How to define hitArea of external assets with alpha channels (for example PNG)?

I copied this from some other forum because it’s explain well

"…Let’s say you are loading in an external asset that has an alpha channel, such as a PNG. Now let’s say that you want that asset to act like a button, with rollover and click actions. No problem. No reason you can’t do that.

But, actually, there is a problem. The hit area for that external asset will be its entire bounding box.

Take this octy image for example:

Even if you rolled over the top left of the image where it’s totally transparent, it will still trigger rollover actions, which is certainly not ideal and could potentially be very confusing for a user…"

If anyone know how to solve the problem i would be greatful =)=)=)

---- there is my code for button just in case if anyone need to know—

btn_album.onRollOver = function() {
btn_album.gotoAndPlay ("_over");
}
btn_album.onRollOut = function() {
btn_album.gotoAndPlay ("_out");
}
btn_album.onRelease = function() {
gotoAndPlay (“album”)
}