Kapitel 13. Skripterstellung


1. Erweiterungen (Plugins)
1.1. Einführung
1.2. Mit Plugins arbeiten
1.3. Neue Plugins installieren
1.4. Plugins schreiben
2. Skript-Fu verwenden
2.1. Skript-Fu?
2.2. Installing Script-Fu scripts
2.3. Typische Fehler
2.4. Verschiedene Skript-Fu-Arten
2.5. Bildabhängige Skripte
3. Ein Skript-Fu-Tutorial
3.1. Der erste Kontakt mit Scheme
3.2. Variablen und Funktionen
3.3. Listen, Listen und noch mehr Listen
3.4. Ihr erstes Fu-Skript
3.5. Unserem Skript Leben einhauchen
3.6. Das Textbox-Skript erweitern
3.7. Ihr Skript und seine Ausgabe

1. Erweiterungen (Plugins)

1.1. Einführung

Eine der besten Eigenschaften vom GIMP ist, wie einfach man die Funktionalität erweitern kann, nämlich mit »Plugins« (Erweiterungen). GIMP-Erweiterungen sind externe Programme, die unter der Kontrolle des GIMP-Hauptprogrammes laufen und sehr eng mit diesem zusammenarbeiten. Plugins können Bilder praktisch genauso verändern, wie Benutzer es können. Ihr Vorteil liegt darin, dass es sehr viel einfacher ist, GIMP um eine Fähigkeit zu erweitern, indem man ein kleines Plugin schreibt, als den extrem umfangreichen und komplexen Code des GIMP-Kernes zu verändern. Viele wichtige Erweiterungen bestehen lediglich aus 100 oder 200 Zeilen C-Code.

Several dozen plugins are included in the main GIMP distribution, and installed automatically along with GIMP. Most of them can be accessed through the Filters menu (in fact, everything in that menu is a plugin), but a number are located in other menus. In many cases you can use one without ever realizing that it is a plugin: for example, the "Normalize" function for automatic color correction is actually a plugin, although there is nothing about the way it works that would tell you this. Even importing and exporting of images is done by plugins.

Everyone can write a GIMP plugin and make it available online. There are many useful plugins that can be obtained this way. Some of them are described elsewhere in the User's Manual.

With this free availability comes a certain degree of risk. The fact that anyone can release plugins means that there is no effective quality control. The plugins distributed with GIMP have all been tested and tuned by the developers. Additional plugins available online, may have been hacked together in a few hours and then abandoned. Some plugin creators don't care about robustness, and even for those who do, their ability to test on a variety of systems in a variety of situations is often quite limited. Basically, when you download a plugin, you are getting something for free, and sometimes you get exactly what you pay for. This is not to discourage you, just to make sure you understand that not all plugins available online will deliver what you expect from them.

[Warnung] Warnung

Plugins, being full-fledged executable programs, can do all of the things that any other program can do. This includes installing back-doors on your system or otherwise compromise its security. Don't install a plugin unless it comes from a trusted source.

[Anmerkung] Anmerkung

Plugins written for a certain version of GIMP may not always work well in other versions. Though in general the GIMP team tries to minimize changes that affect plugins. Usually the only time you can expect serious problems with plugins, is when the major version of GIMP changes. When a plugin made for an older version doesn't work correctly anymore, it needs to be ported. Sometimes this is easy, sometimes not. Bottom line: before trying to install a plugin, make sure that it is compatible with your version of GIMP.

1.2. Mit Plugins arbeiten

Meistens können Sie eine Erweiterung wie jedes andere GIMP-Werkzeug verwenden, ohne sich dessen bewusst zu sein, dass es ein Plugin ist. Allerdings ist es hilfreich, ein paar Dinge über Plugins zu verstehen.

One is that plugins are generally not as robust as the GIMP core. When GIMP crashes, it is considered a very serious thing: it can cost the user a lot of trouble and headache. When a plugin crashes, the consequences are usually not as serious. In most cases you can continue working without worrying about it too much.

[Anmerkung] Anmerkung

Because plugins are separate programs, they communicate with GIMP in a special way: The GIMP developers call it talking over a wire. When a plugin crashes, the communication breaks down, and you may see an error message about a wire read error.

[Tipp] Tipp

Wenn eine Erweiterung abstürzt, erhalten Sie von GIMP die sehr beunruhigende Meldung, dass das Plugin GIMP möglicherweise in einem fehlerhaften Zustand hinterlassen hat und Sie besser Ihre Daten sichern und das Programm beenden sollten. Genau genommen ist das völlig korrekt, da Erweiterungen nahezu alles in GIMP verändern können, aber praktisch zeigt die Erfahrung, dass eine Datenverfälschung tatsächlich sehr selten ist, daher arbeiten viele Benutzer einfach weiter und kümmern sich nicht darum. Unsere Empfehlung lautet: überlegen Sie einfach, wie groß der angerichtete Schaden ist, falls etwas schief geht, und wägen Sie danach ab.

