Kapitel 13. Skriptning

Innehållsförteckning

1. Insticksmoduler
1.1. Introduktion
1.2. Använda insticksmoduler
1.3. Installera nya insticksmoduler
1.4. Skriva insticksmoduler
2. Använda Script-Fu-skript
2.1. Script-Fu?
2.2. Installing Script-Fu scripts
2.3. Så gör man och så gör man inte
2.4. Olika sorters Script-Fu
2.5. Bildberoende skript
3. A Script-Fu Tutorial
3.1. Getting Acquainted With Scheme
3.2. Variables And Functions
3.3. Lists, Lists And More Lists
3.4. Ditt första Script-Fu-skript
3.5. Giving Our Script Some Guts
3.6. Extending The Text Box Script
3.7. Ditt skript och hur det fungerar

1. Insticksmoduler

1.1. Introduktion

En av de trevligaste sakerna med GIMP är hur lätt man kan utöka dess funktionalitet genom att använda insticksmoduler. GIMP-insticksmoduler är externa program som körs under GIMP-huvudprogrammets kontroll och interagerar mycket nära med det. Insticksmoduler kan manipulera bilder på nästan alla sätt som en användare kan. Deras fördel är att det är mycket lättare att lägga till en egenskap till GIMP genom att skriva en liten insticksmodul än att skriva om den enorma mängden komplex kod som utgör kärnan i GIMP. Många betydande insticksmoduler har källkod skriven i C som endast innehåller runt 100-200 rader.

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.

[Varning] Varning

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.

[Notera] Notera

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. Använda insticksmoduler

För det mesta kan du använda en insticksmodul som vilket GIMP-verktyg som helst utan behöva känna till att den är en insticksmodul. Men det finns några saker om insticksmoduler som kan vara bra att känna till.

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.

[Notera] Notera

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.

[Tips] Tips

When a plugin crashes, GIMP gives you a very ominous-looking message telling you that the plugin may have left GIMP in a corrupted state, and you should consider saving your images and exiting. Strictly speaking, this is quite correct, because plugins have the power to alter almost anything in GIMP, but for practical purposes, experience has shown that corruption is actually quite rare, and many users just continue working and don't worry about it. Our advice is that you simply think about how much trouble it would cause you if something went wrong, and weigh it against the odds.

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. Installera nya insticksmoduler

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-liknande system

De flesta insticksmoduler faller in under två kategorier: de mindre som distribueras som en enda .c-fil och de större vars källkod är distribuerad i en mapp som innehåller flera filer, däribland en Makefile.

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

Hur du installerar insticksmoduler i OS X beror mest på hur du installerade GIMP som sådant. Om du var en av de djärva och installerade GIMP genom en av pakethanterarna som fink [FINK] eller darwinports [DARWINPORTS] fungerar insticksinstallationen exakt på samma sätt som redan beskrivits för Linuxplattformen. Den enda skillnaden är att ett par insticksmoduler till och med kan finnas tillgängliga i förrådet för din pakethanterare, så prova det.

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. Skriva insticksmoduler

Om du vill lära dig att skriva en insticksmodul hittar du gott om hjälp på GIMP-utvecklarnas webbplats [GIMP-DEV-PLUGIN]. GIMP är ett komplext program, men utvecklargruppen har gjort stora ansträngningar för att sänka inlärningströskeln för skrivning av insticksmoduler: det finns bra instruktioner och exempel och huvudbiblioteket som insticksmoduler använder som gränssnitt till GIMP (som kallas libgimp) har ett väldokumenterat API. Bra programmerare, som lär sig genom att modifiera befintliga insticksmoduler kan ofta åstadkomma intressanta saker efter bara ett par dagars arbete.