如何获取Maya中两个曲线点之间的arclen？

Question

在Maya 2015年，我可以使用这个命令来获得曲线的ARCLEN：

cmds.arclen('bezier1') 

但现在我想要得到的两点ARCLEN在我的曲线。无论如何得到这个？

Answer 1

使用Maya的API肯定是做@scottiedoo所说的最好的方式，但这是我在不知道API时给出的一个函数，它给了你相同的结果。

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 

Answer 2

使用Maya API，您可以使用MFnNurbsCurve::findLengthFromParam（仅适用于Maya 2016+）。如果两点之间需要它，则用每个参数调用该函数并减去。

如果您不想使用api，那么另一个选项是创建您的原始曲线的副本，并在需要的点使用“分离”它，然后在该新曲线上使用arclen命令来获得您的长度。那是另一种方式。

请注意，分离曲线时，似乎尝试尽量保持曲率尽可能接近原始值，但这并不准确，所以与原始曲线相比，长度可能不同。也许重建曲线以获得更多点可能会提高精度，如果这对您而言是一个重要因素。