One of the goals of &kstars; is to provide the ability to playback complicated behaviours from a script. This will allow you to create virtual tours of the heavens, and will enable teachers to construct classroom demos to illustrate certain astronomical concepts. It is already possible to write such scripts for &kstars;, although not all of the desired functions have been included. Also, while we will eventually have a GUI-based script builder tool, the scripts must currently be written by hand. This chapter will explain how to write &kstars; scripts. The &kde; architecture provides the necessary framework for scriptable applications via the DCOP interface. DCOP stands for Desktop Communication Protocol; through DCOP, &kde; applications can be controlled by other applications, from a terminal prompt, or through a text script. The &kstars; DCOP Interface includes the following functions: lookTowards( const QString direction ): Point the display focus in a direction specified by the argument. This can be the name of any object in the sky, or one of the following directional words or abbreviations: zenith (or z), north (n), northeast (ne), east (e), southeast (se), south (s), southwest(sw), west(w), northwest (nw). setRaDec( double ra, double dec ): Point the display focus at the specified equatorial coordinates. setAltAz(double alt, double az): Point the display focus at the specified horizontal coordinates. zoomIn(): Increase the display's Zoom level. zoomOut(): Decrease the display's Zoom level. defaultZoom(): Reset the display to Zoom level = 3 (the default). setLocalTime(int yr, int mth, int day, int hr, int min, int sec): Set the simulation clock to the specified date and time. waitFor( double t ): Pause for t seconds before continuing with subsequent script commands. waitForKey( const QString k ): Halt the script execution until the user presses the specified key. At this point, you cannot specify combination keystrokes (such as &Ctrl;C); just use simple keys. You can type space to indicate the spacebar. setTracking( bool track ): Toggle whether tracking mode is engaged. changeViewOption( const QString option, const QString value ): Adjust a view option. There are dozens and dozens of options available; basically everything you can change in the Configure &kstars; Window can be changed here as well. The first argument is the name of the option (the names are taken from the kstarsrc configuration file), and the second argument is the desired value. The argument parser is designed to be robust, so if you accidentally send it bad data it should fail gracefully. setGeoLocation( const QString city, const QString province, const QString country ): Change the observing location to the specified city. If no city matching the argument strings is found, then nothing happens. stop() [clock]: Halt the simulation clock. start() [clock]: Start the simulation clock. setScale(float s) [clock]: Set the rate of the simulation clock. s=1.0 corresponds to real time; 2.0 is twice as fast as real-time, etc. You can try out the DCOP functions very easily using the kdcop program. When you run kdcop, you will see a tree-list of all running programs; if &kstars; is running it will be listed. Most of the DCOP functions are listed under the KStarsInterface heading, but the clock functions are listed under clock. Double-click on any function to execute it. If the function requires arguments, a window will open in which you can input the values. DCOP functions can also be called from the UNIX command line, and these can be encapsulated in a script. We will create an example script that switches to Equatorial coordinates, points the display at the Moon, zooms in a bit, and accelerates the clock to 1 hour per second. After tracking the Moon for 20 seconds, the clock is paused and the display zooms out. You can use this script as a template for making new scripts. I will list the entire script first, and then explain its various parts. #!/bin/bash #KStars script: Track the Moon! # KSTARS=`dcopfind -a 'kstars*'` MAIN=KStarsInterface CLOCK=clock#1 dcop $KSTARS $MAIN changeViewOption UseAltAz false dcop $KSTARS $MAIN lookTowards Moon dcop $KSTARS $MAIN defaultZoom dcop $KSTARS $MAIN zoomIn dcop $KSTARS $MAIN zoomIn dcop $KSTARS $MAIN zoomIn dcop $KSTARS $MAIN zoomIn dcop $KSTARS $MAIN zoomIn dcop $KSTARS $CLOCK setScale 3600. dcop $KSTARS $CLOCK start dcop $KSTARS $MAIN waitFor 20. dcop $KSTARS $CLOCK stop dcop $KSTARS $MAIN defaultZoom ## Save this script to a file. The filename can be anything you like; I suggest something descriptive like trackmoon.kstars. Then type the following command to make the script executable: chmod a+x trackmoon.kstars . The script can then be executed at any time by typing ./trackmoon.kstars in the folder which contains the script. Note that the script will only work if an instance of &kstars; is already running. You can use the dcopstart command in a script to launch a new instance &kstars;. Now to the explanation of the script. The top line identifies the file as a BASH shell script. The following two lines are comments (any line beginning with # is a comment, and is ignored by the shell). The next three lines define some convenience variables that will be used later. The KSTARS variable identifies the currently-running &kstars; process, using the dcopfind command. MAIN and CLOCK identify the two DCOP interfaces associated with &kstars;. The remainder of the script is the actual list of DCOP calls. The first command sets the display to use Equatorial coordinates by setting the UseAltAz option to false (again, you can see a list of all options that changeViewOption can use by examining your kstarsrc configuration file). The next command centres the display on the Moon, and automatically engages tracking. We then set the default zoom level, and then zoom in five times. Next, the clock's timescale is set to 1 hour per second (3600 seconds is one hour), and the clock is started (in case it was not already running). The next line pauses the script for 20 seconds while we track the Moon as it moves across the sky. Finally, we stop the clock and reset the zoom level to its default setting. We hope you enjoy the scripting abilities of KStars. If you create an interesting script, please email it to kstars@30doradus.org; we would like to see what you have done, and may post some scripts on our webpage. Also, if you have any ideas for how to improve scripting (or any part of &kstars;), let us know at kstars-devel@lists.sourceforge.net or submit a wishlist item to bugzilla.