Python 如何在numpy中将三维密度贴图的主轴与笛卡尔坐标轴对齐?
我有一个nxnnumpy数组,它包含立方网格上的密度值。我试图将密度图的惯性主轴与网格的笛卡尔x,y,z轴对齐。到目前为止,我有以下几点:Python 如何在numpy中将三维密度贴图的主轴与笛卡尔坐标轴对齐?,python,numpy,image-rotation,ndimage,Python,Numpy,Image Rotation,Ndimage,我有一个nxnnumpy数组,它包含立方网格上的密度值。我试图将密度图的惯性主轴与网格的笛卡尔x,y,z轴对齐。到目前为止,我有以下几点: import numpy as np from scipy import ndimage def center_rho(rho): """Move density map so its center of mass aligns with the center of the grid""" rhocom = np.array(ndimage
import numpy as np
from scipy import ndimage
def center_rho(rho):
"""Move density map so its center of mass aligns with the center of the grid"""
rhocom = np.array(ndimage.measurements.center_of_mass(rho))
gridcenter = np.array(rho.shape)/2.
shift = gridcenter-rhocom
rho = ndimage.interpolation.shift(rho,shift,order=1,mode='wrap')
return rho
def inertia_tensor(rho,side):
"""Calculate the moment of inertia tensor for the given density map."""
halfside = side/2.
n = rho.shape[0]
x_ = np.linspace(-halfside,halfside,n)
x,y,z = np.meshgrid(x_,x_,x_,indexing='ij')
Ixx = np.sum(rho*(y**2 + z**2))
Iyy = np.sum(rho*(x**2 + z**2))
Izz = np.sum(rho*(x**2 + y**2))
Ixy = -np.sum(rho*x*y)
Iyz = -np.sum(rho*y*z)
Ixz = -np.sum(rho*x*z)
I = np.array([[Ixx, Ixy, Ixz],
[Ixy, Iyy, Iyz],
[Ixz, Iyz, Izz]])
return I
def principal_axes(I):
"""Calculate the principal inertia axes and order them in ascending order."""
w,v = np.linalg.eigh(I)
return w,v
#number of grid points along side
n = 10
#note n <= 3 produces unit eigenvectors, not sure why
#in practice, n typically between 10 and 50
np.random.seed(1)
rho = np.random.random(size=(n,n,n))
side = 1. #physical width of box, set to 1.0 for simplicity
rho = center_rho(rho)
I = inertia_tensor(rho,side)
PAw, PAv = principal_axes(I)
#print magnitude and direction of principal axes
print "Eigenvalues/eigenvectors before rotation:"
for i in range(3):
print PAw[i], PAv[:,i]
#sanity check that I = R * D * R.T
#where R is the rotation matrix and D is the diagonalized matrix of eigenvalues
D = np.eye(3)*PAw
print np.allclose(np.dot(PAv,np.dot(D,PAv.T)),I)
#rotate rho to align principal axes with cartesian axes
newrho = ndimage.interpolation.affine_transform(rho,PAv.T,order=1,mode='wrap')
#recalculate principal axes
newI = inertia_tensor(newrho,side)
newPAw, newPAv = principal_axes(newI)
#print magnitude and direction of new principal axes
print "Eigenvalues/eigenvectors before rotation:"
for i in range(3):
print newPAw[i], newPAv[:,i]
(这可能是错误的)但甚至不要从单位向量开始。我得到的是:
Eigenvalues/eigenvectors before rotation:
102.405523732 [-0.05954221 -0.8616362 0.5040216 ]
103.177395578 [-0.30020273 0.49699978 0.81416801]
104.175688943 [-0.95201526 -0.10283129 -0.288258 ]
True
Eigenvalues/eigenvectors after rotation:
104.414931478 [ 0.38786 -0.90425086 0.17859172]
104.731536038 [-0.74968553 -0.19676735 0.63186566]
106.151322662 [-0.53622405 -0.37896304 -0.75422197]
我不确定问题是我的代码还是我关于旋转主轴的假设,但如果有任何帮助,我将不胜感激。是我开发的用于进行这种对齐的代码的链接
给定一组具有坐标(x、y、z)的散射点,目标是将与最小特征值相关联的特征向量与三维笛卡尔轴的x轴相匹配,并将与中值特征值相关联的特征向量与来自同一三维笛卡尔轴的y轴相匹配
为此,我遵循了以下步骤:
旋转是通过这里定义的旋转矩阵来实现的:我知道这很旧,但我想ping一下,看看是否有人能帮上忙。结果证明这是可行的,你只需要在循环中运行几次。插值会引起问题。一旦它应用了3次左右,它就在可接受的公差范围内给出了单位矩阵。谢谢你的努力。我实际上在寻找如何旋转一个nxnxn数组,而不是一组N点(这是一个nx3数组)。我认为问题主要在于插值。我现在有了一些代码来解决这个问题。
Eigenvalues/eigenvectors before rotation:
102.405523732 [-0.05954221 -0.8616362 0.5040216 ]
103.177395578 [-0.30020273 0.49699978 0.81416801]
104.175688943 [-0.95201526 -0.10283129 -0.288258 ]
True
Eigenvalues/eigenvectors after rotation:
104.414931478 [ 0.38786 -0.90425086 0.17859172]
104.731536038 [-0.74968553 -0.19676735 0.63186566]
106.151322662 [-0.53622405 -0.37896304 -0.75422197]