NCL Website header
NCL Home > Documentation > Tools

Viewing and editing your metafiles


The primary token of exchange for graphical imagery in NCL and NCAR Graphics is the NCAR Computer Graphics Metafile (NCGM), which can be produced directly from running NCL scripts or NCAR Graphics programs. This document describes a suite of applications that may be used to display and manipulate NCGMs. Many of the applications described in this document may also ingest valid ANSI-standard Computer Graphics Metafiles (CGMs), provided they are transformed into NCGM format using the tools discussed later in this document.

The primary components covered in this section are:

  • filters to convert CGM files to and from NCAR's privately encoded format
  • ctrans---a translator for sequential frame display to your terminal or workstation window and for output to many hardcopy devices
  • ictrans--- a text-based, interactive version of ctrans that may be used from any graphics terminal and the X11 windowing system
  • idt--- a graphical user interface (GUI) to ictrans
  • med--- a text-based, frame-level interactive NCGM editor
     GUIs are supported by and require the X11 Window System. Text-based          
     interfaces are typically equivalent in function to GUIs but only require
     a standard UNIX environment.                                                  
This document assumes that you have a working knowledge of the X11 windowing system environment for discussions involving GUIs. For X11 references, see X Window System User's Guide, by O'Reilly and Associates, Inc., or contact your system administrator.


The examples in this document assume that you are using csh, the C shell. If you are using ksh, the Korn shell, or sh, the Bourne shell, you may want to get help from your system administrator. If you are not sure what shell you are using, check with your system administrator.

This section discusses the environment variables that must be set. Many of the environment variables are invariant from one session to the next, so you may want to place them in your .login or .cshrc file. Your site administrator may have already preconfigured your environment for you. To determine which variables are already set, type:


Search path

You must make sure that the NCL and NCAR Graphics executables and the current working directory, ".", are on your search path. You must also make sure that the appropriate NCL and NCAR Graphics environment variables are set. Please see the NCL environment variables section for information on setting critical environment variables.

Metafile translators

When using NCL metafile translators you must either set the GRAPHCAP environment variable to reflect the graphics device you are using, or provide the device name as a command line argument. To determine which devices are supported, you can type:

The general form for setting your graphcap is:
setenv GRAPHCAP device_name
As an example, if you are running under X11 you would use:
setenv GRAPHCAP X11
You may want to set this variable in your .login or .cshrc file so that it will always be available. The device_name may also be specified on the command line for the translators, thus overriding the environment variable.

X11 users

If you are an X user, you will also need to set the DISPLAY environment variable. For example, if both your X server and your translator will run on the same local machine then type:

setenv DISPLAY:0
or possibly:
setenv DISPLAY unix:0
Check with your system administrator if you are not sure.

If you are running remotely on a machine named pebbles and your local workstation where your X11 server is running is named bambam, then you need to set your DISPLAY environment variable on pebbles with:

setenv DISPLAY bambam:0
and on bambam execute:
xhost + pebbles

Translator fonts

Finally, you may optionally want to specify a font other than the default to be used for subsequent metafile translation of text. The FONTCAP environment variable may be set:

setenv FONTCAP fontcap_name where fontcap_name is the desired fontcap. For a list of valid fontcaps, execute the fcaps command:

For example:

setenv FONTCAP font10
specifies that the cursive font, font10, be used.

Getting started --- Translating metafiles with ctrans

This section provides several simple examples that demonstrate the basic steps for using the metafile (NCGM) translator ctrans. We assume that your NCARG_ROOT and possibly DISPLAY environment variables have been set correctly as discussed above. Before you start you will need an NCGM. You can create an NCGM by executing:

ncargex bnchmk -clean
which will leave the file bnchmk.ncgm in your current working directory.

