Filter on all tiles ? Isn't it better to just fill the viewable area with a tiledbackground (you can paint a 1-2px width gradient as texture)? Then you just have to change it's opacity with a function, like :
[code:2kt3afzb]+ Function: On function "FadeOut"
+ System: Repeat 100 times
+ System: Is global variable 'Opac' Less than 100
-> Wait: Delay 10 ms
-> System: Add 1 to global variable 'Opac'
-> TiledBackground: Set opacity to global('Opac')[/code:2kt3afzb]
Well, something like that anyway, i guess.