function volume = sphere_imp_zone_volume_3d ( r, h1, h2 )

%*****************************************************************************80
%
%% sphere_imp_zone_volume_3d() computes the volume of a spherical zone in 3D.
%
%  Discussion:
%
%    An implicit sphere in 3D satisfies the equation:
%
%      sum ( ( P(1:DIM_NUM) - CENTER(1:DIM_NUM) )^2 ) = R^2
%
%    Draw any radius of the sphere and note the point P where the radius
%    intersects the sphere.  Now choose two points on the radius line, a
%    distance H1 and H2 from the point P.  Consider all the points on or within
%    the sphere whose projection onto the radius lies between these two points.
%    These points constitute the spherical zone, which can also be considered
%    the difference of two spherical caps.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    07 February 2005
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real R, the radius of the sphere.
%
%    real H1, H2, the distances that define the 
%    thickness of the zone.  H1 and H2 must be between 0 and 2 * R.
%
%  Output:
%
%    real VOLUME, the volume of the spherical zone
%
  h11 = min ( h1, h2 );
  h11 = max ( h11, 0.0 );

  if ( 2.0 * r <= h11 )
    volume = 0.0;
    return
  end

  h22 = max ( h1, h2 );
  h22 = min ( h22, 2.0 * r );

  if ( h22 <= 0.0 )
    volume = 0.0;
    return
  end

  volume = ( 1.0 / 3.0 ) * pi * ( ...
      h22 * h22 * ( 3.0 * r - h22 ) ...
    - h11 * h11 * ( 3.0 * r - h11 ) );

  return
end