Viewing your NCGM

Below are the steps for sequentially displaying the contents of an NCGM on three different devices. Use the command(s) appropriate for your workstation. From a Televideo vt100 graphics terminal:

setenv GRAPHCAP vt100
ctrans bnchmk.ncgm

ctrans -d vt100 bnchmk.ncgm
On a workstation running the X Window System:

setenv GRAPHCAP X11
ctrans bnchmk.ncgm

ctrans -d X11 bnchmk.ncgm
In all three cases, the first of six frames should appear on your screen. You can sequentially advance through the NCGM by pressing the RETURN key after each frame is plotted. In a window-based environment, such as X11, make sure that the mouse sprite is in the graphics window created by ctrans. Also, in a window-based environment, clicking the left mouse button has the same effect as pressing RETURN. You can terminate processing at any time by sending ctrans an interrupt signal (typing CONTROL-c on most systems).

Printing your NCGM

For most of the output devices supported by ctrans, the metafile translation process results in a stream of device-specific commands being written to the standard output. This command stream may be captured for subsequent processing. For example, the command:

ctrans -d ps.mono bnchmk.ncgm >
translates the entire bnchmk.ncgm metafile into PostScript and writes it to the file When the standard output of ctrans is redirected to a file (or a pipe) in this manner, it is not necessary to press the RETURN key between frames. ctrans will process all the frames in the file and then exit without waiting for any user interaction. The file may now be sent to a PostScript printer for printing, possibly with the command:

 The lpr command used above is simply an example. Printing commands
 vary widely from site to site. Your system administrator should be able   
 to provide you with the printing command appropriate for your site.
ctrans can also generate output suitable for printing on HP LaserJet printers that do not support PostScript:

ctrans -d hppcl bnchmk.ncgm > bnchmk.hppcl
For a complete list of supported output devices, execute the gcaps command.

Examining the contents of your NCGM

Sometimes it is desirable for the purposes of debugging to see what CGM commands are actually contained in your metafile. The ctrans clear-text driver may be used to convert your binary-encoded NCGM into a human-readable ASCII representation. For example:

ctrans -d CTXT bnchmk.ncgm | more
converts bnchmk.ncgm into clear text and sends the output to the UNIX pager, more.

The command:

ctrans -d CTXT bnchmk.ncgm | grep -c POLYLINE
counts the number of CGM polyline elements that appear in the file bnchmk.ncgm.

 The ANSI CGM standard defines three encoding schemes for CGMs:            
 binary, character, and clear text. The clear text produced by the          
 ctrans -d CTXT option does not conform to the ANSI definition for clear   
 text encoding and is not acceptable as input to other CGM clear-text      
 interpreters. It is merely intended as a debugging aid for the user.      

Controlling NCGM translation

ctrans offers a suite of command line options for controlling the translation process performed on a metafile which may ultimately determine the appearance of your plot.

Interactive metafile translation

Interactive ctrans (ictrans)

ictrans, a text-based interactive version of the metafile translator ctrans, will read commands from any terminal. With ictrans, you are no longer limited to sequential metafile access. Interactive translation may be desirable if your metafile contains lots of metafile frames or if you simply want to take advantage of some of the functionality in ictrans that is not available in ctrans. You invoke ictrans by typing:

ictrans metafile_name
where metafile_name is the path to a valid NCGM. As with ctrans, you need to specify the output device you want to translate to either on the command line as in:

ictrans -d X11 bnchmk.ncgm
or with the GRAPHCAP variable:

setenv GRAPHCAP X11
ictrans bnchmk.ncgm
Upon invocation, ictrans will respond with an account of the number of frames contained in the metafile followed by the ready prompt ictrans>. Be patient; many of the functions in ictrans require information embedded in the metafile. The time required to extract this information depends on the size of the NCGM.

The command for viewing a metafile frame is plot (you can type p for short). Typing plot at the ictrans> prompt will cause the first frame in the metafile to be displayed. ictrans is case sensitive; typing PLOT will not work. After the frame has been plotted, you can press RETURN to signal ictrans to re-enter command mode (you should see the ictrans> prompt). Entering a second plot command will cause the next frame, if any, to be displayed:

ictrans> p
Note to X users: When ictrans is displaying to an X window, you may find it convenient to operate in movie mode. When ictrans is in movie mode, a RETURN is not necessary to put ictrans into command mode after plotting a frame from your metafile. Enter movie mode by typing:
ictrans> movie 0
You can abbreviate the command plot by typing p; all ictrans commands may be abbreviated up to the point that they are unique. For a complete list of ictrans commands type:
ictrans> help
or just:
ictrans> h

Random access

So far, we have only shown sequential metafile access. ictrans allows for random frame access as well. To view the fifth frame in a file, type:

ictrans> 5p
Relative addressing is also possible. To view the last frame in a file type:
ictrans> $p
Here, the $ is an alias for the last frame in the metafile.

