Filter borders only: avisynth

[themaster1]

I'm looking for a little script to filter borders only: top/bottom (and maybe left/right). I already have one but i wasn't able to tweak it so far; i can filter one border at a time.
The code i use:

###############s
# Border Top
X1 = 0
Y1 = 0
X2 = 720
Y2 = 26
## Border Bottom
#X1 = 0
#Y1 = 418
#X2 = 720
#Y2 =480

# Make the values MOD4
X1 = X1 - (X1 % 4)
Y1 = Y1 - (y1 % 4)
X2 = (X2 + 3) / 4 * 4
Y2 = (Y2 + 3) / 4 * 4
Active = crop(X1,Y1,X2-X1,Y2-Y1)
Above = Y1 > 0 ? crop(0,0,width(),Y1) : NOP
Below = Y2 < height() ? crop(0,Y2,width(),height()-Y2) : NOP
Left = X1 > 0 ? crop(0,Y1,X1,Y2-Y1) : NOP
Right = X2 < width() ? crop(X2,Y1,width()-X2,Y2-Y1) : NOP
Last = Active
##########
# Put filters here:
grayscale()

##########
IsClip(Left) ? StackHorizontal(Left, Last) : NOP
IsClip(Right) ? StackHorizontal(Last, Right) : NOP
IsClip(Above) ? StackVertical(Above, Last) : NOP
IsClip(Below) ? StackVertical(Last, Below) : NOP
return Last

[jagabo]

function filter_borders(clip c, int Left, int Top, int Right, int Bottom)
{
  # Left = size of left border
  # Top = size of top border
  # Right = size of right border
  # Bottom = size of bottom border

  inner = Crop(c, Left, Top, -Right, -Bottom)
  GreyScale(c) # or whatever filters you want
  Overlay(last, inner, Left, Top) # overlay unfiltered inner portion
}

Of course, it's slower since the entire frame is filtered, then the inner portion is overlaid with the unfiltered portion.

[manono]

This may not be exactly what you had in mind, but this is another way to filter only the outside borders, but without making it a function:

A=Last
B=BlankClip(A, color = $ff0000)###Filter here, in this case create a red border
A=A.Crop(16,8,-16,-8)###Define the border area
Overlay(B,A,16,8)

[themaster1]

Thank you both, i'm using jagabo solution for now