tangent of spline point

  • On 02/05/2013 at 11:28, xxxxxxxx wrote:

    User Information:
    Cinema 4D Version:   r14 
    Language(s) :     C++  ;

    hi, can someone tell me how i can get the tangent at point(x) of a spline, where x is a number in GetPointCount? or how i get the spline position that point x has, so i can use GetSplineTangent?



  • On 02/05/2013 at 12:38, xxxxxxxx wrote:

    Read the about SplineObject class in the SDK it has methods to deal with tangents. 
    Or get your spline objects tags, it will return a hidden PointTag like all PointObjects 
    do and a TangentTag. To understand point and tangent tags read about VariableTag 
    in the SDK.

  • On 02/05/2013 at 14:07, xxxxxxxx wrote:

    but how do i access the spline tangent at a real point, not at the position (0...1)along the spline. thats my problem.

  • On 02/05/2013 at 15:05, xxxxxxxx wrote:

    Your first posting implied that you want to get the tangents for the control points of your spline, 
    as you spoke of PointObject.GetPointCount(). Both PointTags and TangentTags hold the data of
    control points/ vertices . In the SplineObject there are the GetTangent methods to access the 
    indexed tangents.

    I am not sure what you do understand as a 'real' point, as you seem to refer to a vector with it,
    while the term real in that context is actually bound to 0.0-1.0 offset - real offset as a reference 
    to the space in real world units (200cm instead of 0.5).
    If you just want to sample an arbitrary point between two vertices aka control points, I am not 
    sure why cannot use the general real offset methods provided by cinema 4d. There is also the
    SplineHelper class providing some convenience methods to handle spline objects. You can also
    get the underlying LineObject of a spline to access the sampled sub points rather than dealing
    with mathematical perfect representation of the spline.

    Happy rendering,

    edit: fixed some bs i wrote ;)

  • On 02/05/2013 at 18:52, xxxxxxxx wrote:

    I'm having a hard time understanding exactly what you want too.

    Here's an example of creating a spline. And setting all of the tangents to a specific Y value.
    If you only want one/or certain tangents to be set instead of all of them. Then use a conditional statement with an array index value inside of the loop. Instead of using "i" for setting all of them:

    //This is an example of creating a spline and setting it's points and tangent handles  
      SplineObject *sp = SplineObject::Alloc(3, SPLINETYPE_BEZIER); //Create a bezier spline with 3 points  
      if(!sp) return NULL;  
      doc->InsertObject(sp, NULL, NULL, 0);  //Add it to the OM  
      Vector *gp = sp->GetPointW();          //Assign the array of spline points to a variable  
      gp[0] = Vector(10,0,-100);             //Place the first spline point here  
      gp[1] = Vector(0,0,0);                 //Place the second spline point here  
      gp[2] = Vector(-10,0,100);             //Place the third spline point here  
      Tangent *tangents = sp->GetTangentW(); //Assign the array of tangents to a variable  
      //Loop through all of the tangents  
      for (LONG i = 0; i < sp->GetTangentCount(); i++)  
          tangents[i].vl.y = 200;           //Set each spline point's left tangent to this position  
          tangents[i].vr.y = -200;          //Set each spline point's right tangent to this position  
      sp->Message(MSG_UPDATE);              //Update the spline's changes


  • On 05/05/2013 at 10:55, xxxxxxxx wrote:

    thank you. i didnt try GetTangentW, because in the sdk it says that it returns the first element. now i get the tangents. however, i had the hope to use the tangents to create a correctly offset position for the points. but, in liniear splines there are no tangents. how can i calculate the direction for the offset so that positive values go into one direction, and negative values go to the opposite direction? (see image)

    i did it like this:

    Vector winkel = VectorToHPB(p2-p1);
    Matrix richtung = HPBToMatrix(winkel, ROTATIONORDER_DEFAULT);

    however, that resulted in point offsets to unwanted directions, like if you changed the green and red for only some of the points in the image. thats why i thought using the tangents could help..

    maybe there is another solution?


  • On 05/05/2013 at 11:57, xxxxxxxx wrote:

    If I wanted to move the points of a spline.
    This is the way I'd do it:

    #include "lib_splinehelp.h"  
      BaseDocument *doc = GetActiveDocument();  
      BaseObject *spline = doc->SearchObject("Spline");  
      PointObject *sp = ToPoint(spline);  
      AutoAlloc<SplineHelp> sh;  
      if(!sh) return FALSE;  
      if(!sh->Exists()) return FALSE;  
      Vector *spPnts = sp->GetPointW();           //Get all the points in the spline  
      Real offsetX = 100.0;  
      Real offsetY = 100.0;                       //Some offset values that can be bound to GUI gizmos  
      Real offsetZ = 100.0;  
      LONG pntCnt = sh->GetPointCount();  
      for(LONG i=0; i<pntCnt; i++)  
          spPnts[i].x += offsetX;  
          spPnts[i].y += offsetY;                 //Move all the points in the spline by this offset amount  
          spPnts[i].z += offsetZ;  
      sp->Message(MSG_UPDATE);                    //Update the changes made to the spline


  • On 06/05/2013 at 00:15, xxxxxxxx wrote:

    yes, i know how to move points, but how do i get the direction when there is no tangent and when  i need to take care that the points need to move all relatively to the right or left side of the spline.. thats the main problem i have here

  • On 06/05/2013 at 03:07, xxxxxxxx wrote:

    You can get the bisecting vector of a point by calculating the arithmetic middle of the adjacent points.

