背后的数学原理:http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070017872_2007014421.pdf
请查看Unity3D Wiki页面(下面的代码示例来自同一篇文章):http://wiki.unity3d.com/index.php/Averaging_Quaternions_and_Vectors
//Get an average (mean) from more then two quaternions (with two, slerp would be used).
//Note: this only works if all the quaternions are relatively close together.
//Usage:
//-Cumulative is an external Vector4 which holds all the added x y z and w components.
//-newRotation is the next rotation to be added to the average pool
//-firstRotation is the first quaternion of the array to be averaged
//-addAmount holds the total amount of quaternions which are currently added
//This function returns the current average quaternion
public static Quaternion AverageQuaternion(ref Vector4 cumulative, Quaternion newRotation, Quaternion firstRotation, int addAmount){
float w = 0.0f;
float x = 0.0f;
float y = 0.0f;
float z = 0.0f;
//Before we add the new rotation to the average (mean), we have to check whether the quaternion has to be inverted. Because
//q and -q are the same rotation, but cannot be averaged, we have to make sure they are all the same.
if(!Math3d.AreQuaternionsClose(newRotation, firstRotation)){
newRotation = Math3d.InverseSignQuaternion(newRotation);
}
//Average the values
float addDet = 1f/(float)addAmount;
cumulative.w += newRotation.w;
w = cumulative.w * addDet;
cumulative.x += newRotation.x;
x = cumulative.x * addDet;
cumulative.y += newRotation.y;
y = cumulative.y * addDet;
cumulative.z += newRotation.z;
z = cumulative.z * addDet;
//note: if speed is an issue, you can skip the normalization step
return NormalizeQuaternion(x, y, z, w);
}
public static Quaternion NormalizeQuaternion(float x, float y, float z, float w){
float lengthD = 1.0f / (w*w + x*x + y*y + z*z);
w *= lengthD;
x *= lengthD;
y *= lengthD;
z *= lengthD;
return new Quaternion(x, y, z, w);
}
//Changes the sign of the quaternion components. This is not the same as the inverse.
public static Quaternion InverseSignQuaternion(Quaternion q){
return new Quaternion(-q.x, -q.y, -q.z, -q.w);
}
//Returns true if the two input quaternions are close to each other. This can
//be used to check whether or not one of two quaternions which are supposed to
//be very similar but has its component signs reversed (q has the same rotation as
//-q)
public static bool AreQuaternionsClose(Quaternion q1, Quaternion q2){
float dot = Quaternion.Dot(q1, q2);
if(dot < 0.0f){
return false;
}
else{
return true;
}
}
还有这篇文章:http://forum.unity3d.com/threads/86898-Average-quaternions
这里是我用于方位估计的四元数平均值的 MATLAB 函数实现。转换成其他语言很容易,但这种特定方法(Markley 2007)需要计算特征向量和特征值。有许多库(包括 Eigen C++)可以为您完成此操作。
您可以阅读文件的描述/标题以查看原始论文中的数学内容。
从http://www.mathworks.com/matlabcentral/fileexchange/40098-tolgabirdal-averaging-quaternions获得的 Matlab 文件:
% by Tolga Birdal
% Q is an Mx4 matrix of quaternions. weights is an Mx1 vector, a weight for
% each quaternion.
% Qavg is the weightedaverage quaternion
% This function is especially useful for example when clustering poses
% after a matching process. In such cases a form of weighting per rotation
% is available (e.g. number of votes), which can guide the trust towards a
% specific pose. weights might then be interpreted as the vector of votes
% per pose.
% Markley, F. Landis, Yang Cheng, John Lucas Crassidis, and Yaakov Oshman.
