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.

Custom data

This section list all types of data usable to enhance GIMP functionalities. If you are interested to contribute default data to GIMP, be aware that we are looking for a very good base set, not an unfinite number of data for all possible usage (even the less common ones).

Furthermore we only accept data on Libre licenses:

• Free Art License
• CC0
• CC BY
• CC BY-SA

Of course you are free to share data usable by GIMP on any license you want on your own. Providing them as third-party GIMP extensions is probably the best idea.

Brushes

GIMP currently supports the following brush formats:

• GIMP Brush (GBR): format to store pixmap brushes
• GIMP Brush Pipe (GIH): format to store a series of pixmap brushes
• GIMP Generated Brush (VBR): format of "generated" brushes
• GIMP Brush Pixmap (GPB): OBSOLETE format to store pixel brushes
• MyPaint brushes v1 (MYB)
• Photoshop ABR Brush
• Paint Shop Pro JBR Brush

We do fully support the GIMP formats obviously, as well as MyPaint brushes, since we use the official libmypaint library. We are not sure how well we support other third-party formats, especially if they had recent versions.

If you are interested in brushes from a developer perspective, you are welcome to read specifications of GIMP formats: GBR, GIH, VBR or the obsolete GPB.

If you want to contribute brushes to the official GIMP, be aware we would only accept brushes in non-obsolete GIMP formats. All these formats can be generated by GIMP itself from images.

If you want to contribute MyPaint brushes, we recommend to propose them to the MyPaint-brushes data project, which is also used by GIMP for its default MyPaint brush set.

Otherwise, you are welcome to provide brush set in any format as third-party extensions.

Dynamics

GIMP supports the GIMP Paint Dynamics format which can be generated from within GIMP.

Patterns

GIMP supports the GIMP Pattern format (PAT, whose specification is available for developers).

This format can be exported by GIMP itself.

Alternatively GIMP supports patterns from GdkPixbuf (TODO: get more information?).

Palettes

GIMP supports the GIMP Palette format which can be generated from within GIMP.

Gradients

GIMP supports the GIMP Gradient format (GGR, whose specification is available for developers) which can be generated from within GIMP.

Alternatively GIMP supports the SVG Gradient format.

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.

Icon themes

Icon sets (a.k.a. "icon themes") have been separated from themes since GIMP 2.10 so you can have any icon theme with any theme.

To know about icons, go to gimp-data/icons.

TODO: Tool presets

TODO: GIMP extensions (.gex)

Core development

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

• GIMP's coding style;
• the directory structure
• our header file inclusion policy

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.

Advanced concepts

XCF

The XCF format is the core image format of GIMP, which mirrors features made available in GIMP. More than an image format, you may consider it as a work or project format, as it is not made for finale presentation of an artwork but for the work-in-progress process.

Developers are welcome to read the specifications of XCF.

Locks

Items in an image can be locked in various ways to prevent different types of edits.

This is further explained in the specifications of locks.

UI Framework

GIMP has an evolved GUI framework, with a toolbox, dockables, menus…

This document describing how the GIMP UI framework functions and how it is implemented might be of interest.

Contexts

GIMP uses a lot a concept of "contexts". We recommend reading more about how GimpContexts are used in GIMP.

Undo

GIMP undo system can be challenging at times. This quick overview of the undo system can be of interest as a first introduction.

Parasites

GIMP has a concept of "parasite" data which basically correspond to persistent or semi-persistent data which can be attached to images or items (layers, channels, paths) within an image. These parasites are saved in the XCF format.

Parasites can also be attached globally to the GIMP session.

Parasite contents is format-free and you can use any parasite name, nevertheless GIMP itself uses parasite so you should read the descriptions of known parasites.

Metadata

GIMP supports Exif, IPTC and XMP metadata as well as various image format-specific metadata. The topic is quite huge and complex, if not overwhelming.

This old document might be of interest (or maybe not, it has not been recently reviewed and might be widely outdated; in any case, it is not a complete document at all as we definitely do a lot more nowadays). TODO: review this document and delete or update it depending of whether it still makes sense.

Tagging

Various data in GIMP can be tagged across sessions.

This document on how resource tagging in GIMP works may be of interest.