[英]Angle between three points in python - Why this result?
I tried to calculate the angle ABC between points A, B and C. I know the math are pretty basic but I don't understand why my function give the wrong result.我试图计算点 A、B 和 C 之间的角度 ABC。我知道数学很基础,但我不明白为什么我的函数给出了错误的结果。 First, here is the code (
a
contains a list [x, y, z]
)首先,这是代码(
a
包含一个列表[x, y, z]
)
def angle(a, b, c):
# Create vectors from points
ba = [a[0] - b[0], a[1] - b[1], a[2] - b[2]]
bc = [c[0] - b[0], c[1] - b[1], c[2] - b[2]]
# Normalize vector
nba = sqrt(ba[0]**2 + ba[1]**2 + ba[2]**2)
ba = [ba[0]/nba, ba[1]/nba, ba[2]/nba]
nbc = sqrt(bc[0]**2 + bc[1]**2 + bc[2]**2)
bc = [bc[0]/nbc, bc[1]/nbc, bc[2]/nbc]
# Calculate scalar from normalized vectors
scal = ba[0]*bc[0] + ba[1]*bc[1] + ba[2]*bc[2]
# calculate the angle in radian
angle = acos(scal)
This function gives wrong result.这个函数给出了错误的结果。 In fact, it gives the good result if I change the second vector from
bc
to cb
:事实上,如果我将第二个向量从
bc
更改为cb
,它会给出很好的结果:
cb = [b[0]-c[0], b[1]-c[1], b[2]-c[2]]
I don't understand why, as if I follow math, my first solution should work well and give the good result...我不明白为什么,就像我遵循数学一样,我的第一个解决方案应该运行良好并给出好的结果......
Firstly, your code is very non-pythonic.首先,您的代码非常非 Pythonic。 Here is a suggestion:
这是一个建议:
from math import sqrt, acos
def angle(a, b, c):
# Create vectors from points
ba = [ aa-bb for aa,bb in zip(a,b) ]
bc = [ cc-bb for cc,bb in zip(c,b) ]
# Normalize vector
nba = sqrt ( sum ( (x**2.0 for x in ba) ) )
ba = [ x/nba for x in ba ]
nbc = sqrt ( sum ( (x**2.0 for x in bc) ) )
bc = [ x/nbc for x in bc ]
# Calculate scalar from normalized vectors
scalar = sum ( (aa*bb for aa,bb in zip(ba,bc)) )
# calculate the angle in radian
angle = acos(scalar)
return angle
Secondly, your code is probably returning the correct angle, but maybe not the angle you expected.其次,您的代码可能返回正确的角度,但可能不是您期望的角度。
Assuming this scenario:假设这个场景:
A-----C
| /
| /
| /
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B
The angle you're calculating is the bottom angle at B, not the top-left angle at A which is usually what people want when they pass three vectors (a,b,c) into a function that returns angles.您计算的角度是 B 处的底角,而不是 A 处的左上角,这通常是人们将三个向量 (a,b,c) 传递给返回角度的函数时想要的。
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