For convenience, several frames may be listed on a single command line. To view the first through third, the fifth, and the last frame, type:

ictrans> 1,3 5 $p
After processing each frame, ictrans waits for you to press RETURN before proceeding with the next frame. You can read the man page for ictrans by typing:
ictrans> ! man ictrans
(Typing ! returns you to the UNIX shell for one command.)

Saving a metafile

ictrans allows you to save the metafile describing a frame, or set of frames, to a file. If this file already exists, the saved frames are appended at the end. If the file does not exist, it will be created for you. To save the current frame to a file named save.ncgm, type:

ictrans> save save.ncgm
The current frame is the last frame plotted. (If no frames have been plotted the current frame is the first frame in the metafile.)

If you type:

ictrans> s save.ncgm
ictrans will tell you that the command is ambiguous because the s command could stand for either save or spooler. You can, however, abbreviate save with sa. The save command uses identical syntax to the plot command for addressing frames (as do all ictrans commands). To save the third through last frame to the file foo.ncgm , type:
ictrans> 3,$s foo.ncgm
The newly created file, foo.ncgm, may now be translated just like any other NCGM. If you type:
ictrans> file foo.ncgm
foo.ncgm will be read into the translator for processing. The file command is used to change the current file being processed.

Printing metafiles

If the necessary hardware and software exist at your site, ictrans may have been configured to translate metafile frames and send them to a printer. To determine whether ictrans has been configured for printing to a spooled device, type:

ictrans> spooler
If ictrans responds with anything other than "" (null), then there exists at least one device to which ictrans can send translated code. The current printing device name is given by the spooler command when spooler is executed without any arguments as above. (The ictrans command alias, without any arguments, lists all available printing devices, if any.) For example, if spooler responds with a device named "postscript," then to send the first three frames of your file to this device after translation, type:
ictrans> 1,3Print
Don't forget to capitalize the P in Print. The P is capitalized so Print can be abbreviated and not conflict with p for plot.

For more information about configuring ictrans for printing metafiles, see the documents on ictrans and ncarv_spool.

Zooming in on your plot

The ictrans zoom command allows you to zoom or pan any area of your plot. For example, the following command will result in the bottom-left quarter of the current frame being drawn so that it fills the entire graphics window:

ictrans> ;zoom .0 .0 .5 .5
The two pairs of numbers, (.0, .0) and (.5, .5), specify the lower left and upper right coordinates, respectively, of the frame to be viewed. The new coordinate specification remains in place until changed with a subsequent zoom command. To restore normal viewing type:
ictrans> zoom .0 .0 1. 1.

Batch processing

It is sometimes desirable to access ictrans functionality in a noninteractive, or batch, environment. For example, you may want be able to send selected metafile frames to a printer from inside a shell script. You might be able to use ctrans to accomplish this task, but ctrans provides no flexible mechanism for choosing which frames get sent; ctrans will send the entire file. As we have seen, ictrans provides a rich addressing model for selecting pieces of a metafile for processing. To facilitate the use of ictrans in a batch mode, ictrans accepts all of its commands on the UNIX command line by preceding them with a -e flag.

For example:

ictrans -e 'spooler postscript' -e '$ Print' bnchmk.ncgm
would translate the last frame (the $ frame) in the metafile, presumably into PostScript, and send it to the spooled device named "postscript". Don't forget to enclose all -e options in single quotes or the shell will mistakenly interpret special characters, like $, for you.

Interactive display tool --- idt

Yet another interactive metafile translator is idt. What makes idt different from ictrans is that idt has a point-and-click or graphical user interface (GUI). It may only be used on a workstation (or X terminal) supporting the X11 windowing system.

