Using overlay - the basics

[Ilmion]

I looked in the manual, I did some research in the forum and the wiki don't say much (but a little helpus tough), but I Still haven't found what I'm looking for (is my English too poor that I have to look in songs lyrics...)

What I'm looking for is :
What are exactly Overlay ?
How can they help me making a Game ? (examples)
How to use them efficiently ? (Tips and tricks)

[Radiant]

What exactly do you need for your game?

If you can tell us what you're trying to do, we can tell you if overlays are, or are not, useful in doing that.

[Ilmion]

If you've read the other thread I started (Snow Effect vs plugin) you could say that I would like to make snow effect, but It is not only that. If I want to improve my self with AGS, I want to try everything there is in it.

I've learn recently that there is Overlay in AGS, I would like to try those and do it the right way. If there is thing you need to do and things you should not do. How the use of Overlay would make me a better game maker.

[Radiant]

The easiest way to do snow (other than via the plugin) is either using RawDraw, or putting the flakes on a GUI.

Frankly I've never used any overlays; I tend to use GUIs as they are more customizable.

[FSi++]

Overlays are pretty much like GUIs in the way they are displayed over other things (though GUIs go over overlays); the difference is you can't have buttons and stuff on overlays and they're unclickable.
An example of (crappy) rain done with overlays can be seen in Ache Quest (is it even downloadable?).
I used overlays for scrollers in another game (it never passed demo stage, so relax).
Oi, another thing: you can't turn overlays on and off (iirc) nor change its graphics on the fly (i.e. you have to destroy one first, then create another - same for disabling/enabling overlays, you just create or destroy them). And in AGS 2.72 you have a cap on a number of overlays displayed (including one used for characters' speech) (i think 16 or similar not very big number).