Why capture screens, windows, and dialogs?
Though many technical writers hate to admit it, a picture is often worth a thousand words. Most good technical documentation includes images, generically known as screen captures or screen shots, that show windows and dialogs as they appear in the application that is being documented. Including images in documentation makes users more comfortable with an application because such images show menus, dialogs, and general program interaction in context, so that users can see exactly what to do and what an application will look like while they are working with it.
Capturing such images is not just a writing task. Most marketing material for graphical applications includes screen shots of significant portions of an application's interface, or the application with complex work underway, to highlight the appearances of capabilities of an application. Anyone who has ever filed a defect report against a piece of software knows that a screen shot that actually shows the problem establishes more credibility than a written or verbal description. Highlighting a problem via a screen shot also makes it easier for developers to focus on the actual source of a problem.
UNIX and Linux screen capture overview
All modern Linux® and UNIX® systems provide some sort of graphical
desktop, which usually includes at least one graphical screen
capture utility—most often one that is specific to a particular
desktop environment. For example, GNOME provides gnome-screenshot, and KDE provides
KSnapshot. However, every Linux and UNIX system that runs the X
Window System (in other words, all of them) includes a
xwd, that makes
it easy to capture an entire screen, a specific window, a window
showing pull-down menus, and even specific dialogs.
The next sections of this article explore various ways of using
xwd tool to capture different
types of images from a UNIX or Linux system's graphical display.
The X Window Dump (
xwd) utility has been part of the X Window
System since the mid-1980s and the early days of Massachusetts Institute of Technology (MIT) Project
Athena. It is still typically installed as a part of most X11
xwd utility enables you to capture an entire
screen (by capturing the X11 root window) or a specific window
identified by using a mouse or a window ID on the command
line. Images of a single window can consist of just that window
or can also include menus and overlaid dialogs.
The simplest possible
xwd command line
looks like the following:
xwd > filename.xwd
When executed, this command displays a set of crosshairs. You
use the crosshairs to select the window that you want to capture an image of by
right-clicking on that window. The command then writes its output to the
filename.xwd file using shell redirections. The
xwd command also
provides a command-line option,
that eliminates the need for shell redirection by enabling you to
specify the name of an output file on the command line, as shown below:
xwd -out filename.xwd
However you specify the output file, the output of the
is a window image in a generic format (known as xwd format),
which you can easily convert to more standard graphical formats (such as, portable
network graphics (PNG); joint photographic experts group (JPEG); or
graphics interchange format (GIF)) using easily-obtained utilities (as
discussed in the next section).
xwd images to more popular formats
Several image processing and conversion toolkits are available for almost all Linux and UNIX systems. The most common of these are the ImageMagick and Netpbm toolkits (see Resources for more information). Of these, the most widely used is probably the ImageMagick toolkit, which is included on vendor distributions of Linux tools for various UNIX systems (such as, IBM AIX Toolbox for Linux Applications CD).
The ImageMagick toolkit contains a number of different utilities
for graphics conversion and manipulation. It includes a utility
(import) that provides a command-line alternative to the
utility. Of these utilities,
the most important when working with
xwd files is the
convert utility, which enables you to convert
images from one format to another. You can specify the format that
you want to convert an image to by preceding the name of the
output file with the abbreviation for an image format or by
specifying the target image format as the file extension of your
output file. For example, both the following commands
produce an output file in PNG format:
convert filename.xwd PNG:filename.image convert filename.xwd filename.png
The ability to specify the output format as a prefix is convenient if you do not want your output file to have a format-specific file extension. Figure 1 shows a screen capture of a simple GNOME terminal window that has been converted into PNG format.
