Python如何处理复杂的计算？

Question

这可能是一个非常简单的问题，但似乎我无法看到它。 我有一个顺时针排序的点列表，想要根据这个使用以下函数计算这些点的质心（凸多边形）：

和

def calculateCentroid(raLinks,raNodes, links, nodes):  

orderedPointsOfLinks = orderClockwise(raLinks,raNodes, links, nodes)

arg1 = 0
arg2 = 0
Xc = 0
Yc = 0
i = 0
for point in orderedPointsOfLinks:
    arg1 += point.Y*(orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].X)
    arg2 += (orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].Y)*point.X
    Xc += (point.X+(orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].X))*(((orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].Y)*point.X)-(point.Y*(orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].X)))
    Yc += (point.Y+(orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].Y))*(((orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].Y)*point.X)-(point.Y*(orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].X)))    
    i+=1

area = (arg1-arg2)*0.5
print area

X = -Xc/(6*area)
Y = -Yc/(6*area)
print X , "   ", Y

使用Arcpy计算面积和centorid表示上述函数计算的面积是正确的，但质心是错误的。

Xc和Yc有什么问题我无法修复它？

如果我按以下方式更改for循环，它可以工作：

    for point in orderedPointsOfLinks:
        y0 = point.Y
        x0 = point.X
        x1 = orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].X
        y1 = orderedPointsOfLinks[i+1 if i+1<len(orderedPointsOfLinks) else 0].Y
        a = x0*y1 - x1*y0
        area += a
        Xc += (x0+x1)*a
        Yc += (y0+y1)*a
         i+=1
    area *= 0.5
    print area

    X = Xc/(6*area)
    Y = Yc/(6*area)
    print X , "   ", Y

这是检查代码的节点列表：

[(371623.876, 6159668.714),(371625.994, 6159661.094), (371624.319, 6159654.634), (371619.654, 6159649.86), (371614.194, 6159647.819), (371608.401, 6159648.449), (371601.544, 6159652.652), (371598.77, 6159658.058), (371599.318, 6159665.421), (371603.025, 6159671.805), (371611.372, 6159674.882 ), (371619.417, 6159673.065)]

Answer 1

资源

尝试：

import numpy

tp = [(371623.876, 6159668.714),(371625.994, 6159661.094), (371624.319, 6159654.634), (371619.654, 6159649.86),\
      (371614.194, 6159647.819), (371608.401, 6159648.449), (371601.544, 6159652.652), (371598.77, 6159658.058), \
      (371599.318, 6159665.421), (371603.025, 6159671.805), (371611.372, 6159674.882 ), (371619.417, 6159673.065),(371623.876, 6159668.714)]



# cx = sigma (x[i]+x[i+1])*((x[i]*y[i+1]) - (x[i+1]*y[i] ))
# cy = sigma (y[i]+y[i+1])*((x[i]*y[i+1]) - (x[i+1]*y[i] ))
cx = 0
cy = 0

p = numpy.array(tp)

x = p[:, 0]
y = p[:, 1]

a = x[:-1] * y[1:]
b = y[:-1] * x[1:]

cx = x[:-1] + x[1:]
cy = y[:-1] + y[1:]



tp = tp[:-1] #dont need repeat


def area():
    tox=0
    toy=0
    for i in range(len(tp)):
        if i+1 == len(tp):
            tox += tp[-1][0]*tp[0][1]
        else:
            tox += tp[i][0]*tp[i+1][1]

    for i in range(len(tp)):
        if i+1 == len(tp):
            toy += tp[-1][1]*tp[0][0]
        else:
            toy += tp[i][1]*tp[i+1][0]
    return abs(tox-toy)*0.5

ar = area()
Cx = abs(numpy.sum(cx * (a - b)) / (6. * ar))
Cy = abs(numpy.sum(cy * (a - b)) / (6. * ar))
print Cx,Cy

警告 ！

tp[0] == tp[-1]

所以：第一个和最后一个坐标是相同的值......