GIMP and Standards

Table of Contents

GIMP is supposed to integrate well into your workflow. Thus is needs to interoperate with other applications and your desktop. Standards are there to make this happen, so we try to follow established (and sometimes even proposed) standards. Below you will find a list with links to specifications that a GIMP developer may find useful.

GIMP Formats

GIMP project created its own formats for various purposes. The most well known is the XCF format, which is our own project format.

Image File Formats

These are image formats supported in GIMP or not. Note that GIMP has a policy to support every image format, however good or bad it is, at least as an import format. For instance, being able to restore old images in outdated and long-forgotten formats is definitely a real use case.

APNG
Extension of the PNG format, adding support for animated images. It is backward compatible with PNG-supporting software where it would simply show the first frame instead of an animation. While adoption was stalling for years, it gained more support across browsers recently. It is not supported in GIMP yet. The biggest reason is that it relied on a patched libpng, using the same namespace as legacy libpng and not distributed commonly by distributions. This is apparently how it is shipped, bundedl, in Firefox historically. Nevertheless it seems that some browsers, like Chromium, have managed to support it using only legacy libpng. It would need to be studied.
AVIF (AV1 Image File Format)
The royalty free counterpart of HEIC, using AV1 image encoding and the same HEIF container format. It supports up to 12-bit per channel, animation, HDR images and more. GIMP uses libheif for support, though there are talks that moving to libavif could be better to support some features, such as animated images.
BMP
Legacy image format with no or low compression and very simple to implement. There doesn’t seem to be centralized format specifications though we find various pages on the web describing its structure. From a quick skimming, the Wikipedia page itself might be all there is to know to encode and decode a BMP file.
DDS (DirectDraw Surface)
Microsoft format for storing data compressed with the previously proprietary S3 Texture Compression (S3TC) algorithm, which can be decompressed in hardware by GPUs. It seems to be quite used in the video game industry in particular for assets such as textures and other images mapped on 3D objects.
GIF (Graphics Interchange Format)
Describes the GIF file format, used in the GIF plug-in. GIF is bad for various reasons, so modern programs should not use it for anything else than supporting people’s files and use cases (i.e. import and export, as we do in GIMP). Yet it kept quite a popularity for very long as the one image format to do small cycle animations on the web. These last years, it slowly loses its throne to new formats such as WebP, though it is still quite omnipresent.
HEIC
Modern image format, supporting up to 12-bit per channel, animation, HDR images and more, using HEVC image encoding and the HEIF container format. Unfortunately HEVC has patent issues, unlike its AVIF counterpart, using AV1, which was created specifically to be royalty-free by the Alliance for Open Media (AOMedia). This is why many Free Software distributors do not wish to package HEIC support, hence why our file-heif plug-in runs runtime tests to determine support for AVIF or HEIC, making it in particular able to support only AVIF. GIMP uses libheif for support. HEVC specifications are freely available.
HEIF
The image and animation container used for AVIF and HEIC. As ISO standard, the specifications are not freely available. GIMP supports this container by supporting AVIF and HEIC image formats.
JPEG (Joint Photographic Experts Group)
Describes the JPEG JFIF file format, used in the JPEG plug-in. JPEG is lossy regarding pixel data (though there is a lossless JPEG standard, currently unsupported by libjpeg-turbo, the library we rely on), and only up to 8-bit per channel. Once again, not entirely true: 12-bit support is added as an extension of the JPEG specification and libjpeg-turbo has support, but it needs to be set at compile time and only one variant — 8 or 12-bit — at the same time. Plans exist to provide both 8 and 12-bit support to a program but there is no timeline when it will happen. Anyway similarly to GIF for small animated images, this format (in the 8-bit variant) is still the most popular format for photography on the web, despite its quality issues.
JPEG 2000
New-ish format, still by the Joint Photographic Experts Group, which supports lossy and lossless compression, high bit depth (up to 38-bits per channel!). It compresses a bit better than legacy JPEG but this is not really its focus. This format clearly didn’t gain a lot of popularity, at least towards the general public. Its biggest usage though is for the cinema market as it is the image format used for image tracks in the DCP container format, which is the de-facto standard in the cinema industry these days as it replaced films reels in theaters using digital movie projectors. As ISO standards, the specifications are not freely available. For support, GIMP uses OpenJPEG, one of the reference implementations.
JPEG XL
New format, still by the Joint Photographic Experts Group, which supports lossy and lossless compression, high bit depth, and supposed to compress a lot better than legacy JPEG. As ISO standards, the specifications are not freely available. The reference implementation though, libjxl, is what we use in GIMP.
JNG (JPEG Network Graphics) Format
GIMP doesn’t use this format yet but it would be nice to extend the MNG plug-in to use it and to add a dedicated JNG plug-in.
MNG (Multiple-image Network Graphics) Format
Describes the MNG file format, used in the MNG plug-in.
OpenRaster
Meant to be an open exchange format for raster editing software. It is still a bit bare and evolution has stalled these last years. GIMP contributors originally contributed as part of its creation process.
PDF (Portable Document Format)
File format for document layout. It contains vector as well as raster images, text and more. GIMP uses libpoppler for import and libcairo-pdf for export. It doesn’t support any of the PDF/X variants yet, but this will need to happen for good printing support. One of the Free Software with good PDF/X support is Scribus, making it a good complementary software to GIMP for doing printing jobs.
Portable Network Graphics (PNG)
Describes the PNG file format, used in the PNG plug-in. GIMP also reads patterns in the PNG file format and it stores thumbnails as PNG images. It supports up to 16-bit per component and proper alpha channel. It is widely used on the web, especially for flat colored images (logos or other design) where it can even sometimes end up in smaller files (or close size, yet better quality) than JPEG, and even for photography or complex images when quality matters more than size (thanks to its lossless property regarding pixel data).
PSD (Photoshop Document)
Core file format for Photoshop project files. A public specification is available though it is not always very complete, missing features or details.
Scalable Vector Graphics (SVG) 1.1
Describes the SVG file format, used in the SVG plug-in. GIMP uses SVG for import and export of paths and it also loads gradients from SVG files.
TIFF 6.0
Describes the TIFF file format, used in the TIFF plug-in. See also the Unofficial TIFF Home Page.
Digital Negative (DNG)
Specifies DNG, a format, proposed by Adobe, aiming to become a standard for storing raw data from digital cameras. As for most other “RAW image formats”, GIMP doesn’t support them natively anymore and instead promotes passing through a RAW image developer, such as darktable or RawTherapee. GIMP has an infrastructure system redirecting automatically RAW images to one of these software if available, then get the result back for further processing. Other RAW developer software may be added to GIMP by creating proper plug-ins with a GimpLoadProcedure flagged with the gimp_load_procedure_set_handles_raw() function.
WebP
Modern image format, supporting lossy and lossless compression, animation and transparency. It is often described as a replacement for GIF, JPEG and PNG, but not that much further as it doesn’t support high bit depth or many color models. This makes it good for web usage, but less for advanced image usages where other recent formats make take the higher road. GIMP supports this format through the libwebp reference library.