This section provides examples of using idt's more common features. For a complete description of idt functionality, see the idt man page.

Once again we assume that you have set your NCARG_ROOT and DISPLAY environment variables correctly as discussed in the "Environment" section. Furthermore, idt requires a device specification of X11. One way to accomplish this is by setting your GRAPHCAP environment variable to X11:

setenv GRAPHCAP X11

A note on entering text: In many of the examples that follow, you will be asked to enter text into a "text widget" that appears on your screen as part of the application. To enter text into a text widget, you must move the mouse sprite so that it is completely inside the surrounding text widget box. Depending on your system's configuration, the sprite should change appearances to confirm its position within the text widget. The Athena Text widget provides many keyboard editing commands. These commands allow you to edit the buffer. The default key bindings are patterned after those in the emacs editor. For example, typing CONTROL-k will delete from the cursor to the end of the line, and DELETE key will delete the previous character. If you are familiar with the emacs editor, you should have no trouble entering text into NCAR Graphics GUIs. If you are not familiar with emacs, you may have to experiment a little to get the hang of it.

Translating metafiles (idt)

You are now ready to examine a metafile on your X11 workstation or X11 terminal. See the preceding section "Getting started---Translating metafiles with ctrans" for instructions on creating a metafile. The following examples assume you have already created a metafile and its name is bnchmk.ncgm. We also assume that you have not modified any of the X11 resources that describe the appearance of idt.

Invoke idt by typing:

idt bnchmk.ncgm
Two windows should appear on your screen. The position and dimensions of these windows may be adjusted with the mouse after it appears, or it may be pre-set through your X resource file. The details for manipulating window geometry after the window appears depend on the window manager you are running. See your system administrator, or consult the document "The X Window System at NCAR," which offers some guidelines for customizing your X11 environment.

The idt control panel

The control panel (Figure 1) provides a simple mechanism for managing displays (Figure 2). A display represents a metafile, a drawing canvas, and a set of functions for determining how the metafile is plotted on the drawing canvas. The control panel simply allows you to create and destroy displays. Before you can "create" a display, you need to "select" a metafile. A metafile can be selected either by using the <select file> button on the control panel or by placing its name on the command line. Invoking idt with a metafile name as a command line argument not only "selects" the metafile but also "creates" the display.

Figure 1.

The display panel

On most systems, the name of the metafile appears in the title bar at the top of the display. The first row in the display contains a scroll bar for randomly selecting a frame to be translated. The label bar to the right displays the number of the frame to which you have scrolled. The display is intended to resemble a video tape recorder. Thus, the second row contains <playback>, <jogback>, <stop>, <jog>, and <play> buttons, respectively. The third and fourth rows contain more complex display commands. See Figure 2.

Figure 2.
If you select the <jog> button (the fourth button from the left on the second row), the first frame in the file should be displayed. Selecting this button a second time will cause the second frame to be displayed, and so forth. You can go backward through the file one frame at a time by selecting the <jogback> button, the second button from the left on the second row. The buttons on either end of the second row are for continuous playing in either the reverse or forward directions, respectively. The <stop> button in the middle halts either <play> or <playback>.

There are two ways to randomly access frames in your metafile: the first is by using the scrollbar in the upper left corner of the display panel. The second is by using the <goto> button in the third row. When you select <goto>, a small popup dialog box appears and prompts you for the number of the frame to display. If you enter 2 and then select <ok> from the <goto> popup, the second frame in the metafile will be displayed.

idt segments <start segment> <stop segment>

It is sometimes convenient, particularly with large NCGMs, to work with only a subset of the frames contained in the file. The <start segment> and <stop segment> buttons allow you to define such a subset, called a segment. Initially, the defined segment is the entire file. The <start segment> and <stop segment> buttons allow you to redefine the starting and ending frame boundaries, respectively, of the current segment. Subsequent idt commands will treat the metafile as if it only contained the frames defined by the current segment. For example, if you select the <play> button after setting <stop segment> to 4, playing will halt after the fourth frame in the file is displayed.

