graphics - Creating ghost trail effects

Does anyone know how to create a ghost trail effect? For a simple example look at this screenshot:

You can find the actual animation here. What I would ultimately like to see it happen is to make the object move based on whatever equations you specify it. For instance, to make it move around a circle the object should have the position (cos[t], sin[t]). Or, lets say you have a list of specified coordinates {(x1,y1), (x2,y2), ..., (xn,yn)}, All I want to be able to see is the trace as an object takes in the coordinates I specify.

Here is a simple ball moving without the ghosting effect.

Animate[
 Graphics[
  Disk[{Cos[u], Sin[u]}, .25],
  PlotRange -> {{-2, 2}, {-2, 2}},
  ImageSize -> 400, 
  Axes -> True
 ],
 {u, 0, 6}

]

Answer

Here is a simple approach to create a ghost trail:

obj[{xfunc_, yfunc_}, rad_, lag_, npts_][x_] := MapThread[
 {Opacity[#1, ColorData["SunsetColors", #1]],
  Disk[{xfunc@#2, yfunc@#2}, rad Exp[#1 - 1]]} &, 
 Through[{Rescale, Identity}[Range[x - lag, x, lag/npts]]]]


frames = Most@Table[Graphics[obj[{Sin[2 #] &, Sin[3 #] &}, 0.1, 1, 500][u], 
  PlotRange -> {{-2, 2}, {-2, 2}}, Axes -> False, ImageSize -> 300,
  Background -> Black] ~Blur~ 3, {u, 0, 2 Pi, 0.1}];

Export["trail.gif", frames, "DisplayDurations" -> .03]