Because of the way plugins communicate with GIMP, they do not have any mechanism for being informed about changes you make to an image after the plugin has been started. If you start a plugin, and then alter the image using some other tool, the plugin may crash. Even if it doesn't, doing this may cause incorrect results. You should avoid running more than one plugin at a time on an image, and avoid doing anything to the image until the plugin has finished working on it. If you ignore this advice, not only could you screw up the image, you may also screw up the undo system, so that you won't be able to recover from your mistake.

1.3. Neue Plugins installieren

The plugins that are distributed with GIMP don't require installation. Plugins that you download yourself do. Usually the default location is in GIMP's user directory in a folder under /plug-ins, where the folder name needs to be the same as the plugin filename. You can find the default locations where GIMP searches for plugins in GIMP's folder preferences. There you can also add new locations where GIMP should look for plug-ins. There are several scenarios, depending on what OS you are using and how the plugin is structured.

1.3.1. Linux / Unix-like systems

Die meisten Plugins gehören in eine von zwei Kategorien: kleine, deren Quellcode in einer einzigen C-Datei ausgeliefert wird, und größere, deren Quellcode in einem Verzeichnis mit mehreren Dateien inklusive einem Makefile ausgeliefert wird.

For a simple one-file plugin, call it borker.c, installing it is just a matter of running the command gimptool-2.0 --install borker.c. This command compiles the plugin and installs it in your personal plugin directory, ~/gimp-2.10/plug-ins unless you have changed it. This will cause it to be loaded automatically the next time you start GIMP. You don't need to be root to do these things; in fact, you shouldn't be. If the plugin fails to compile, well, be creative.

1.3.2. Windows

Most GIMP plugins available on Windows supply either an installer, or can be downloaded in a pre-compiled binary format ready to copy to a folder of your choice that is recognized by GIMP.

If an installer is available, that should do all the work for you selecting an appropriate folder and copying all relevant files. If not, you may have to check in GIMP's folder preferences where the plugins should be copied to. Remember, each plugin needs to be in its own folder with the same name as the plugin.

1.3.3. Apple Mac OS X

How you install plugins on OS X mostly depends on how you installed GIMP itself. If you were one of the brave and installed GIMP through one of the package managers like fink [FINK] or darwinports [DARWINPORTS], the plugin installation works exactly the way it is described for the Linux platform already. The only difference is, that a couple of plugins might be even available in the repository of your package manager, so give it a try.

If, on the other hand, you prefer to grab a prebuilt GIMP package like GIMP.app, you most likely want to a prebuilt plugin too. You can try to get a prebuilt version of the plugin of your dreams from the author of the plugin. Building your own binaries unfortunately involves installing GIMP.

1.3.4. Running the installed plugin

Once you have installed the plugin, how do you activate it? The menu path is determined by the plugin itself, so to answer this you need to either look at the documentation for the plugin (if there is any), explore the menus, or use GIMP's command search function by pressing / and then entering the name of the plugin. If you know how to read source code you could also check that to see in what menu it registers itself.

For more complex plugins, organized as a directory with multiple files, there usually is a file inside called either INSTALL or README, with instructions. If not, the best advice is to toss the plugin in the trash and spend your time on something else: any code written with so little concern for the user is likely to be frustrating in myriad ways.

If you install a plugin in your personal plugin directory that has the same name as one in the system plugin directory, only one can be loaded, and it will be the one in your home directory. You will receive messages telling you this each time you start GIMP. This is probably a situation best avoided.

1.4. Plugins schreiben

Wenn Sie lernen möchten, wie man eine Erweiterung schreibt, können Sie reichlich Hilfestellungen auf der GIMP-Entwicklerwebseite [GIMP-DEV-PLUGIN] finden. GIMP ist ein komplexes Programm, aber das Entwicklerteam hat erhebliche Anstrengungen unternommen, die Lernkurve fürs Schreiben von Plugins flach zu halten. Es gibt ausgezeichnete Unterweisungen und Beispiele, und die Programmbibliothek, die Plugins zur Kommunikation mit GIMP verwenden (»libgimp«), besitzt eine gut dokumentierte Schnittstelle (API). Gute Programmierer, die dadurch lernen, dass sie vorhandene Erweiterung modifizieren, sind oft schon nach wenigen Tagen in der Lage, erstaunliche Dinge zu erreichen.