Continuous playing <loop>

The <loop> button toggles the continuous play mode of idt on and off for subsequent "plays" When idt is in continuous play mode, the currently defined segment is treated as if it were circular. There is no longer a beginning and ending frame in the segment. Selecting <play> (or <playback>) will cause idt to display all the frames in the current segment, and then return to the first (or last) frame and start over. Playing will not stop until aborted by selecting the <stop> button.

Printing frames <print>

Your site may have facilities for printing translated metafiles. idt may provide an interface to these devices, depending on how the application has been configured. Selecting the <print> button will present you with a menu of the known printing devices for which idt has been configured. If you select <print> and a menu appears, you may select a printer by dragging the mouse sprite down to the desired menu item and releasing the mouse button. Selecting a printer sends the current frame to that device. New printers may be added dynamically to idt. Consult the man page on ncarv_spool for details.

Saving metafiles <save>

The <save> button allows you to write the metafile describing the current frame to a file. This file can translated at a later time. When you select <save>, a dialog popup appears asking you for the name of the file to save to. Enter the name of the file and select <ok>. If the file already exists and it is a valid NCAR CGM, the current frame will be appended to this file. If the file exists and it is not an NCAR CGM, it will be overwritten. If the file does not exist, it will be created.

Enlarging a plot <zoom>

The <zoom> button is used for zooming and panning an area displayed in the drawing canvas. For example, suppose you want the upper left corner of the currently displayed plot to be blown up to fill the entire drawing canvas. Select <zoom> from the display panel. The mouse sprite should change its shape, signaling to you that you are in zoom mode. Move the sprite to the center of the drawing canvas. Click and hold any of the mouse buttons. Move the sprite to the upper left corner of the viewing window and release the mouse button. The rectangular area described by the sprite motions will be redrawn to fill the entire viewing window. To unzoom your plot, select the <unzoom> button from the fourth row of the display panel.

Desktop animation <animate>

One of idt's most useful features is its ability to perform animation on the desktop. Before we discuss how to use this powerful tool, it may help to briefly explain the hows and whys of desktop animation.

You may have already noticed when experimenting with the <play> and <playback> buttons that no matter how fast your X server is, the ensuing sequence of images displayed to your screen does not make a very acceptable animation: To your eye it appears as though images flicker and fail to proceed along in a smooth, animated motion. This is due to an anomaly known as "temporal aliasing." Temporal aliasing results partially from the fact that whatever is drawn on the drawing canvas (lines, filled areas, etc.) is immediately visible. Another factor is the speed at which objects are drawn. As the complexity of an image increases, so does the drawing time.

One way to help alleviate temporal aliasing is to postpone the display of an image until it is completely drawn. This is how the animate function works in idt; images (or frames) are predrawn into an area of memory managed by the X server. When it is time to display each frame in an animation sequence, the entire frame can be posted to the drawing canvas instantly. The rate at which successive images can be posted in this manner is not affected by the complexity of the image, but rather on its spatial resolution (width and height in pixels). If the images are small enough in terms spatial resolution, the X server will be able to display them rapidly and at a constant rate to produce a smooth, flicker-free animation.

There is one drawback to this approach, however: the amount of RAM (random access memory) required to store each predrawn image. RAM is a limited resource on any workstation or X terminal. Be aware that it is the X server memory that is of concern here. X servers run on the workstation that is physically attached to your graphics display.

On most systems, the amount of memory required by an animation sequence can be computed by multiplying the number of pixels in the drawing canvas by the number of frames in the sequence. For example, if your drawing canvas is 512 by 512 pixels and you have 10 frames, then the total memory required is about 512 * 512 * 10 = 2,621,440 bytes. The amount of memory that is actually available to your X server will vary depending on how much physical memory is installed on your machine and what other processes are currently running. So how do you determine how "big" an animation you can run? Experimentation is probably the best method. Start with something small, just a few frames, then move up. If the animation gets too big, there will be a noticeable degradation in performance.

One final word of caution: UNIX processes typically do not free up dynamically allocated memory for other processes to use until they exit. Using idt to animate imagery may cause your X server to grow substantially. On some systems, this is a trivial issue because each console login session starts a new X server. On other systems, it may be necessary to restart your server manually if it grows too big. Check with your system administrator. Your system administrator should also be able to tell you how to monitor the size of your X server.

