I have been using the Colefax spline macro for decades.
I have a new animation spline that gives unexpected results and I can't 
figure out why. (see below)
As I walk through the spline clock, this one iteration reverses 
direction 180 degrees and moves back towards the origin. Then it 
reverses direction again and continues normally, but the damage is done.
By tweaking the spline points, and the loop increment, I can move the 
reversed frame (currently #4 in the code below) but I can't get rid of 
it.  Help?

The interesting output is this.  Note the one y=90 frame.

PushStool: 0.00 StoolVec=<200.00, 1.00, 200.00>
PushStool: 0.00 StoolRot=<0.00, -90.00, 0.00>
PushStool: 0.01 StoolVec=<197.84, 1.00, 200.00>
PushStool: 0.01 StoolRot=<0.00, -90.00, 0.00>
PushStool: 0.02 StoolVec=<195.68, 1.00, 200.00>
PushStool: 0.02 StoolRot=<0.00, -90.00, 0.00>
PushStool: 0.03 StoolVec=<194.36, 1.00, 200.00>
PushStool: 0.03 StoolRot=<0.00, 90.00, 0.00>
PushStool: 0.04 StoolVec=<193.47, 1.00, 200.00>
PushStool: 0.04 StoolRot=<0.00, -90.00, 0.00>
PushStool: 0.05 StoolVec=<191.31, 1.00, 200.00>
PushStool: 0.05 StoolRot=<0.00, -90.00, 0.00>
PushStool: 0.06 StoolVec=<189.15, 1.00, 200.05>
PushStool: 0.06 StoolRot=<0.00, -87.56, 0.00>
PushStool: 0.07 StoolVec=<186.99, 1.00, 200.16>
PushStool: 0.07 StoolRot=<0.00, -86.61, 0.00>
PushStool: 0.08 StoolVec=<184.83, 1.00, 200.30>
PushStool: 0.08 StoolRot=<0.00, -86.26, 0.00>
PushStool: 0.09 StoolVec=<182.68, 1.00, 200.44>
PushStool: 0.09 StoolRot=<0.00, -86.15, 0.00>
PushStool: 0.10 StoolVec=<180.52, 1.00, 200.58>
PushStool: 0.10 StoolRot=<0.00, -86.19, 0.00>



------------ >% code %< ---------------
// splinetest.pov
// with which Colefax's spline macro and I disagree

#include "colors.inc"
#include "splineColefax.inc"

#declare feet=12;
#declare foot=12;
#declare inches=1;
#declare inch=1;

#macro vdebug(vec)
  concat("<", str(vec.x,0,2), ", ",str(vec.y,0,2), ", ", str(vec.z,0,2), 
">")
#end

#declare arrow=
difference {
  box { <-1, -1, -1> <1,1,1> }
  plane { x, 0 rotate y*45 translate x*-1 }
  plane { -x, 0 rotate y*-45 translate x*1 }
  scale 0.2
}

#declare StoolPushStartVec=<200*inches, 1*inch, 200*inches>;
#declare StoolAtCookiesVec=<1*inches, 1*inch, 200*inches>;

#declare stoolToCookiesPathSpline=create_spline(
  array[10]{StoolPushStartVec,
            StoolPushStartVec+<-4*inches, 0, -0*inches>,
            StoolPushStartVec+<-6*inches, 0, -0*inches>,
            StoolPushStartVec+<-9*inches, 0, -0*inches>,
            StoolPushStartVec+<-60*inches, 0, 0>,
            StoolAtCookiesVec+<70*inches, 0, -25*inches>,
            StoolAtCookiesVec+<40*inches, 0, 0>,
            StoolAtCookiesVec+<8*inches, 0, 0>,
            StoolAtCookiesVec+<4*inches, 0, 0>,
            StoolAtCookiesVec}
  create_default_spline + spline_loop (no) + spline_tension (0.2) + 
spline_sampling(100.0)
  )
  //preview_spline (stoolToCookiesPathSpline, spline_preview_pigment
(pigment {Red}))

#local _i=0;
#while (_i<0.1)
  evaluate_spline(stoolToCookiesPathSpline, spline_clock(_i))
  #declare StoolVec=spline_pos;
  #declare StoolRot=spline_heading+<0,0,0>; 
  object {
    arrow
    texture{ pigment{color rgb <1-_i*10/2,0.75+_i*10/4,_i*10>}}
    rotate StoolRot
    translate StoolVec
  }

  #debug concat("PushStool: ", str(_i,0,2), " StoolVec=", vdebug
(StoolVec), "\n")
  #debug concat("PushStool: ", str(_i,0,2), " StoolRot=", vdebug
(StoolRot), "\n")
  #local _i=_i+0.01;
#end
  
  evaluate_spline(stoolToCookiesPathSpline, spline_clock(0.1))
  sphere {
    spline_pos, 0.4
    pigment {Blue}
  }

camera {
    location <StoolPushStartVec.x, 18*inches, StoolPushStartVec.z>+<-10
*inches, 0, 0>
    look_at StoolPushStartVec+<-10*inches, 0, 0>
}
  light_source {  <StoolPushStartVec.x, 38*inches, StoolPushStartVec.z> 
color rgb 1}

Post a reply to this message

Dick Balaska <dic### [at] buckosoftcom> wrote:
> I have been using the Colefax spline macro for decades.
> I have a new animation spline that gives unexpected results and I can't
> figure out why. (see below)
> As I walk through the spline clock, this one iteration reverses
> direction 180 degrees and moves back towards the origin. Then it
> reverses direction again and continues normally, but the damage is done.
> By tweaking the spline points, and the loop increment, I can move the
> reversed frame (currently #4 in the code below) but I can't get rid of
> it.  Help?

The type of spline used is a bitch, and does things like that.

This type of spline is designed to make as smooth a curve, with as smooth an
acceleration, as possible, while going through the specified points. It will
resist any attempt to force it into jerky movements, and instead prefer to swing
back and forth at any point where you try to do that.

Moving the 2nd point a bit towards the starting point and moving the 4th point a
good deal away should help you solve the issue, but it might not be the
trajectory you want.

Also note that it won't help you to add more control points: The more you try to
pin down the spline to some desired trajectory, the more likely it will exhibit
noticeable swings.

I'd personally recommend using a different spline type, one that does not go
through the control points but rather touches the lines between adjacent control
points tangentially; those are much less prone to undesired swings.

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