back of beyond screen transition effect

bob_transition.lua

function change_mode(int)
 local modes={
  {menu_init,menu_update,menu_draw},
  {ow_init,ow_update,ow_draw},
  {shmup_init,shmup_update,shmup_draw},
  {gameover_init,gameover_update,gameover_draw}
 }
 second_swipe=false
 next_init=modes[int][1]
 next_update=modes[int][2]
 next_draw=modes[int][3]
 next_init()
 next_update()
 transition_init()
end

function transition_init()
 clr_lines={}
 t_stars={}
 for i=1,128 do
  local x=127+rnd(128)
  add(clr_lines,{
   x=x,
   o_x=x,
   y=i,
  })
  local z=flr(rnd(4))
  add(t_stars,{
   x=ceil(rnd(128)),
   y=i,
   col=star_cols[z+1],
   spd=5-z
  })
 end

 if debug_mode then
  next_init()
  _update=next_update
  _draw=next_draw
 else
  _update=transition_update
  _draw=transition_draw
 end
end

function transition_update() end

function transition_draw()
 camera()
 if second_swipe then
   if (next_update==ow_update) update_camera()
   next_draw()
 end
 local finished=true
 for i=1,128 do
  local clr_line=clr_lines[i]
  local star=t_stars[i]
  star.x-=star.spd
  clr_line.x-=10
  if (clr_line.x>=0) finished=false
  if (star.x<0) star.x=128
  if second_swipe then
   line(0,clr_line.y,clr_line.x,clr_line.y,0)
   if (star.x<clr_line.x) pset(star.x,star.y,star.col)
  else
   line(clr_line.x,clr_line.y,127,clr_line.y,0)
   if (star.x>clr_line.x) pset(star.x,star.y,star.col)
  end
 end
 if finished and not second_swipe then
  second_swipe=true
  for clr_line in all(clr_lines) do
   clr_line.x=clr_line.o_x
  end
 elseif finished then
  next_update()
  _update=next_update
  _draw=next_draw
 end
end