Figure 1. Converted screen capture of a terminal window
Capturing entire screens
The previous example of using the
xwd command required that you
select a specific window interactively. In some cases, you may
want to capture an entire screen to show the contents and
placement of multiple application windows simultaneously. The
command provides the
-root option to
enable you to do this by capturing the X Window System root
window, which is the full-display window atop all other X
Window System windows and dialogs displayed. The following command
captures the entire screen and writes it to the full-screen.xwd file:
xwd -root -out full-screen.xwd
When used with the
-root option, the
xwd command is non-interactive because you are capturing
everything. If you need to capture a specific window in a specific
state, such as displaying a menu or some other interactive action,
you can do this by delaying the time at which the root window is
captured (giving you time to set up the screen in the way
that you want). To do so, use the
found on all UNIX and Linux systems, which simply
waits for a specified number of seconds, as in the following
sleep 15 ; xwd -root -out full-screen.xwd
This command sequence gives you 15 seconds to display whatever
menus or dialogs you want to capture before executing the
Capturing specific windows
When executed without arguments, the
xwd command requires that you
interactively select the window that you want to capture which
can be a problem if you want specific menus to be displayed. In
most cases, the action of selecting a window is a mouse event that
cannot proceed when any other pending mouse events, such as
selecting a menu item, are in progress. Trying to capture a window
with a pending mouse event results in an error message, as in
the following example:
$ sleep 10 ; xwd -out window2.xwd xwd: error: Can't grab the mouse.
You can work around this problem by taking advantage of other
command-line options for the
xwd command. The first one to try is
-id option, which lets you specify
the X Window System ID of the window that you want to capture and
therefore does not require that you select a window. The easiest
way to determine the X Window System ID of a window is to use the
xwininfo command, which provides that
information plus a huge amount of information about the location
and appearance of a specific window. When you execute the
xwininfo command, a set of
crosshairs is displayed. Use the crosshairs to click on the window for which you want
information. A sample
and its output are shown below in Listing 1.
Listing 1. Abbreviated output from the
$ xwininfo xwininfo: Please select the window about which you would like information by clicking the mouse in that window. xwininfo: Window id: 0x2c435b4 (has no name) Absolute upper-left X: 1811 Absolute upper-left Y: 99 Relative upper-left X: 1811 Relative upper-left Y: 99 ... Corners: +1811+99 -423+99 -423-116 +1811-116 -geometry 1126x835+1811+99
Using the window ID from this information, you can combine
imposing a delay via the
option to capture a window and any menus or other transient
graphics by using a command such as the following:
sleep 15 ; xwd -id 0x2c435b4 -out window-with-menu.xwd
Figure 2. Screen capture of a terminal window with menu
One side effect of identifying a window by ID is that a screen
capture based on the window ID may not capture other windows, such
as dialogs, that are overlaid on the specified window. All
windows, even dialogs, have their own IDs, and you can only
specify one ID at a time. Whether or not you capture overlying
windows is dependent on the graphical environment that you
are using. To work around potential problems capturing overlying
windows, you can use the
-screen option, which specifies that the
contents of a window are to be captured relative to the root
window, as in the following example:
sleep 15 ; xwd -id 0x2c435b4 -screen -out window-with-menu.xwd
-screen option captures a window as
it appears atop the root window and therefore includes any
overlaid dialogs, in-progress menu actions, and so on.
xwd command is available for every
Linux and UNIX distribution and, as a command-line application,
provides an easy, graphical user interface (GUI)-independent
screen and window capture tool. Combining
xwd with freely available graphics toolkits
such as ImageMagick and Netpbm makes it easy to capture screen
images on your platform of choice and deliver them in any graphics
formats that you need for documentation, marketing, bug reporting,
and other purposes where a picture may indeed save you a thousand
words of description.
The command-line screen and window capture tool discussed in this article are both easy to use and available on all Linux and UNIX systems, but different Linux and UNIX systems also offer a variety of graphical tools to further simplify capturing screen and window images. The next article in this series discusses the graphical screen capture tools that are available on different Linux and UNIX systems.
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- The ImageMagick Toolkit provides a popular set of command-line tools for graphics capture, conversion, and manipulation.
- The Netpbm toolkit provides another popular set of command-line tools for graphics capture, conversion, and manipulation.
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