Below are the steps, and some guidelines, for successfully animating a sequence of metafile imagery.

The first step is to determine how many and which frames you want to animate from your file. As discussed, it's likely that it will not be possible to animate your entire file at once. Fortunately, idt provides a convenient mechanism for selecting only a portion of your image file: segments (see the section on idt segments). When you enter animate mode, only the currently defined segment is preprocessed and loaded into server memory. By default, the current segment is the entire metafile, so you may need to define a new segment. Defining a new segment is a simple process and was discussed earlier in the "idt segments" section. We'll assume we are still working with the bnchmk.ncgm file, which only contains six frames. For this example, we will animate the entire file. You may want to check the value of the <start segment> and <stop segment> to verify that the current segment is indeed the entire file.

The next step is to select the <animate> button. idt will begin processing the frames in the segment and loading them into server memory. For our example, this should only take a few seconds. For a really large segment, this process might take several minutes (Note: the total preprocessing time also depends on the size of the drawing canvas.). The text window in the control panel will keep you informed of idt's progress.

When the preprocessing is complete, the text window in the control panel will display the message "Done loading x images," where x is the number of frames in your segment. Now you can start your animation. The animation is controlled the same way that the display of normal metafile imagery is controlled. Selecting the <play> button will "play" your animation in the forward direction. The <playback> button will cause your animation to play backward. If <loop> is turned on, the animation will loop repeatedly until the <stop> button is selected.

One animation feature that is only available in animation mode is the <delay> button. If your animation is moving too fast, you can slow it down by setting a delay time, in 1/100's of a second. By default, the delay time is 0.

To terminate your animation and go back to normal metafile processing, select the <animate> button a second time.

Terminating a plotting session <done>

When you are done viewing a particular metafile, you can terminate the display session by selecting the <done> button. Terminating a display session does not terminate the entire application. To exit idt, you may select <quit> from the main control panel at any time.

The idt file browser

Once idt is up and running, you may want to take advantage of idt's file browser for selecting metafiles for translation. idt has a built-in file finder that you can access by selecting the <select file> button from the control panel. Selecting this button at any time causes a popup menu labeled "file finder" to appear. The names of the files in your current working directory should be listed in the text box in the middle of this file selector. If you created the bnchmk.ncgm metafile earlier, you should see it listed somewhere in this box. You may need to use the scrollbar to bring it into view. When you have found it, select bnchmk.ncgm with the mouse. The string bnchmk.ncgm should appear in the box labeled "selection," indicating that bnchmk.ncgm is the current selection.

The uppermost box of the popup file selector contains a text widget with an asterisk (*) displayed. The asterisk is the metacharacter used in both the Bourne shell and the C shell for matching zero or more characters in a filename. Hence all the files in the current directory are displayed in the middle box. You can change the mask for the file finder by simply editing the file finder text widget and selecting <finder>. For example, if you enter:

and select <finder> or press RETURN, all of the files in the current directory with the ".ncgm" name extension will be displayed. Or, if you enter:
all of the files in the user directory /u1/joe will be displayed.

When the desired filename is displayed in the selection box, you may confirm your choice by selecting <ok>. You have now informed idt of the name of the next metafile to process. idt will pop up a new display panel with which to plot the new metafile.

Metafile editing

The med program

The med program is a frame-level metafile editor similar in style and syntax to ed, the UNIX line editor. med allows you to interactively modify metafiles at the frame level. It is important to understand that the smallest atomic object that med can work with is a frame. med does not allow you to change the appearance of individual frames, except when you composite frames on top of each other.

All of med's editing functions are performed in a temporary buffer. This greatly improves the speed with which med can perform its actions. Changes made in the buffer have no effect on the original file until the buffer is written to disk with the write command. If you terminate a med session without saving the buffer, all changes will be lost.