Generic File Formats

Extensible Markup Language (XML)
Describes the XML markup language, used to store the menu layout, the startup tips, help indices and other things.

Metadata File Formats

Exif (Exchangeable image file format)
Widely used metadata format.
Extensible Metadata Platform (XMP)
Describes XMP, a labeling technology that allows to embed data about a file, known as metadata, into the file itself.

Palette File Formats

The Swatchbooker project has a very nice listing with technical information of existing color palette formats.

Below are more details on formats currently supported (at least in parts) by GIMP:

GIMP’s Palette Format (GPL)
GIMP’s own palette format, used to store palettes created in GIMP.
Adobe Color Table (ACT)
Section “Color Table” in “Adobe Photoshop File Formats Specification” (November 2019). Only uint8 RGB colors are supported. No specific color spaces can be specified.
Adobe Color Swatches (ACO)
Section “Color Swatches” in “Adobe Photoshop File Formats Specification” (November 2019). RGB, HSB, CMYK, Lab and Grayscale colors are supported, with uint16 components. No specific color spaces can be specified.
Version 2 of the format allows to set names to colors (they are currently ignored by GIMP).
Adobe Color Books (ACB)
Section “Color Books” in “Adobe Photoshop File Formats Specification” (November 2019). RGB, CMYK and CIE Lab colors are supported, with uint8 components. No specific color spaces can be specified. Color entries have names.
Adobe Swatch Exchange (ASE)
Specifications were not found online but there are various blog posts by third-parties. AFAWK, it supports RGB, CMYK, Grayscale and CIE Lab in floating point precision. The palette has a name, colors have names too and each color in the palette has its own color model (possibly mixing RGB, CMYK, Grayscale and Lab entries).
No specific color spaces can be specified.
Finally color entries have a type between “Process”, “Global” and “Spot” which seems to imply it can be used for spot colors, though there is not much information on how and if it is for specific catalogs or vendor-agnostic. It seems it would require additional information for this field to be actually useful.
RIFF Palette File Format
Section “Palette File Format” (3-18) of the “Multimedia Programming Interface and Data Specifications 1.0”.
GIMP currently supports the “Simple PAL Format” (RGB only) but not the “Extended PAL Format” which can additionally contain YUV and XYZ palettes.
Only uint8 RGB colors are supported. No specific color spaces can be specified.
See also Microsoft docs on PALETTEENTRY structure.
JASC Palettes (Paint Shop Pro)
A plain text file with header “JASC-PAL” line followed by “0100” line, followed by number of colors (in plain text, not binary), followed by as many lines as the number of colors. Each of these lines is a triplet of integers (in plain text) separated by space. Line separation is Windows style CR-LF.
Only uint8 RGB colors are supported. No specific color spaces can be specified.
SwatchBooker format (SBZ)
Though it is sometimes considered deprecated because the corresponding software’s development is abandoned these days, it is the most complete of all currently supported formats.
Palettes can have a name and a description (with possible localization). Color entries support (possibly mixed) sRGB, RGB, HSV, HSL, CMY, CMYK and spot colors (possibly with multiple model representation for a given spot color), though the catalog does not seem to be associated to spot color identifiers.
And finally, when relevant, a space can be attributed to color values through embedded ICC profiles.
Sample files can be found from our merge request as well as from the FreieFarbe project.