  • On 06/05/2013 at 04:26, xxxxxxxx wrote:

    i guess you mean something like

    tRot = Vector((getAngle(points[i-1],points[i]).x+getAngle(points[i+1],points[i]).x)/2,0,0);
    Matrix richtung = HPBToMatrix(tRot, ROTATIONORDER_DEFAULT);
    tPos += Vector(0,0,_ipOffset)*richtung;

    this still makes some objects move to the one direction end others to the opposite direction...

  • On 06/05/2013 at 14:13, xxxxxxxx wrote:

    double post, sorry.

  • On 06/05/2013 at 14:15, xxxxxxxx wrote:

    Not exactly. But I expressed myself not 100% correct. You can get a point that lies on the bisecting
    line this way:

    p1 = points[i]
    p2 = (points[i - 1] + points[i + 1]) * (1 / 2.0)   // I think division is not supported by the vector class
    bv = (p2 - p1).GetNormalized()

    Then again you can cross it with one of the adjacent lines and cross it
    again and you have the tangent:

    n = bv.Cross(points[i] - points[i - 1])
    tangent = bv.Cross(n)


  • On 06/05/2013 at 21:32, xxxxxxxx wrote:

    thank you niklas. i'll try if this helps me solve my problem and report back...

  • On 07/05/2013 at 01:44, xxxxxxxx wrote:

    hm, this still produces the wrong direction for some points. i have no idea how to get this working...

  • On 07/05/2013 at 03:18, xxxxxxxx wrote:

    Hi ello,

    here, some example code quickly plucked together.

    import c4d
    class Tag(c4d.plugins.TagData) :
        SIZE_SPHERE = c4d.Vector(10)
        COLOR_SPHERE = c4d.Vector(1, 0.66, 0.02)
        LENGTH_TANGENT = 20
        COLOR_TANGENT = c4d.Vector(0.1, 0.95, 0.5)
        def Draw(self, tag, op, bd, bh) :
            if not isinstance(op, c4d.SplineObject) :
                return True
            bd.SetMatrix_Matrix(op, op.GetMg())
            segments = [op.GetSegment(i) for i in xrange(op.GetSegmentCount())]
            if not segments:
                segments = [{'cnt': op.GetPointCount(), 'closed': op.IsClosed()}]
            pi = 0
            for segment in segments:
                cnt = segment['cnt']
                closed = segment['closed']
                start = pi
                end = pi + cnt
                i_start = start
                i_end = end
                if not closed:
                    i_start += 1
                    i_end -= 1
                    bd.DrawSphere(op.GetPoint(pi), self.SIZE_SPHERE, self.COLOR_SPHERE, 0)
                    bd.DrawSphere(op.GetPoint(pi + cnt - 1), self.SIZE_SPHERE, self.COLOR_SPHERE, 0)
                if start < end:
                    for i in xrange(i_start, i_end, 1) :
                        left = i - 1
                        right = i + 1
                        if left < start:
                            left = end - 1
                        if right >= end:
                            right = start
                        print left, i, right
                        pleft = op.GetPoint(left)
                        pright = op.GetPoint(right)
                        point = op.GetPoint(i)
                        mid = (pleft + pright) * 0.5
                        bv = point - mid
                        n = bv.Cross(point - pleft)
                        tangent = n.Cross(bv).GetNormalized() * self.LENGTH_TANGENT
                        pmin = point - tangent
                        pmax = point + tangent
                        bd.DrawLine(pmin, pmax, 0)
                pi += cnt
            print "-------------------"
            return True
    if __name__ == "__main__":
        c4d.plugins.RegisterTagPlugin(100003, "Spline Tangents", c4d.TAG_VISIBLE, Tag, "Tsplinetangents", None)



  • On 07/05/2013 at 03:20, xxxxxxxx wrote:

    Originally posted by xxxxxxxx

    this still makes some objects move to the one direction end others to the opposite direction...

    Uhm, I just see: The result in the image is correct. What else did you expect?
    The only thing left would be to rotate them about 90° to get the actual tangent (see my example code).

  • On 07/05/2013 at 04:05, xxxxxxxx wrote:

    thank you very much, but when i rotate the direction the problem still remains. it just doesnt move left/right but back and forward.. still some point to the opposite..

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