e4cb7e12b4
Also remove the now deprecated Jenkins tutorial. We have not had this CI system running for some time now, and the Gitlab CI has totally replaced it.
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This manual holds information that you will find useful if you develop a GIMP plug-in or want to contribute to the GIMP core.
People only interested into plug-ins can probably read just the Plug-in development section. If you wish to contribute to all parts of GIMP, the whole documentation is of interest.
[TOC]
Plug-in development
Concepts
Basics
Plug-ins in GIMP are executables which GIMP can call upon certain conditions. Since they are separate executables, it means that they are run as their own process, making the plug-in infrastructure very robust. No plug-in should ever crash GIMP, even with the worst bugs. If such thing happens, you can consider this a core bug.
On the other hand, a plug-in can mess your opened files, so a badly developed plug-in could still leave your opened images in an undesirable state. If this happens, you'd be advised to close and reopen the file (provided you saved recently).
Another downside of plug-ins is that GIMP currently doesn't have any sandboxing ability. Since we explained that plug-ins are run by GIMP as independant processes, it also means they have the same rights as your GIMP process. Therefore be careful that you trust the source of your plug-ins. You should never run shady plug-ins from untrusted sources.
GIMP comes itself with a lot of plug-ins. Actually nearly all file format support is implemented as a plug-in (XCF support being the exception: the only format implemented as core code). This makes it a very good base to study plug-in development.
Procedural DataBase (PDB)
Obviously since plug-ins are separate processes, they need a way to communicate with GIMP. This is the Procedural Database role, also known as PDB.
The PDB is our protocol allowing plug-ins to request or send information from or to the main GIMP process.
Not only this, but every plug-in has the ability to register one or several procedures itself, which means that any plug-in can call features brought by other plug-ins through the PDB.
libgimp and libgimpui
The GIMP project provides plug-in developers with the libgimp library.
This is the main library which any plug-in needs. All the core PDB
procedures have a wrapper in libgimp so you actually nearly never need
to call PDB procedures explicitly (exception being when you call
procedures registered by other plug-ins; these won't have a wrapper).
The libgimpui library is an optional one which provides various
graphical interface utility functions, based on the GIMP toolkit
(GTK). Of course, it means that linking to this library is not
mandatory (unlike libgimp). Some cases where you would not do this
are: because you don't need any graphical interface (e.g. a plug-in
doing something directly without dialog, or even a plug-in meant to be
run on non-GUI servers); because you want to use pure GTK directly
without going through libgimpui facility; because you want to make
your GUI with another toolkit…
The whole C reference documentation for both these libraries can be
generated in the main GIMP build with the --enable-gi-docgen autotools
option or the -Dgi-docgen=enabled meson option (you need to have the
gi-docgen tools installed).
TODO: add online links when it is up for the new APIs.
Programming Languages
While C is our main language, and the one libgimp and libgimpui are
provided in, these 2 libraries are also introspected thanks to the
GObject-Introspection (GI)
project. It means you can in fact create plug-ins with absolutely any
language with a GI binding
though of course it may not always be as easy as the theory goes.
The GIMP project explicitly tests the following languages and even provides a test plug-in as a case study:
- C (not a binding)
- Python 3 (binding)
- Lua (binding)
- Vala (binding)
- Javascript (binding, not supported on Windows for the time being)
One of the big advantage of these automatic bindings is that they are full-featured since they don't require manual tweaking. Therefore any function in the C library should have an equivalent in any of the bindings.
TODO: binding reference documentation.
Note: several GObject-Introspection's Scheme bindings exist though
we haven't tested them. Nevertheless, GIMP also provides historically
the "script-fu" interface, based on an integrated Scheme implementation.
It is different from the other bindings (even from any GI Scheme
binding) and doesn't use libgimp. Please see the Script-fu
development section.
Tutorials
TODO: at least in C and in one of the officially supported binding (ideally even in all of them).
Porting from GIMP 2 plug-ins
Script-fu development
Script-fu is its own thing as it is a way to run Scheme script with
GIMP. It is itself implemented as an always-running plug-in with its own
Scheme mini-interpreter and therefore Script-fu scripts do not use
libgimp or libgimpui. They interface with the PDB through the
Script-fu plug-in.
Tutorials
Porting from GIMP 2 scripts
GEGL operation development
Custom data
Brushes
Dynamics
Patterns
Themes
Icon themes
GIMP extensions (.gex)
Continuous Integration
For most of its continuous integration (macOS excepted), GIMP project uses Gitlab CI. We recommend looking the file .gitlab-ci.yml which is the startup script.
The main URL for our CI system is build.gimp.org which redirects to Gitlab pipelines page.
Note that it is important to keep working CI jobs for a healthy code source. Therefore when you push some code which breaks the CI (you should receive a notification email when you do so), you are expected to look at the failed jobs' logs, try and understand the issue(s) and fix them (or ask for help). Don't just shrug this because it works locally (the point of the CI is to build in more conditions than developers usually do locally).
Of course, sometimes CI failures are out of our control, for instance when downloaded dependencies have issues, or because of runner issues. You should still check that these were reported and that packagers/maintainers of these parts are aware and working on a fix.
Automatic pipelines
At each commit pushed to the repository, several pipelines are currently running, such as:
- Debian testing autotools and meson builds (autotools is still the official build system while meson is experimental).
