The Ai Barndoor filter can be found within the spot lights filter list in the Arnold attributes

A tutorial that demonstrates how to use the Barndoor filter can be found here


The images below show the various attributes of the Ai Barndoor filter (the spotlight model is purely for illustrative purposes). Click on the images to see the animation. 

Top Left

 

Top Right

 

 

Top Edge

 

Bottom Left

 

Bottom Right

 

Bottom Edge

 

Left Top

 

Left Bottom

 

Left Edge

 

Right Top

 

Right Bottom

 

Right Edge