From: eLinux.org

X11

X11 also known as X.org, XFree
or XServer is the most used graphics on Linux, at least on desktop.

Contents

Architecture

X11 works in a client-server architecture with communication going via
fast UNIX Sockets if local or TCP/IP if remote, it’s totally transparent
to users.

Protocol is simple and optimized, often used via libraries like Xlib or
Xcb (asynchronous). Most common commands like mouse movement and expose
events are small so they don’t impact too much. Since communication
happens using file descriptors (either local unix sockets or tcp) one
can easily integrate it in event loops (or main loops) with easy polling
with poll(2) or select(2).

Extensions and Hardware Acceleration

It is extensible and can make use of hardware acceleration. With
extensions like XRender one can optimize 2d rendering paths. With Xvideo
it’s possible to use extra planes and also pass through raw YUV data.
Others like Composite can allow a composite manager to do actual
drawings, possible adding nice effects like semi-transparent windows or
shadows. One can use OpenGL (and OpenGL-ES) with X11.

One extension that worth special note is XShm, or the shared memory
extension, that is can be used to avoid sending images or other heavy
data over the wire. When using XShm images, one just need to send the
image identifier and other image parameters, not the image pixels. This,
however, have the impact that image creation is more expensive since
Linux Kernel needs to zero memory to avoid data disclosure, but it’s
negligible.

Strong Points

Recent efforts are being made to optimize X11 more and its Linux
integration even better, like Kernel Mode Setting, which will avoid
flickers during system boot and also reduce X11 server complexity.

Although one can write directly to wire or use low level libraries like
Xlib or Xcb, usually one write X11 applications using various helpers
and toolkits like GTK,
Qt,
Evas/EFL, FLTK
and more. These libraries are the base of most graphical user interfaces
available for Linux, including big projects as Firefox and
Thunderbird, Pidgin Instant
Messenger, GNOME
Desktop, KDE Desktop,
Enlightenment Desktop and more. These
applications and libraries are primarily developed with focus on X11, so
they’re always up to date and require no porting at all and receive much
more testing. This ready/availability of most famous applications is the
strongest point of using X11, even on embedded systems as showed by
Maemo, OpenMoko,
OLPC and more.

Weak Points

X11 was for a long time neglected from embedded systems tagged as slow
and big. While not the very truth, it do have some true facts.

  • X11 is usually bigger than alternatives like raw framebuffer and
    DirectFB, but it’s not too bad is you
    consider minimum system, see Thomas Petazzoni’s talk at ELC-E 2008:
    Choosing embedded graphical
    libraries    .
  • Due chicken-egg problem and apparently bit more complicated api for
    X drivers, embedded hardware manufactures provide more drivers for
    DirectFB than X11.

There are benchmarks that show that software-on-software comparison X11
does not add much overhead compared to DirectFB and raw framebuffer:
http://profusion.mobi/node/11

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