% "Averaging quaternions." Journal of Guidance, Control, and Dynamics 30,
% no. 4 (2007): 1193-1197.
function [Qavg]=quatWAvgMarkley(Q, weights)
% Form the symmetric accumulator matrix
A=zeros(4,4);
M=size(Q,1);
wSum = 0;
for i=1:M
q = Q(i,:)';
w_i = weights(i);
A=w_i.*(q*q')+A; % rank 1 update
wSum = wSum + w_i;
end
% scale
A=(1.0/wSum)*A;
% Get the eigenvector corresponding to largest eigen value
[Qavg, ~]=eigs(A,1);
end
for i = 1:M
循环?你不能只是加权 Q
的值然后计算 A=Q'*Q
吗? - fdermishinQ = (weights .* Q) ./ sum(weights); A = transpose(Q) * Q;
- Fritzimport numpy as np
def quatWAvgMarkley(Q, weights):
'''
Averaging Quaternions.
Arguments:
Q(ndarray): an Mx4 ndarray of quaternions.
weights(list): an M elements list, a weight for each quaternion.
'''
# Form the symmetric accumulator matrix
A = np.zeros((4, 4))
M = Q.shape[0]
wSum = 0
for i in range(M):
q = Q[i, :]
w_i = weights[i]
A += w_i * (np.outer(q, q)) # rank 1 update
wSum += w_i
# scale
A /= wSum
# Get the eigenvector corresponding to largest eigen value
return np.linalg.eigh(A)[1][:, -1]
np.linalg.eigh(np.einsum('ij,ik,i->... jk',q,q,w))[1] [:,-1]
来实现此操作,这比原来的方法快几百倍。累加器的缩放全部在特征值中(不包括规范正交特征向量),因此可以省略。 - reve_etrangeq
是上面答案中的 Q
(而w
是 weights
)? - ketza我尝试按照这里的建议对四元数进行旋转,但对于我要做的事情(模型变形)没有效果,因此我最终决定通过每个四元数来变换向量,然后求平均值(直到我能找到更好的解决方案)。
您不能直接相加四元数。但是,您可以找到一个四元数,它可以在两个角度之间连续旋转,包括中间位置。四元数插值通常被称为“slerp”,并且有一个维基页面来介绍它。这是动画制作中非常有用的技巧。在某些方面,四元数插值(slerp)是使用计算机图形学中四元数的主要原因。
有一份来自2001年的技术报告指出,平均数实际上是一个相当好的近似值,只要四元数彼此接近。(对于-q=q的情况,您可以通过在它们之前乘以-1来翻转指向另一个方向的那些四元数,以便涉及的所有四元数都处于同一半球中。)
更好的方法在这篇2007年的论文中概述,其中涉及使用奇异值分解(SVD)。这正是Nathan提到的同一篇论文。我想补充说明的是,不仅有C++实现,还有Matlab实现。通过执行与Matlab代码一起提供的测试脚本,我可以说它对涉及的四元数的小扰动(0.004 *均匀噪声)给出了相当不错的结果:
qinit=rand(4,1);
Q=repmat(qinit,1,10);
% apply small perturbation to the quaternions
perturb=0.004;
Q2=Q+rand(size(Q))*perturb;
everything okay, max angle offset == 9.5843
qinit to average: 0.47053 degrees
qinit to simple_average: 0.47059 degrees
average to simple_average: 0.00046228 degrees
loop implementation to matrix implementation: 3.4151e-06 degrees
%% Generate random unity quaternion
rng(42); % set arbitrary seed for random number generator
M = 100;
qinit=rand(1,4) - 0.5;
qinit=qinit/norm(qinit);
Qinit=repmat(qinit,M,1);
%% apply small perturbation to the quaternions
perturb=0.05; % 0.05 => +- 10 degrees of rotation (see angles_deg)
Q = Qinit + 2*(rand(size(Qinit)) - 0.5)*perturb;
Q = Q ./ vecnorm(Q, 2, 2); % Normalize perturbed quaternions
Q_inv = Q * diag([1 -1 -1 -1]); % calculated inverse perturbed rotations
%% Test if everything worked as expected: assert(Q2 * Q2_inv = unity)
unity = quatmultiply(Q, Q_inv);
Q_diffs = quatmultiply(Qinit, Q_inv);
angles = 2*acos(Q_diffs(:,1));
angles_deg = wrapTo180(rad2deg(angles));
if sum(sum(abs(unity - repmat([1 0 0 0], M, 1)))) > 0.0001
disp('error, quaternion inversion failed for some reason');
