Hi all,

I would like to make a image of magnetic field loops (similar to these:
https://ichef.bbci.co.uk/news/304/media/images/58947000/jpg/_58947314_162169main_trace_solar_flare_lg-1.jpg
https://martianchronicles.files.wordpress.com/2010/05/loops.gif
http://www.space.com/images/i/000/015/747/original/sun-as-art-swirls.jpg
more information here: https://en.wikipedia.org/wiki/Coronal_loop
)
The only difference is that I want them on a disk.

So far I tried to make the loops with multiple "torus" elements,
which are light sources at the same time.
I added more of these in larger sizes to give it a glow/halo effect.
This is all very similar to the lightsabre object example.

However, already with a few of these (8, see code below), the rendering is
getting slow. I think the reason is that the ~130 torus objects are overlapping
each other. This leads to a lot of root tests (742109138, see output below).
I was wondering if you could suggest a smarter technique?

It would also be good to be more flexible than with the torus.
I attached my code below.

Cheers,
       Johannes

$ povray +W400 +H400 -V +Olightarc.png +WT4 -d lightarc.pov
.....
----------------------------------------------------------------------------
Render Statistics
Image Resolution 400 x 400
----------------------------------------------------------------------------
Pixels:           160000   Samples:               0   Smpls/Pxl: 0.00
Rays:             301581   Saved:                 0   Max Level: 1/5
----------------------------------------------------------------------------
Ray->Shape Intersection          Tests       Succeeded  Percentage
----------------------------------------------------------------------------
Box                          401896791          652287      0.16
Cone/Cylinder                    89831           70491     78.47
CSG Intersection             401896791       120763539     30.05
Torus                        401896791       129236959     32.16
Torus Bound                  401896791       141316940     35.16
Bounding Box                 742109138       532510992     71.76
----------------------------------------------------------------------------
Roots tested:             141316940   eliminated:             16711239
Media Intervals:             125201   Media Samples:           1627613 (13.00)
Shadow Ray Tests:          17116032   Succeeded:              16027193
Shadow Cache Hits:              363
Transmitted Rays:            141581
----------------------------------------------------------------------------
----------------------------------------------------------------------------
Render Time:
  Photon Time:      No photons
  Radiosity Time:   No radiosity
  Trace Time:       0 hours  3 minutes 56 seconds (236.095 seconds)
              using 4 thread(s) with 781.471 CPU-seconds total



Code:

// Magnetic field on a disk
//
#include "colors.inc"
#include "rand.inc"

// options of this script
#declare number_of_subarcs=8;
#declare number_of_arcs=8;
#declare disk_radius=4;

camera {
 location <4,1,4>
 look_at <0,0,0>
}
background {color Black}

// set up disk
cylinder {
 <0,0,0>
 <0,-0.04,0>
 disk_radius
 pigment { color White }
 finish { phong 0 diffuse 0.2 ambient 0 }
}

// a single arc is a thin half-torus
#declare arc=
difference {
 torus {
  5, 0.04           // major and minor radius
  rotate -90*x      // make vertical
 }
 box {<-6,-6,-1>, <6,0,1>}
}
// this thicker torus is used to make a halo glow around the thing arcs
#declare thickarc=
difference {
 torus {
  5, 0.5
  rotate -90*x
 }
 box {<-6,-6,-1>, <6,0,1>}
}

#declare r1=seed(5); // defines the shape
// now comes the combination of objects
#declare glowarc= union {
light_source { <0, 5, 0> color rgb <0,0,.5> }
// bright white arc
object {arc pigment{Clear} hollow
 interior { media{ emission <1,1,1>*100 } }
}
/*object {
 thickarc pigment{Clear} hollow
 interior { media{emission <.01,.01,1>*2 density { spherical scale <5,100,100> }
} }
 translate <0,0.01,0>
}*/
// fainter blue arcs
#declare i=0;
#while (i<number_of_subarcs)
 #declare i = i + 1;
 // we jitter them slightly
 #declare myscale = 1/Rand_Normal(1,0.05,r1);
 #declare xrot = Rand_Normal(0, 7, r1);
 #declare yrot = Rand_Normal(0, 3, r1);
 #declare zrot = 0;
 object {
  arc pigment{Clear} hollow
  scale myscale
  rotate <xrot,yrot,zrot>
  interior { media{
   emission <0,0,1>*pow(10,1+1*rand(r1))
  } }
 }
 // make fuzzy, weak halos with thickarc objects
 object {
  thickarc pigment{Clear} hollow
  scale myscale
  rotate <xrot,yrot,zrot>
  interior { media{emission <.01,.01,1>*(0.4/number_of_subarcs)} }
 }
#end
}

#declare r2=seed(2); // defines the locations

// now we can create a couple of these
#declare i=0;
#while (i<number_of_arcs)
 // find a uniform location on the disk to put it
 #declare myx = (rand(r2)-0.5)*3*2;
 #declare myy = (rand(r2)-0.5)*3*2;
 #declare d = sqrt(pow(myx,2)+pow(myy,2));
 #declare phi = atan2(myy,myx);
 // make sure the radius is not outside the disk
 #if (d < disk_radius-1)

 // orient the arc so it tends to point to the disk center
 #declare phi2 = phi + 45*(rand(r2)-0.5)/180*3.14;
 object{ glowarc
  scale 0.05+0.1*rand(r2)
  rotate y*(phi2/3.14*180+90)
  translate <d*sin(phi),0,d*cos(phi)>
 }
 #declare i = i + 1;
 #end
#end

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