Extensions
Scheifler and Gettys designed the X server to be simple but extensible. As such, much functionality now resides in extensions to the protocol.
At the protocol level, every extension can be assigned new request/event/error packet types. Extension features are accessed by client applications through extension libraries. Adding extensions to current X server implementations is reportedly difficult due to a lack of modularity in the server design. It is a long term goal of the XCB project to automate generating both the client and server sides of extensions from XML protocol descriptions.
The following table provides a partial catalog of extensions that have been developed, sorted roughly by recency of introduction:
| Extension | Description and notes |
|---|---|
| Composite | Off-screen rendering of entire window hierarchies, allowing applications and composition managers to do effects anywhere along the way. Required for things like alpha transparency for windows and drop shadows. |
| Damage | Tracks modified regions of windows, and minimizes bandwidth use required to keep the display up to date. |
| XFixes | Several protocol changes. |
| Extended-Visual-Information (EVIE) | Allows client to determine information about core X visuals beyond what the core protocol provides. |
| Distributed Multihead (DMX) | Communicates with DMX X server. |
| X-Video Motion Compensation (XvMC) | Offloading video motion compensation to a GPU that supports it. |
| GLX | Support for rendering OpenGL within windows. |
| XRender | Hardware accelerated image compositing with alpha blending. |
| Resize and Rotate (RANDR) | Dynamically change the size, reflection, rotation and refresh rate of an X screen. |
| Xinerama | Splitting the desktop across multiple monitors. |
| Display Power Management Signaling (DPMS) | Allows controlling monitor power saving modes. |
| X keyboard extension | Enhanced keyboard layout handling. |
| DOUBLE-BUFFER | Gives flicker-free animation. |
| RECORD | Used in server testing |
| MIT-SHM | Use of shared memory to improve performance. |
| SYNC | Provides timers and synchronizes clients (e.g. running on different hosts and operating systems) from within the X server. Created because of errors introduced by the network. |
| XTEST | For generating fake input. Use i.e. the xte application from the XAutomation package. |
| XInputExtension | Support for input devices such as graphic tablets; implementable as libXi |
| BIG-REQUESTS | Enables requests exceeding 262140 bytes in length. |
| XC-MISC | Fixes resource ID range overrun |
| X video extension | Support for hardware video overlays and hardware-based video scaling on playback. Also called Xv (not to be confused with the xv program). |
| Shape | Support for non-rectangular and partially transparent (binary, no alpha opacity) windows. |
| MIT-SCREEN-SAVER | Launches a program when the X server turns on the built-in screen saver |
| SECURITY | Provides enhanced server security |
| X-Resource | Allows querying the X server about its resource usage |
| XFree86-Bigfont | |
| XFree86-DGA | Provides direct linear framebuffer access (direct graphics access). To be mostly removed in X.Org 7.6 |
| XFree86-VidModeExtension | Dynamically configures modelines and gamma. |
Read more about this topic: X Window System Protocols And Architecture
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