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What is a compositing manager?

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I''ve mainly heard this term in reference to Apple''s Mac OS X Aqua GUI and Xorg''s X server. OS X uses OpenGL for compositing, and Xorg has a composition manager that uses OpenGL. What I''m curious is, what exactly is the compositing manager and what differentiates it what the window manager?

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This is something of an oversimplification, but a rough answer would be that a compositing manager handles display of the window images whereas a window manager handles the windows themselves. While on earlier GUI systems the window manager was sufficient to do rendering as well, due to the lack of effects such as transparency, on more recent GUI systems such as Aqua, there''s sufficient extra work to be done that putting management of those functions in its own subsystem is justifiable.

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So it''s basically a renderer for the windows?

Whereas the window manager keeps track of things like Z-order, position, minimized/maximized status, etc?

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IIRC, (and I may not, the last time I read up on the OS X window management system was many moons ago), the composition manager is not just a renderer, it also pre-processes some input before that information is passed to the window manager. But it''s primary usage is to do the pretty special effects like transparency or the weird genie animation, etc.

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