Color Management

sRGB Color Space
Describes sRGB, a color space proposed as a standard default color space for the Internet and other interested vendors.
ICC Specification
Specifies the profile format defined by the International Color Consortium (ICC). The intent of this format is to provide a cross-platform device profile format that can be used to translate color data between device colorspaces.
ICC Profiles In X Specification
This is a specification for associating ICC color profiles with X screens. GIMP 2.4 implements this proposed standard.

Color Compositing

Our compositing and blending algorithms are not necessarily exactly the below W3C specifications, yet they contain base concepts and definitions of the process behind compositing images into one combined result:

Desktop Standards

AppStream
Metadata to describe a software. It is used massively on Linux by software centers (such as GNOME Software or KDE Discover), which is where they get a software name, description, screenshots, release information and more. Our AppStream file is visible in desktop/org.gimp.GIMP.appdata.xml.in.in. We also generates the release note tab of the “Welcome Dialog” from the release data in our AppStream file. Finally we also use this format as metadata for the new extension format of GIMP.
Desktop Entry Specification
This document describes desktop entries: files describing information about an application such as the name, icon, and description. GIMP installs such a .desktop file.
Desktop Message Bus
D-Bus is a message bus for the desktop. If available, GIMP uses it to detect if another GIMP instance is already running. In the future, GIMP might make even more use of D-Bus.
File URI Specification
Specifies how URIs for normal UNIX filenames (file: URIs) are interpreted and created. This functionality is provided by GLib,
GNOME Human Interface Guidelines
We don’t follow this spec to the word but we try to adopt as much of these guidelines as makes sense.
Recent File Storage Specification
Provides a standard mechanism for storing a list of recently used files. Supported since GIMP version 2.1.6.
Shared MIME Database
The shared MIME database contains common MIME types, descriptions, and rules for determining the types of files. GIMP file plug-ins should use the MIME types and descriptions defined here.
Startup Notification
Specifies a mechanism allowing a desktop environment to track application startup to provide user feedback. GTK provides support for this protocol.
Thumbnail Managing Standard
Deals with the permanent storage of previews for file content. In particular, it tries to define a general and widely accepted standard for this task. GIMP 2.0 implements this standard and dropped support for the old-fashioned .xvpics.

Standards specific to the X window system

Clipboards
Not a formal specification, but explains the consensus of the Qt and GTK developers on how the X clipboard works.
Clipboard Manager
The Clipboard Manager specification describes how applications can actively store the contents of the clipboard when the application is quit. This requires that a compliant clipboard manager is running.
Drag-and-Drop Protocol for the X Window System
XDND defines a standard for drag and drop on X11. It is implemented by GTK.
Direct Save Protocol for the X Window System
XDS defines an extension to XDND that allow users to save a file by simply dragging it to a file manager window. GIMP 2.4 supports this protocol.
Extended Window Manager Hints
The Window Manager Specification is meant to unify the GNOME and KDE window manager hint conventions.
Inter-Client Communication Conventions Manual (ICCCM)
This spec defines the interaction between X11 clients. In particular it talks about selections, cut buffers, window and session management, manipulation of shared resources and device color characterization.
XSETTINGS
The XSETTINGS protocol provides a mechanism for applications written with different toolkits to share simple configuration settings such as double-click-times and background colors. GTK hides this from us.

Programming Standards

GIMP Coding Style guidelines
Our project coding style guidelines. It explains how the GIMP source code should be formatted.
GNU coding standards
A guide to writing portable, robust and reliable programs. Also defines the GNU coding style. Our own Coding Style originally derivates from the GNU Coding style.
ISO/IEC 9899
ISO 9899 is the international standard for the C programming language.