Under normal invocation, med will enter command mode and display the prompt med>. The prompt is a signal that med is ready to accept commands from the user. The basic format of a med command is:

where address is an optional field listing a range of frames on which to perform the named command. Frames are numbered starting with 1. A comma-separated pair of numbers specifies an inclusive range of frames on which to operate. If no address is given, the current frame is assumed as default. As with ictrans, med commands may be abbreviated up to the point that they are unique. A complete description of addressing syntax is given in the man page for med.

Assuming you have an NCAR CGM named bnchmk.ncgm, invoke med with the command:

med bnchmk.ncgm
med will respond with the number of frames in the file. The current frame is set to the last frame in the file. (Note that this is different from ictrans, which uses the first frame in the file as the default current frame). If bnchmk.ncgm was created with the ncargex utility described earlier, med should report the existence of six frames.

Listing buffer contents (print)

To list the contents of the buffer at any time, use the print command. The print command will report a frame's offset, that is, its relative position in the file, the number of 1440-byte records contained in the frame, the offset of the first record of the frame in the file, and an optional description of the frame if it exists. To list the entire contents of the buffer containing the bnchmk.ncgm metafile type:

med> 1,6print
A listing for each of the six frames will appear.

Writing frames to a file (write)

To write the first three frames in bnchmk.ncgm to a file called test.ncgm, type:

med> 1,3write test.ncgm
The first three frames in bnchmk.ncgm will be written to test.ncgm. If the file already exists, it is overwritten.

Appending frames to a file (append)

To append a frame to an existing file, use the append command. If you use the write command as in the previous example, the file will be overwritten and will only contain the sixth bnchmk.ncgm frame. To append the sixth frame of bnchmk.ncgm to the end of the file test.ncgm, type:

med> 6 append test.ncgm
The file test.ncgm now contains the first through third and the sixth frame from the file bnchmk.ncgm.

Reading a file into the buffer (read)

To read the contents of another NCGM into the current buffer without destroying the buffer contents, use the read command:

med> read test.ncgm
This will read all of the frames contained in the file test.ncgm and append them after the current frame. med will report the number of frames read successfully. A destination other than the current frame offset may be specified as the target for the incoming frames. The command:
med> 1 read test.ncgm
will append the contents of the file test.ncgm after the first frame in the buffer. The first frame in test.ncgm will become the second frame in the buffer.

Splitting a large metafile (split)

Suppose you have a single NCGM that contains a large number of frames. Since large NCGMs can be quite unwieldy, it is often more convenient to have several smaller files than one huge one. The split command can be used to divide a single file into an ordered sequence of smaller ones. For example, suppose you were editing a large metafile with 1,000 frames in it. You could use the command:

med> 1,$ split 10
to create a sequence of files named med001 through med010, each containing one hundred frames. med001 would contain the first hundred frames from the original file, med002 would contain the second hundred frames, and so forth.

The number of split files created does not have to be an integral divisor of the total number of frames. med will do its best to keep the same number of frames in each split file.

Copying frames (copy)

To duplicate a set of frames and place them somewhere else in the metafile, use the copy command. The general format of the copy command is:

[source_frames] copy [destination]

where source_frames is a list of frames to copy and destination is the destination frame of the copy. The frames copied are appended after destination. For example, to make a copy of the first two frames and place them at the end of the file, use:
med> 1,2 copy $
In this example, the $ is an alias for the last frame in a file. This is helpful if you do not know how many frames are contained in the NCGM.

Deleting frames (delete)

To remove some frames from the buffer, use the delete command. To delete the last two frames in the buffer:

med> $-1,$ delete
The $-1 denotes the offset of the last frame minus 1, that is, the second from last frame. The second $ denotes the last frame, so $-1,$ denotes the last two frames in the buffer.

Moving frames (move)

The move command can be used to change the relative position of frames in a metafile. Like the copy command, the general format of the move command is:

[source_frames] move [destination]
To move the first frame to the end of the file, making the second frame the first frame and the original first frame the last frame, type:
med> 1 move $

Compositing frames (merge)

The med utility can do a limited amount of compositing with the merge command. merge will append the drawing instructions of one frame to the end of another frame. When plotted, the resulting image appears as if one frame were composited on top of another. The command:

med> 1,2 merge
appends the drawing instructions contained in frame 2 to frame 1. This changes frame 1, and frame 2 is unaltered.

