"twister" <twi### [at] o2pl> wrote in message 
news:web.499c2f4a4c7fcf678896485d0@news.povray.org...
> "Chris B" <nom### [at] nomailcom> wrote:
>> I imagine that you are describing something lying flat on the XZ-plane
>> winding around like a track or a roadway or something like that?
>>
>> If that is what you want, then you could define the centre line as a 
>> POV-Ray
>> spline, then generate a POV-Ray prism object that follows the line of the
>> spline with appropriate offsets to the left and right (to keep the width
>> constant). To do this you could build the prism using two #while loops. 
>> The
>> first would follow the spline from 0 to 1 and offset to the left, the 
>> second
>> would follow the spline back from 1 to 0, offsetting to the right.
>>
>> To offset from a spline you need to get the direction of the spline at 
>> the
>> point you're interested in. You can do this using a point slightly ahead 
>> of
>> or slightly behind the current position. You can then use the 
>> VPerp_To_Plane
>> function from "math.inc" with the 'up' vector (y) to get a vector 
>> pointing
>> out to the left (or right). Add this vector times a distance to the 
>> current
>> position to get the new position that is offset from the spline.
>
> Yes, you're right! I want something that is quite like a road object with
> constant width and placed on X-Z plane.
> I tried to use prism, but it didn't work as required (probably something 
> was
> wrong). I used a pair describing each point: point-coordinates and its 
> vector
> of "movement". The vector allowed me to get side points that are in 
> constant
> distance from center of line, but I still can't get smooth 'road'. Could 
> you
> provide me some example as I am a bit new to povray or point some 
> reference how
> to handle it?
>

The example below illustrates two alternative techniques. The first simply 
draws a succession of spheres that follow the spline, then slices a thin 
Yellow slice out of the middle. The second uses the technique I described 
above, with two #while loops to create a prism that follows one side of a 
spline up and the other side back (Orange).  Note that the second #while 
loop effectively runs backwards (from 100 to 0).

Regards,
Chris B.

camera {location <0,5,-3> look_at 0}
light_source {<30,70,-10> color rgb 1}

#include "math.inc"

#declare MyWidth = 0.6;
#declare MySpline = spline {
  cubic_spline
  -.25, <3,0,0.2>
  0.00, <2,0,0.2>
  0.25, <0,0,1>
  0.50, <-2,0,0>
  0.75, <-3,0,1>
  1.00, <-3,0,2>
  1.25, <-3,0,4>
}

difference {
  union {
    #declare I = 0;
    #while (I < 1)
      sphere {
        MySpline(I),.15
      }
      #declare I = I + 0.01;
    #end
  }
  plane { y,-0.01}
  plane {-y,-0.01}
  pigment { rgb <1,1,0> }
  translate 0.5*y
}


prism {
  linear_spline
  0,0.5,202
  #declare I = 0;
  #while (I <= 100)
    #declare ThisPosition = MySpline(I/100);
    #declare AheadVector = MySpline((I+1)/100)-ThisPosition;
    #declare LeftVector = vnormalize(VPerp_To_Plane(AheadVector,y));
    #declare SidePosition = ThisPosition + LeftVector*MyWidth/2;
    <SidePosition.x,SidePosition.z>
    #declare I = I + 1;
  #end
  #declare I = 100;
  #while (I >=0)
    #declare ThisPosition = MySpline(I/100);
    #declare AheadVector = MySpline((I+1)/100)-ThisPosition;
    #declare LeftVector = vnormalize(VPerp_To_Plane(AheadVector,-y));
    #declare SidePosition = ThisPosition + LeftVector*MyWidth/2;
    <SidePosition.x,SidePosition.z>
    #declare I = I - 1;
  #end
  pigment { rgb <1,0.1,0> }
}

Post a reply to this message