Chapitre 13. Compléter GIMP

Table des matières

1. Greffons
1.1. Introduction
1.2. Utiliser les greffons
1.3. Installation de nouveaux greffons
1.4. Écrire des greffons
2. Utiliser les Script-Fu
2.1. Script-Fu ?
2.2. Installing Script-Fu scripts
2.3. Ce qu’il faut faire et ne pas faire
2.4. Différentes sortes de Script-Fu
3. Didacticiel pour Script-Fu
3.1. Faisons connaissance avec Scheme
3.2. Variables et fonctions
3.3. Listes, des listes et encore des listes
3.4. Votre premier script de Script-Fu
3.5. Donner un peu plus de corps à votre script
3.6. Compléter le script Text Box
3.7. Votre script et son fonctionnement

1. Greffons

1.1. Introduction

L’un des points les plus agréables du GIMP est qu’on peut aisément lui ajouter des fonctionnalités en utilisant des greffons. Les greffons de GIMP sont des programmes extérieurs qui tournent sous le contrôle de l’application principale du GIMP en interaction très proche avec lui. Les greffons peuvent manipuler une image presque de la même manière qu’un utilisateur. Leur avantage est qu’il est beaucoup plus simple d’ajouter une possibilité à GIMP en écrivant un petit greffon plutôt qu’en modifiant l’énorme quantité de lignes de codes qui constituent le noyau de GIMP. De nombreux greffons sont écrits en langage C et ne font que 100-200 lignes.

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.

[Avertissement] Avertissement

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.

[Note] Note

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. Utiliser les greffons

La plupart du temps vous utiliserez un greffon tout comme les autres outils de GIMP, sans même savoir qu’il s’agit d’un greffon. Mais il y a quelques petites choses qu’il est utile de savoir sur les greffons.

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.

[Note] Note

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 ».

[Astuce] Astuce

Quand un greffon se plante, GIMP vous affiche un message menaçant qui vous informe que le greffon peut l’avoir laissé dans un état corrompu, vous devrez alors sauvegarder vos images et quitter GIMP. En théorie, cela est vrai, parce que les greffons peuvent altérer à peu près tout dans GIMP, mais en pratique, l’expérience montre que la corruption est plutôt rare, et beaucoup d’utilisateurs continuent de travailler sans se soucier de cela. Notre conseil est de réfléchir aux conséquences d’un éventuel problème et de peser le pour et le contre.

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. Installation de nouveaux greffons

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

La plupart des greffons peuvent être classés en deux catégories : les petits dont le code source tient dans un seul fichier .c, et les plus gros dont le code source est distribué dans un répertoire contenant de nombreux fichiers dont un 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. Fenêtres

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, 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. Écrire des greffons

Si vous voulez apprendre comment écrire un greffon, vous trouverez de l’aide sur le site des développeurs du GIMP[GIMP-DEV-PLUGIN] (en anglais). GIMP est un programme complexe, mais l’équipe de développement a fait d’ardents efforts pour faciliter l’apprentissage de l’écriture des greffons : de bonnes instructions, des exemples et la principale bibliothèque que les greffons utilisent comme interface avec GIMP (appelée « libgimp  ») a une API très bien documentée. Les bons programmeurs, qui apprennent en modifiant les greffons existants, peuvent accomplir des choses intéressantes après quelques jours de travail.