 This command may produce unexpected results depending on how          
 color and GKS point, line, and text attributes are specified in the   

Editing a new file (edit)

The edit command causes the entire contents of the buffer to be erased and replaced with a new metafile. The command:

med> edit test.ncgm
will read the file test.ncgm into the buffer.

The edit command has a safeguard to prevent you from accidentally destroying changes made to a buffer that have not been saved. If unsaved changes exist, the edit command will fail and print a warning message. To prevent this, save the file before executing edit:

med> w
or append a ! to the edit command. This tells edit to disregard any unsaved changes:
med> edit ! test.ncgm

Obtaining help (help)

The help command, without any arguments, provides a list of all available commands and a brief synopsis of what each command does. It will also give you the appropriate syntax for a particular command. For example, if you type:

med> help delete
med will describe the syntax for the delete command.

Batch processing

Like ictrans, med offers a batch or command line processing capability that is sometimes preferable to the interactive approach.

Like ictrans, med performs commands from the command line using the -e option.

For example, when issued from the command line, the command:

med -e '1,$ split 3 bnch' bnchmk.ncgm
would split the file bnchmk.ncgm into three smaller files: bnch001.ncgm, bnch002.ncgm, and bnch003.ncgm.

The command:

med -e 'r bnch001.ncgm' -e 'r bnch002.ncgm' -e 'r bnch003.ncgm' -e 'w all.ncgm'
would concatenate the bnch* files into a single metafile, all.ncgm. The all.ncgm file should be identical to the original bnchmk.ncgm.

Interoperability with other tools

NCL and NCAR Graphics provide utilities to facilitate interoperability with other software. In essence, these tools may provide a bridge between NCL/NCAR Graphics and some of the more ubiquitous software packages that may be available at your site. This section identifies these tools and takes a brief look at how they are commonly used.

Non-NCAR Computer Graphics Metafiles

CGMs that were not created by NCL or NCAR Graphics are not suitable for processing by NCL/NCAR Graphics applications. NCGMs contain information not present in vanilla CGMs; this information helps to speed up the parsing of the file. This parsing information must be added to "vanilla" CGMs before they can be processed by NCL and NCAR Graphics applications. The command:

cgm2ncgm < vanilla.cgm > ncar.ncgm
will read in a vanilla CGM named vanilla.cgm and produce an NCAR CGM named ncar.ncgm.

Similarly, NCGMs cannot be parsed by non-NCAR Graphics applications. To convert the NCGM ncar.ncgm to the vanilla CGM vanilla.cgm, use:

ncgm2cgm < ncar.ncgm > vanilla.cgm
 A converted CGM may not produce entirely correct results when           
 translated. NCAR Graphics translators operate on a subset of elements   
 of the CGM standard (as do many CGM translators) and thus may           
 encounter valid CGM elements that they do not know how to translate.    
 Similarly, NCGMs converted to vanilla CGMs are not guaranteed to be     
 correctly translated or even parsed by third-party translators.         

Rasterizing NCGMs

Many commercial and public-domain scientific visualization packages accept raster imagery as an input source. The primary image format used by NCAR Graphics is the NCGM. However, you may use ctrans to convert your NCGM into a number of different raster image formats. The general way to make such a conversion is:

ctrans -d format -res resolution metafile > raster_file
For example, the command:
ctrans -d hdf -res 512x512 myfile.ncgm > myfile.hdf
converts the NCGM myfile.ncgm into an HDF raster file, myfile.hdf, with a 512x512 pixel resolution. HDF files are accepted by the NCSA application XImage.

The command:

ctrans -d avs -res 200x200 myfile.ncgm > myfile.avs
converts myfile.ncgm into a 200x200-pixel AVS file. The AVS raster file format is suitable for ingestion by the Application Visualization System. Finally, an SGI raster file may be created with a command like:
ctrans -d sgi -res 200x200 myfile.ncgm > myfile.sgi
SGI raster files are understood by Silicon Graphics Inc.'s Explorer.