# tangent of spline point

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,
Ferdinand

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
``````

-ScottA

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?

cheers,
Ello

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;
sh->InitSpline(sp);
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
``````

-ScottA

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)
``````

Best,
Niklas

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)

bd.SetPen(self.COLOR_TANGENT)
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)
``````

Best,
-Niklas