Developing Imaging Applications with XIE
In this article I'll introduce the X Image Extension (XIE), and illustrate how it might be used by a C programmer to add image display support to a simple application. The following assumptions are made about the reader:
You are comfortable reading C language code.
You are comfortable with (or at least have a basic understanding of) Xlib development, e.g., creating windows, drawing text and graphics. Since the example program uses Motif, some exposure to Xt and a widget set such as Motif or Xaw would also be helpful, but is not required.
I've been using XIE and Linux together since late 1994. At that time, I had to build my own X server (an early beta version of X11R6) to support XIE, plus I had to port the client library (libXIE.a) to Linux since an X environment that supported XIE was not yet available in any of the Linux distributions. Now, all currently available Linux distributions provide a more than adequate platform for XIE client development, as well as the runtime support needed for clients that use XIE.
In X, a client (i.e., an application) connects to an X server, which you can think of as essentially nothing more than a display with a keyboard and a mouse attached to it. The core X protocol provides all the functionality needed by a client to produce a user interface on the server. Using the core X protocol, a client can:
Create, move and destroy windows.
Render graphics and text into windows and off-screen pixmaps.
Receive notification of events occurring on the server which are of interest to the client. Such events include button presses, mouse movement, keyboard presses, window exposure notification and so forth.
For most of you, the client and server are both running on the same machine, i.e., on a single Linux system, but it doesn't have to be that way. The X Window System protocol is designed so that communication between the client and server can be carried out over a network connection, e.g., TCP/IP. (If run locally, it is a local, i.e., AF_UNIX, connection.) Therefore, you can have the situation as illustrated in Figure 1. There, the X server is my Linux machine at home. The clock application and terminal emulator (xterm) are processes running on my Linux machine locally. In addition, I have a PPP connection to my ISP (which is running Solaris or some other Linux-wannabe) and within the xterm I am executing a telnet session between my local machine and the ISP. The other two windows (xiegen) display an application executing remotely on my ISP's machine. This application is displaying its client windows on my Linux machine, responding to keyboard and mouse events that occur on my Linux machine, courtesy of the X protocol. Prior to executing the remote application, I needed to set the DISPLAY environment variable to the IP address of my Linux machine which acts as the server. Xlib (on the remote host) reads this variable on client startup and uses its value to open a connection to the X server running on my Linux machine. The :0 characters in the following line indicate a logical display on the server:
$ typeset -x DISPLAY=22.214.171.124:0
Note that except for the screen, mouse and keyboard, the client generating xiegen's output in Figure 1 is interacting with resources on the remote Solaris host. If my remote client opens a file, /etc/passwd for example, it is opening /etc/passwd on the Solaris host, not the Linux host.
In reality, running console-based UNIX or Linux applications from a dumb terminal over an RS-232 connection has much in common with running UNIX or Linux applications from an X server over a network connection, except that when using X, the graphics support is much better.
Additional functionality can be added by vendors to the core X protocol via extension protocols. XIE is one example. Other extension protocols include the Phigs graphics extension to X (PEX), and the shape extension, which allows X to display non-rectangular windows. There are many other extensions; execute the xdpyinfo command to take a look at which ones are supported by your X server.
XIE is an extension that was released with the first version of X11R6 back in July 1994. XIE was developed in an attempt to provide clients with support in the following areas:
Transmission of image data between the client and the server (in either direction)
Image enhancement and manipulation
The core X protocol provides only minimal support for the transmission, manipulation and display of image data. Let's look at each of these areas in more detail, and discuss what core X is missing with regard to imaging support, and what XIE brings to the table.