A basic "compositing manager" for the X Window System that adds transparency/alpha blending (compositing) to windows.

Normally, you'd get this same functionality for free via your desktop environment's window manager which probably includes a "compositing manager".

This project is useful to use when you're running in a virtual X environment like Xvfb (X virtual framebuffer) or Xephyr and need to work with multiple windows that overlay on top of each other.

To paraphrase from the Adding Transparency page in the Xplain series by Jasper St. Pierre; a "compositing manager" is needed because by default, every pixel on the screen is owned by a single window at a time. At any given point in time, you can point to a pixel, and the X server can tell you which window is responsible for painting it. Windows in the X11 protocol sense, aren't a tangible thing. They simply make their mark on the X server's front "root" buffer, as a series of pixels which they own. The issue is that when the bottommost window is being clipped by another window on top, we don't have access to the pixels that are being occluded at all; they simply don't exist anymore. The topmost window owns that pixel.

To accomplish transparency, we redirect the output of all windows to off-screen buffers and then composite them together to form the final image.