Help about fake depth of field effect

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Canali

Hi.
I have a main drawing routine that is the "standard":

Repeat
...
...
DrawImage ...
Flip;Cls
Until KeyHit (KEY_ESCAPE)

Before flipping the backbuffer to the front, I would like to QUICKLY "blur" a part of the backbuffer (for instance the top 100 lines) to obtain something similar to a depht of field.

Anyone has a QUICK routine to accomplish this, somehting like:
Blur (x1,y1,x2,y2,Q) where x1,y1 are the top left corner of the area of the backbuffer, x2,x2 are the lower right corner and Q the quantity of blur passes?

Thanks.
And sorry for my poor english but I'm Italian.
:)