Kapittel 13. Å utvide GIMP


1. Programtillegg
1.1. Innleiing
1.2. Å bruke programtillegg
1.3. Å installera nye programtillegg
1.4. Å skrive programtillegg
2. Å bruke Script-Fu
2.1. Script-Fu?
2.2. Installing Script-Fu scripts
2.3. Råd og vink
2.4. Ulike typar av Script-Fu
3. Ei innføring i Script-Fu
3.1. Bli kjent med Scheme
3.2. Variablar og funksjonar
3.3. Lister, lister og atter lister
3.4. Det første Script-Fu skriptet ditt
3.5. Blås liv i skriptet
3.6. Utvid skriptet Text Box
3.7. Skriptet og resultatet av det

1. Programtillegg

1.1. Innleiing

Ein av fordelane med GIMP er at det er lett å utvida funksjonane ved hjelp av såkalla programtillegg (engelsk: «Plugins»). Programtillegga samarbeider svært nær med GIMP, men det er GIMP som har kontrollen med dei. Ofte gjer ikkje programtillegga anna enn det du også kan gjere direkte, men på ein for deg enklare måte. Den store fordelen med desse programma, er at det er lett å lage tilleggsfunksjonar utan å måtte gjere endringar i det nokså kompliserte programmet GIMP er. Mange verdfulle programtillegg har ei kjeldekode på berre 100 - 200 linjer. Dei fleste er skrivne i språket «C» eller ein variant av dette.

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.

[Åtvaring] Åtvaring

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.

[Notat] Notat

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. Å bruke programtillegg

Oftast vil du kunne bruke eit programtillegg i GIMP utan å bry deg om eller merka at det eigentleg er eit tillegg. Det er likevel nokre småting med programtillegga som det kan vere nyttig å kjenne litt til.

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.

[Notat] Notat

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

Når eit programtillegg krasjar, dukkar det opp ei melding frå GIMP om at programtillegget kan ha øydelagt GIMP. Du får beskjed om å lagre alt du held på med og avslutta GIMP. Dette kan i teorien vere heilt rett. Programtillegga er i stand til å endre nesten kva det skal vere i GIMP. Praktisk røynsle har derimot vist at det er sjeldan at programtillegga gjer nemnande skade. Som oftast kan du halde fram som om ingenting var hendt. Kanskje etter å ha vurdert kva du taper dersom programmet likevel skulle vere øydelagt og arbeidet ditt forsvinna.

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. Å installera nye programtillegg

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

Dei fleste programtillegga kan delast i to grupper, dei små med kjeldekoden levert i ei enkelt .c-fil, og dei større der kjeldekoden kjem i ei mappe med fleire ulike filer, deriblant også ei «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

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. Å skrive programtillegg

Har du ønskje om å lære å skrive programtillegg, kan du finne mykje hjelp på utviklingssidene til GIMP[GIMP-DEV-PLUGIN]. GIMP er eit nokså komplisert program, men utviklarane har lagt mykje arbeid i å flate ut læringskurva for å lage programtillegg. Du finn fyldige instruksjonar og gode eksempel på dei nemnde sidene. Hovudbiblioteket («libgimp») som programtillegga nyttar for å kommunisera med GIMP, er i tillegg veldokumentert. Ein mykje brukt måte for å lære seg programmeringa, er å modifisere eksisterande program. Dyktige programmerarar er dermed ofte i stand til å lage svært så interessante ting berre etter eit par netters arbeid.