Gimp/devel-docs

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 section. If you wish to contribute to all parts of GIMP, the whole documentation is of interest.

[TOC]

Plug-in and Filters development

All needed information for Plug-in and Filters development is documented on the Resource Development section of GIMP Developer website, with exception of the following:

Porting from GIMP 2 plug-ins

Take a look at our porting guide.

Themes

GTK3 uses CSS themes. Don't be fooled though. It's not real CSS in that it doesn't have all the features of real web CSS, and since it's for desktop applications, some things are necessarily different. What it means is mostly that it "looks similar" enough that people used to web styling should not be too disorientated.

You can start by looking at the official documentation for theme migration (from GTK+2 to 3), which gives a good overview, though it's far from being perfect unfortunately.

Another good idea would be to look at existing well maintained GTK3 themes to get inspiration and see how things work.

Finally you can look at our existing themes, like the System theme. Note though that this System theme is pretty bare, and that's its goal (try to theme as few as possible over whatever is the current real GTK system theme).

As a last trick for theme makers, we recommend to work with the GtkInspector tool, which allows you to test CSS rules live in the CSS tab. You can run the GtkInspector by going to the File > Debug menu and selecting Start GtkInspector menu item.

It also allows you to find the name of a widget to use in your CSS rules. To do so:

• Start the GtkInspector;
• go on the "Objects" tab;
• click the "target" 🞋 icon on the headerbar's top-left, then pick in GIMP interface the widget you are interested to style;
• the widget name will be displayed on the top of the information area of the dialog.
• Feel free to browse the various sections to see the class hierarchy, CSS nodes and so on.
• The second top-left button (just next to the target icon) allows you to switch between the details of the selected widget and the widget hierarchy (container widgets containing other widgets), which is also very useful information.

Additionally you can quickly switch between the light and dark variant of a same theme by going to "Visual" tab and switching the "Dark Variant" button ON or OFF.

Core development

When writing code, any core developer is expected to follow:

• GIMP's coding style;
• the directory structure

GIMP's developer site contain various valuable resources.

Finally the debugging-tips file contain many very useful tricks to help you debugging in various common cases.

Newcomers

If this is your first time contributing to GIMP, you might be interested by build instructions.

You might also like to read these instructions on the process of submitting patches.

TODO: Core Contributors

As a core dev, you can trigger .appimage, .flatpak standalone packages, .exe Windows installer or Microsoft Store/.msixbundle to be generated with the MR code as explained in gitlab-mr.md.

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
data/	Data files: dynamics, gradients, palettes…
desktop/	Desktop integration files
devel-docs/	Developers documentation
docs/	Users documentation
etc/	Configuration files installed with GIMP
extensions/	Source code of extensions
gimp-data/	Raster or image data files
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
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

Auto-generated Files

Please notice that some files in the source are generated from other sources. All those files have a short notice about being generated somewhere at the top. Among them are the files ending in pdb.[ch] in the libgimp/ directory and the files ending in cmds.c in the app/pdb/ subdirectory. Those are generated from the respective .pdb files in pdb/groups.

Other files are:

• AUTHORS from authors.xml

You should also check out gimp-module-dependencies.svg. TODO: this SVG file is interesting yet very outdated. It should not be considered as some kind dependency rule and should be updated.