[SDL] Cloud effect

[Salvatore Di Fazio]

Hi guys,
does anybody know where I can find an example of 2d clouds like this:

http://fi.wikipedia.org/wiki/Kuva:SNES_Chrono_Trigger.png


or a shadow like this:

http://www.mobygames.com/game/snes/tales-of-phantasia/screenshots/gameShotId,53196/

or like this:

http://www.mobygames.com/game/snes/ogre-battle/screenshots/gameShotId,74153/


I think it's an effect made by the alpha channel

tnx