gimp/devel-docs/README.md

---
title: Developers documentation
---

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](#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](https://gi.readthedocs.io/en/latest/) (**GI**)
project. It means you can in fact create plug-ins with absolutely any
[language with a GI binding](https://wiki.gnome.org/Projects/GObjectIntrospection/Users)
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](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-c.c) (not a binding)
* [Python 3](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-py3.py)
  (binding)
* [Lua](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-lua.lua)
  (binding)
* [Vala](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-vala.vala)
  (binding)
* [Javascript](https://gitlab.gnome.org/GNOME/gimp/-/blob/master/extensions/goat-exercises/goat-exercise-gjs.js)
  (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](#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](/.gitlab-ci.yml) which is the startup script.

The main URL for our CI system is
[build.gimp.org](https://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](/.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:

1. go to the [Pipelines](https://gitlab.gnome.org/GNOME/gimp/-/pipelines)
   page.
2. Hit the "*Run pipeline*" button.
3. Choose the branch or tag you wish to build.
4. 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](/.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](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](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](https://gitlab.gnome.org/Infrastructure/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**                                                    |


### Advanced concepts