如何在 Maya 中获得两个曲线点之间的 arclen？

Question

在 Maya 2015 中，我可以使用以下命令获取曲线的 arclen：

cmds.arclen('bezier1')

但现在我想得到曲线中两点的弧线。有没有办法得到这个？

Answer 1

使用 Maya API，您可以使用 MFnNurbsCurve::findLengthFromParam（仅限 Maya 2016+）。如果你需要在两点之间，然后用每个参数调用这个函数并减去。

如果您不想使用 api，那么另一个选项是创建原始曲线的副本并在需要的点处使用“分离”它，然后在新曲线上使用 arclen 命令来获取您的长度.所以这是另一种方式。

请注意，在分离曲线时，它似乎试图使曲率尽可能接近原始曲线，但这并不准确，因此与原始曲线相比，长度可能不同。如果这对您来说是一个重要因素，也许重建曲线以获得更多点可能会提高准确性。

Answer 2

正如@scottiedoo 所说，使用 Maya 的 API 无疑是最好的方法，但这是我在不了解 API 时创建的一个函数，它可以为您提供相同的结果。

from maya import cmds

def computeCrvLength(crv, startParam = None, endParam = None):
    '''
    Compute the length of a curve between the two given UParameters. If the both
    UParameters arguments are set to None (default), will compute the length of
    the whole curve.

    Arguments:
    - crv = string; an existing nurbCurve
    - startParam = 0 <= float <= 1 or None; default = None; point parameter
      value, if not None, will compute the points only between the startPt and
      EndPt values.
    - endParam = 0 <= float <= 1 or None; default = None; point parameter
      value, if not None, will compute the points only between the startPt and
      EndPt values.

    Returns:
    - The length of the curve between the given UParameters
    - The length of the curve from its start to the startParam
    - The length of the curve from its start to the endParam
    '''

    ###### Exceptions
    if cmds.objExists(crv) == False:
        cmds.error ('The curve "%s" does\'nt exists.' % crv)

    if cmds.filterExpand (crv, sm = 9) == None:
        cmds.error ('The object "%s" is not a nurbCurve.' % crv)

    if startParam != None:
        if (0 <= startParam <= 1) == False:
            cmds.error ('The start point parameter value must be between 0 and 1.')

    if endParam != None:
        if (0 <= endParam <= 1) == False:
            cmds.error ('The end point parameter value must be between 0 and 1.')

    if (startParam == None and endParam != None) or (startParam != None and endParam == None):
        cmds.error ('The start and end points parameters must be both None or ' + 
                    'both have values.')

    if startParam != None and endParam != None:
        if endParam < startParam:
            cmds.error ('The end point parameter value cannot be less or ' + 
                        'equal to start point parameter value.')

    ###### Function
    if startParam == None and endParam == None:

        crvLength = cmds.arclen (crv, ch = False)
        distCrvToStartParam = 0
        distCrvToEndParam = crvLength

    else:

        tmpArclenDim = cmds.arcLengthDimension (cmds.listRelatives(crv, s = True)[0]
                                                + '.u[0]')
        cmds.setAttr (cmds.listRelatives(tmpArclenDim, p = True)[0] +
                      '.uParamValue', startParam)
        distCrvToStartParam = cmds.getAttr (tmpArclenDim + '.al')
        cmds.setAttr (cmds.listRelatives(tmpArclenDim, p = True)[0] +
                      '.uParamValue', endParam)
        distCrvToEndParam = cmds.getAttr (tmpArclenDim + '.al')
        cmds.delete (tmpArclenDim)
        crvLength = (distCrvToEndParam - distCrvToStartParam)

    return crvLength, distCrvToStartParam, distCrvToEndParam