avisynth script : zoom mixed with letterbox aspect

Hi,

I'd like to make an avisynth script to encode automaticaly my video to get a kind of zoom mixed with letterbox aspect.
n=number of frames

the initial script isn't the point; let's start with this simple script :

v = dss2("vid.ts", fps=25)
a = DirectShowSource("vid.ts", video=False)
AudioDub(v,a)

Frame Sart (frame 0)
video occupies full player screen like this :


I frame 0 >> frame 60

video dimension is smoothly shifting to letterbox aspect

II frame 60 >> frame n-60

v = dss2("vid.ts", fps=25)
a = DirectShowSource("vid.ts", video=False)
AudioDub(v,a)
vid=LanczosResize(last,500,280)
vid=AddBorders(vid,70,40,70,40)
return vid



III frame n-60 to frame n

video smoothly return to initial dimension and will look again like this :


The crucial parts I'd like to get some help are I & III

Thanks,

Sorry for my bad english