-- Title: Orbit an object
-- Author: Chris Laurel

-- constants
km = 1.0/9460730.4725808
sec = 1.0/86400
deg = math.pi / 180


-- Smoothstep function is 0 at x = 0, 1 at x = 1
-- derivative is zero at both x = 0 and x = 1
function smoothstep(x)
   return (3 - 2 * x) * x * x
end

-- Smoothstep2 is exactly like smoothstep, except that
-- the derivative is 1 at x=1
function smoothstep2(x)
   return (2 - x) * x * x
end


-- Circularly orbit the observer frame center. The period of the
-- orbit is given in seconds.
function orbit_object(period)
   local obs = celestia:getobserver()
   local obsFrame = obs:getframe()
   local center = obsFrame:getrefobject()

   -- If there's no followed object, use the current selection
   if not center then
       center = celestia:getselection()
   end

   if not center then return end

   -- v1 is the vector from the viewer to the center
   -- up is the camera up direction
   -- v2 is normal to both v1 and up
   -- Orbit in the plane containing v1 and normal to the up direction
   local now = celestia:gettime()
   local v1 = obs:getposition() - center:getposition(now)
   local distance = v1:length()
   v1 = v1:normalize()

   local up = obs:getorientation():transform(celestia:newvector(0, 1, 0))
   local v2 = v1 ^ up
   v2 = v2:normalize()

   start = celestia:getscripttime()

   while true do
      local t = (celestia:getscripttime() - start) / period
      if t < 1 then t = smoothstep2(t) end
      local theta = 2 * math.pi * t

      local v = math.cos(theta) * v1 + math.sin(theta) * v2

      obs:setposition(center:getposition() + v * distance)
      obs:lookat(center:getposition(), up)

      wait(0)
   end
end

orbit_object(30)