np.ndarray 由 np.ndarray 索引

Question

def compute_normal(vertices, faces):
    norm = np.zeros(vertices.shape, dtype=vertices.dtype)
    tris = vertices[faces]
    ## skip the rest of the code ##

其中vertices是形狀 (A,3) 的np.ndarray ， faces是形狀 (B,3) 的np.ndarray ， tris變成形狀 (B,3,3)。 （順便說一句，B>A）

tris = vertices[faces]實際上在做什么？

Answer 1

也許這個例子可以幫助你：

import numpy as np
np.random.seed(123)

vertices = np.random.rand(3, 3)            # shape -> (3, 3)
faces = np.random.randint(3, size=(4, 3))  # shape -> (4, 3)

print(vertices)
# [[0.69646919 0.28613933 0.22685145]
#  [0.55131477 0.71946897 0.42310646]
#  [0.9807642  0.68482974 0.4809319 ]]

print(faces)
# [[1 0 2]
#  [0 1 2]
#  [1 0 0]
#  [0 0 1]]

print(vertices[faces]) # shape -> (4, 3, 3)
# [[[0.55131477 0.71946897 0.42310646]   # faces==1 -> take second row of vertices
#   [0.69646919 0.28613933 0.22685145]   # faces==0 -> take first  row of vertices
#   [0.9807642  0.68482974 0.4809319 ]]  # faces==2 -> take third  row of vertices

#  [[0.69646919 0.28613933 0.22685145]   # faces==0 -> take first  row of vertices
#   [0.55131477 0.71946897 0.42310646]   # faces==1 -> take second row of vertices
#   [0.9807642  0.68482974 0.4809319 ]]  # faces==2 -> take third  row of vertices

#  [[0.55131477 0.71946897 0.42310646]   # faces==1 -> take second row of vertices
#   [0.69646919 0.28613933 0.22685145]   # faces==0 -> take first  row of vertices
#   [0.69646919 0.28613933 0.22685145]]  # faces==0 -> take first  row of vertices

#  [[0.69646919 0.28613933 0.22685145]   # faces==0 -> take first  row of vertices
#   [0.69646919 0.28613933 0.22685145]   # faces==0 -> take first  row of vertices
#   [0.55131477 0.71946897 0.42310646]]] # faces==1 -> take second row of vertices