// Last edited on 2018-10-27 18:07:43 by stolfilocal

  // The files "field_{NN}.inc" should define a macro
  // called "field" with parameters {(xp,yp,zp,tt,zrad)}
  // where {xp,yp,zp} are the space coordinates,
  // {tt} is time in the animation (if any).
  
  // On entry, {xp,yp,zp} should be scaled so that {xp,yp} range 
  // from {-1} to {+1}, and the layers are spaced by {zrad} in the 
  // Z direction.

  // The procedure {field} should return the activity (for ECR) or potential (for EEG) 
  // in the neighborhood of the point {(xp,yp,zp)} of the plot cylinder.
  
  // Since movies start at {tt=0} and end at {tt=1},
  // the fild should be periodic in {tt} with period 1.
  // It may be independent of {tt} (i.e. the flow may be steady-state).

  // The files "field_{NN}.inc" should also define the scalar
  // {field_max} with the estimated maximum magnitude of the field's value,
  // for arrow length scaling purposes.

#macro field_eval_blobs(xp,yp,zp, NB, bctr,brad,bval, zrad)
  // Evaluates a field defined by {NB} blobs whose
  // parameters are in {bctr,brad,bval}. Namely {brad[i]} is the
  // center of the blob, {brad[i]} is the XY radius, and {bval[i]} 
  // is the field value at the center.  Every blob will have radius
  // {zrad} in the Z direction.
  
  #local fld = 0.0;
  
  #local i = 0;
  #while (i < NB)
    #local bc = bctr[i]; #local br = brad[i]; #local bv = bval[i];
    #local dx = (xp - bc.x)/br;
    #local dy = (yp - bc.y)/br;
    #local dz = (zp - bc.z)/(1.5*zrad);
    #local d2 = dx*dx + dy*dy + dz*dz;
    #if (d2 < 1.0)
      #local bump = (1 - d2)*(1 - d2);
      #local fld = fld + bv*bump;
    #end
    #local i = i + 1;
  #end

  fld
  
#end