- Windows builds (cross or natively compiled).
Additionally, we test build with alternative tools or options (e.g. with
Clang instead of gcc compiler) or jobs which may take much longer,
such as package creation as scheduled pipelines (once every few days).
The above listing is not necessarily exhaustive nor is it meant to be. Only the .gitlab-ci.yml script is meant to be authoritative. The top comment in this file should stay as exhaustive as possible.
Manual pipelines
It is possible to trigger pipelines manually, for instance with specific jobs, if you have the "Developer" Gitlab role:
- go to the Pipelines page.
- Hit the "Run pipeline" button.
- Choose the branch or tag you wish to build.
- Add relevant variables. A list of variables named
GIMP_CI_*are available (just set them to any value) and will trigger specific job lists. These variables are listed in the top comment of .gitlab-ci.yml.
Merge request pipelines
Special pipelines happen for merge request code. For instance, these also include a (non-perfect) code style check.
Additionally you can trigger Windows installer or flatpack standalone packages to be generated with the MR code as explained in gitlab-mr.md.
Release pipeline
Special pipelines happen when pushing git tags. These should be tested
before a release to avoid unexpected release-time issues, as explained
in release-howto.txt.
Exception: macOS
As an exception, macOS is currently built with the Circle-CI service.
The whole CI scripts and documentation can be found in the dedicated
gimp-macos-build
repository.
Eventually we want to move this pipeline to Gitlab as well.
Core development
Newcomers
Core Contributors
Directory structure of GIMP source tree
GIMP source tree can be divided into the main application, libraries, plug-ins, data files and some stuff that don't fit into these categories. Here are the top-level directories:
| Folder | Description |
|---|---|
| app/ | Source code of the main GIMP application |
| app-tools/ | Source code of distributed tools |
| build/ | Scripts for creating binary packages |
| cursors/ | Bitmaps used to construct cursors |
| data/ | Data files: brushes, gradients, patterns, images… |
| desktop/ | Desktop integration files |
| devel-docs/ | Developers documentation |
| docs/ | Users documentation |
| etc/ | Configuration files installed with GIMP |
| extensions/ | Source code of extensions |
| icons/ | Official icon themes |
| libgimp/ | Library for plug-ins (core does not link against) |
| libgimpbase/ | Basic functions shared by core and plug-ins |
| libgimpcolor/ | Color-related functions shared by core and plug-ins |
| libgimpconfig/ | Config functions shared by core and plug-ins |
| libgimpmath/ | Mathematic operations useful for core and plug-ins |
| libgimpmodule/ | Abstracts dynamic loading of modules (used to implement loadable color selectors and display filters) |
| libgimpthumb/ | Thumbnail functions shared by core and plug-ins |
| libgimpwidgets/ | User interface elements (widgets) and utility functions shared by core and plug-ins |
| m4macros/ | Scripts for autotools configuration |
| menus/ | XML/XSL files used to generate menus |
| modules/ | Color selectors and display filters loadable at run-time |
| pdb/ | Scripts for PDB source code generation |
| plug-ins/ | Source code for plug-ins distributed with GIMP |
| po/ | Translations of strings used in the core application |
| po-libgimp/ | Translations of strings used in libgimp |
| po-plug-ins/ | Translations of strings used in C plug-ins |
| po-python/ | Translations of strings used in Python plug-ins |
| po-script-fu/ | Translations of strings used in Script-Fu scripts |
| po-tags/ | Translations of strings used in tags |
| po-tips/ | Translations of strings used in tips |
| po-windows-installer/ | Translations of strings used in the Windows installer |
| themes/ | Official themes |
| tools/ | Source code for non-distributed GIMP-related tools |
| .gitlab/ | Gitlab-related templates or scripts |
The source code of the main GIMP application is found in the app/ directory:
| Folder | Description |
|---|---|
| app/actions/ | Code of actions (GimpAction* defined in app/widgets/) (depends: GTK) |
| app/config/ | Config files handling: GimpConfig interface and GimpRc object (depends: GObject) |
| app/core/ | Core of GIMP core (depends: GObject) |
| app/dialogs/ | Dialog widgets (depends: GTK) |
| app/display/ | Handles displays (e.g. image windows) (depends: GTK) |
| app/file/ | File handling routines in core (depends: GIO) |
| app/file-data/ | GIMP file formats (gbr, gex, gih, pat) support (depends: GIO) |
| app/gegl/ | Wrapper code for babl and GEGL API (depends: babl, GEGL) |
| app/gui/ | Code that puts the user interface together (depends: GTK) |
| app/menus/ | Code for menus (depends: GTK) |
| app/operations/ | Custom GEGL operations (depends: GEGL) |
| app/paint/ | Paint core that provides different ways to paint strokes (depends: GEGL) |
| app/pdb/ | Core side of the Procedural Database, exposes internal functionality |
| app/plug-in/ | Plug-in handling in core |
| app/propgui/ | Property widgets generated from config properties (depends: GTK) |
| app/tests/ | Core unit testing framework |
| app/text/ | Text handling in core |
| app/tools/ | User interface part of the tools. Actual tool functionality is in core |
| app/vectors/ | Vectors framework in core |
| app/widgets/ | Collection of widgets used in the application GUI |
| app/xcf/ | XCF file handling in core |