You can't set the Irrlicht window to transparent, in the sense that you can see through it to windows underneath. And, certainly, this is not the function of the alpha component of the background colour.
You can set Irrlicht IGUIElement windows transparent, but not the main-window (so the desktop shines through as often seen with modern Window-Managers). I don't know what would be needed to support that (would be cool sometimes...).
System-dependent. For linux compositing managers, you'd need to set a NETWM hint for requesting transparency. How it would work with opengl I don't know.
Whether the main framebuffer uses an alpha channel.
In some situations it might be desireable to get a color buffer with an alpha channel, e.g. when rendering into a transparent window or overlay. If this flag is set the device tries to create a framebuffer with alpha channel. If this flag is set, only color buffers with alpha channel are considered. Otherwise, it depends on the actual hardware if the colorbuffer has an alpha channel or not.
i think this is what i need but when i set the flag true it doesnt seems to work. i think i need to activate the tranparency in Linux window manager so it works
This flag only does something useful under Linux anyway. At least it will chosse a video setting and context which supports the alpha layer. But the window setup is not yet properly made to support transparent windows. At least no in general. But if you know how to create a transparent window, you can use that combined with the alpha mode setting to render into the transparent window.
