有沒有一種有效的方法來找到 python 中 3 維中的點最近的線段？

Question

我對 3D 有意見

p = [0,1,0]  

以及由它們的起點和終點坐標定義的線段列表。

line_starts = [[1,1,1], [2,2,2], [3,3,3]]  
line_ends = [[5,1,3], [3,2,1], [3, 1, 1]]

我嘗試調整本文詳細介紹的前兩種算法： Find the shortest distance between a point and line segments (not line)

但是對於超過 1k 的點和線段，算法要么非常慢，要么不適用於 3 維。 有沒有一種有效的方法來計算從點到線段的最小距離，並返回線段上該點的坐標？

例如，我能夠從上面鏈接的帖子中調整這段代碼，但它非常慢。

import math
import numpy as np

def dot(v,w):  
    x,y,z = v  
    X,Y,Z = w   
    return x*X + y*Y + z*Z  

def length(v):  
    x,y,z = v  
    return math.sqrt(x*x + y*y + z*z)  

def vector(b,e):  
    x,y,z = b  
    X,Y,Z = e  
    return (X-x, Y-y, Z-z)  

def unit(v):   
    x,y,z = v  
    mag = length(v)   
    return (x/mag, y/mag, z/mag)  

def distance(p0,p1):  
    return length(vector(p0,p1))  

def scale(v,sc):  
    x,y,z = v  
    return (x * sc, y * sc, z * sc)  

def add(v,w):  
    x,y,z = v  
    X,Y,Z = w  
    return (x+X, y+Y, z+Z)  


'''Given a line with coordinates 'start' and 'end' and the 
coordinates of a point 'pnt' the proc returns the shortest   
distance from pnt to the line and the coordinates of the   
nearest point on the line.  
1  Convert the line segment to a vector ('line_vec').  
2  Create a vector connecting start to pnt ('pnt_vec').  
3  Find the length of the line vector ('line_len').  
4  Convert line_vec to a unit vector ('line_unitvec').  
5  Scale pnt_vec by line_len ('pnt_vec_scaled').  
6  Get the dot product of line_unitvec and pnt_vec_scaled ('t').   
7  Ensure t is in the range 0 to 1.  
8  Use t to get the nearest location on the line to the end  
   of vector pnt_vec_scaled ('nearest').  
9  Calculate the distance from nearest to pnt_vec_scaled.  
10 Translate nearest back to the start/end line.   
Malcolm Kesson 16 Dec 2012'''

def pnt2line(array):  
    pnt = array[0]  
    start = array[1]   
    end = array[2]  
    line_vec = vector(start, end)  
    pnt_vec = vector(start, pnt)  
    line_len = length(line_vec)  
    line_unitvec = unit(line_vec)  
    pnt_vec_scaled = scale(pnt_vec, 1.0/line_len)  
    t = dot(line_unitvec, pnt_vec_scaled)        
    if t < 0.0:  
        t = 0.0  
    elif t > 1.0:  
        t = 1.0  
    nearest = scale(line_vec, t)  
    dist = distance(nearest, pnt_vec)  
    nearest = add(nearest, start)  
    return (round(dist, 3), [round(i, 3) for i in nearest])  



def get_nearest_line(input_d):  
    ''' 
    input_d is an array of arrays  
    Each subarray is [point, line_start, line_end]  
    The point must be the same for all sub_arrays  
    '''   
    op = np.array(list(map(pnt2line, input_d)))  
    ind = np.argmin(op[:, 0])  
    return ind, op[ind, 0], op[ind, 1]  



if __name__ == '__main__':  
    p = [0,1,0]    
  
    line_starts = [[1,1,1], [2,2,2], [3,3,3]]    
    line_ends = [[5,1,3], [3,2,1], [3, 1, 1]]  

    input_d = [[p, line_starts[i], line_ends[i]] for i in range(len(line_starts))]  
    print(get_nearest_line(input_d))  

Output：

(0, 1.414, [1.0, 1.0, 1.0])

在這里，（0 - 第一條線段最接近，
1.414 - 到線段的距離，
[1.0, 1.0, 1.0] - 線段上離給定點最近的點）

問題是上面的代碼非常慢。 此外，我有大約 10K 點和一組固定的 10K 線段。 對於每個點，我必須找到最近的線段，以及線段上最近的點。 現在處理 10K 點需要 30 分鍾。

有沒有一種有效的方法來實現這一目標？

Answer 1

你可以試試這個：

import numpy as np


def dot(v, w):
    """
    row-wise dot product of 2-dimensional arrays
    """
    return np.einsum('ij,ij->i', v, w)


def closest(line_starts, line_ends, p):
    """
    find line segment closest to the point p 
    """

    # array of vectors from the start to the end of each line segment
    se = line_ends - line_starts
    # array of vectors from the start of each line segment to the point p
    sp = p - line_starts
    # array of vectors from the end of each line segment to p
    ep = p - line_ends

    # orthogonal projection of sp onto se
    proj = (dot(sp, se) / dot(se, se)).reshape(-1, 1) * se
    # orthogonal complement of the projection
    n = sp - proj
    
    # squares of distances from the start of each line segment to p
    starts_d = dot(sp, sp)
    # squares of distances from the end of each line segments to p
    ends_d = dot(ep, ep)
    # squares of distances between p and each line
    lines_d = dot(n, n)

    # If the point determined by the projection is inside
    # the line segment, it is the point of the line segment
    # closest to p; otherwhise the closest point is one of
    # the enpoints. Determine which of these cases holds 
    # and compute the square of the distance to each line segment. 
    coeffs = dot(proj, se)
    dist = np.select([coeffs < 0, coeffs < dot(se, se), True],
                     [starts_d, lines_d, ends_d])

    # find the index of the closest line segment, its distance to p,
    # and the point in this line segment closest to p
    idx = np.argmin(dist)
    min_dist = dist[idx]
    if min_dist == starts_d[idx]:
        min_point = line_starts[idx]
    elif min_dist == ends_d[idx]:
        min_point = line_ends[idx]
    else:
        min_point = line_starts[idx] + proj[idx]
    return idx, min_dist**0.5, min_point

例子：

line_starts = np.array([[1,1,1], [2,2,2], [3,3,3]])  
line_ends = np.array([[5,1,3], [3,2,1], [3, 1, 1]])
p = np.array([0,1,0])

idx, dist, point = closest(line_starts, line_ends, p)
print(f"index = {idx}\ndistance = {dist}\nclosest point = {point}")

它給：

index = 0
distance = 1.4142135623730951
closest point = [1 1 1]

由於在您的情況下線段是固定的，並且只有點在變化，因此可以針對這種情況進行優化。