else
disp(['everything okay, max angle offset == ' num2str(max(angles_deg))])
end
%% Calculate average using matrix implementation of eigenvalues algorithm
[average,~] = eigs(transpose(Q) * Q, 1);
average = transpose(average);
diff = quatmultiply(qinit, average * diag([1 -1 -1 -1]));
diff_angle = 2*acos(diff(1));
%% Calculate average using algorithm from https://dev59.com/N2ct5IYBdhLWcg3wEJcm#29315869
average2 = quatWAvgMarkley(Q, ones(M,1));
diff2 = quatmultiply(average, average2 * diag([1 -1 -1 -1]));
diff2_angle = 2*acos(diff2(1));
%% Simply add coefficients and normalize the result
simple_average = sum(Q) / norm(sum(Q));
simple_diff = quatmultiply(qinit, simple_average * diag([1 -1 -1 -1]));
simple_diff_angle = 2*acos(simple_diff(1));
simple_to_complex = quatmultiply(simple_average, average * diag([1 -1 -1 -1]));
simple_to_complex_angle = 2*acos(simple_to_complex(1));
%% Compare results
disp(['qinit to average: ' num2str(wrapTo180(rad2deg(diff_angle))) ' degrees']);
disp(['qinit to simple_average: ' num2str(wrapTo180(rad2deg(simple_diff_angle))) ' degrees']);
disp(['average to simple_average: ' num2str(wrapTo180(rad2deg(simple_to_complex_angle))) ' degrees']);
disp(['loop implementation to matrix implementation: ' num2str(wrapTo180(rad2deg(diff2_angle))) ' degrees']);
当计算无约束平均值时,四元数不是用于旋转的理想自由度集。
以下是我大部分时间使用的内容(
[MethodImpl(MethodImplOptions.AggressiveInlining)]
internal static Vector3 ToAngularVelocity( this Quaternion q )
{
if ( abs(q.w) > 1023.5f / 1024.0f)
return new Vector3();
var angle = acos( abs(q.w) );
var gain = Sign(q.w)*2.0f * angle / Sin(angle);
return new Vector3(q.x * gain, q.y * gain, q.z * gain);
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
internal static Quaternion FromAngularVelocity( this Vector3 w )
{
var mag = w.magnitude;
if (mag <= 0)
return Quaternion.identity;
var cs = cos(mag * 0.5f);
var siGain = sin(mag * 0.5f) / mag;
return new Quaternion(w.x * siGain, w.y * siGain, w.z * siGain, cs);
}
internal static Quaternion Average(this Quaternion refence, Quaternion[] source)
{
var refernceInverse = refence.Inverse();
Assert.IsFalse(source.IsNullOrEmpty());
Vector3 result = new Vector3();
foreach (var q in source)
{
result += (refernceInverse*q).ToAngularVelocity();
}
return reference*((result / source.Length).FromAngularVelocity());
}
internal static Quaternion Average(Quaternion[] source)
{
Assert.IsFalse(source.IsNullOrEmpty());
Vector3 result = new Vector3();
foreach (var q in source)
{
result += q.ToAngularVelocity();
}
return (result / source.Length).FromAngularVelocity();
}
internal static Quaternion Average(Quaternion[] source, int iterations)
{
Assert.IsFalse(source.IsNullOrEmpty());
var reference = Quaternion.identity;
for(int i = 0;i < iterations;i++)
{
reference = Average(reference,source);
}
return reference;
}`