Glycin

Glycin allows to decode, edit, and create images and read metadata. On Linux, decoding happens in sandboxed modular image loaders. We use the term 'image loader' for simplicity, but image loaders also handle image editing and creation.

• glycin – The Rust image library
  • libglycin – C-Bindings for the library
  • libglycin-gtk4 – C-Bindings to convert glycin frames to GDK Textures
  • libglycin-rebind - Safe Rust bindings to libglycin
• glycin-loaders – Glycin loaders for several formats
• glycin-thumbnailer – Glycin thumbnailer using the installed loaders

Bindings for other languages:

• GJS

Other rust crates:

• glycin-utils – Utilities to write loaders for glycin
• glycin-common – Components shared between the glycin-utils and glycin crates.

Usage and Packaging

The Rust client library is available as glycin on crates.io. For other programming languages, the libglycin C client library can be used. For the client libraries to work on Linux, loader binaries must also be installed. The loader binaries provided by the glycin project cover a lot of common image formats (see below). Both, the loader binaries and libglycin can be built from the released glycin tarballs. By using -Dglycin-thumbnailer=false, -Dglycin-loaders=false, -Dlibglycin=false, or -Dlibglycin-gtk4=false it is possible to build only specific components. In distributions, the loaders are usually packaged as glycin-loaders, and libglycin as libglycin-2. However, each loader binary could be also packaged as its own package.

Example

let file = gio::File::for_path("image.jpg");
let image = Loader::new(file).load().await?;

let height = image.info().height();
let texture = image.next_frame().await?.texture();

Limitations

Glycin sandboxing and modularity is based on technologies like memfds, unix sockets, and linux namespaces. Therefore, these features are only available on Linux. Other operating systems are supported by embedding the image loaders directly into the glycin/libglycin libraries. This happens automatically when building with a non-linux target.

Supported Image Formats

The following formats are supported by the glycin image loaders provided in the loaders directory. You can learn more about the supported features for each format on the glycin website.

Format	Glycin Loader	Decoder
Animated PNG image	glycin-image-rs	png (Rust)
AVIF image (.avif)	glycin-heif	libheif (C++)
Windows BMP image (.bmp)	glycin-image-rs	image (Rust)
GIF image (.gif)	glycin-image-rs	gif (Rust)
HEIF image (.heic)	glycin-heif	libheif (C++)
JPEG-2000 JP2 image	glycin-image-rs	hayro-jpeg2000 (Rust)
JPEG image (.jpg)	glycin-image-rs	zune-jpeg (Rust)
JPEG XL image (.jxl)²	glycin-jxl	libjxl (C++)
PNG image (.png)	glycin-image-rs	png (Rust)
Quite OK Image Format (.qoi)	glycin-image-rs	qoi (Rust)
SVG image	glycin-svg	librsvg (C/Rust)
Compressed SVG image	glycin-svg	librsvg (C/Rust)
TIFF image (.tiff)	glycin-image-rs	tiff (Rust)
Windows icon (.ico)	glycin-image-rs	image (Rust), bmp (Rust), png (Rust)
WebP image (.webp)	glycin-image-rs	image-webp (Rust)
Adobe DNG negative¹³	glycin-raw	libopenraw (Rust)
Canon CR2 raw image¹³	glycin-raw	libopenraw (Rust)
DirectDraw surface (.dds)	glycin-image-rs	image (Rust)
image/x-epson-erf type¹³	glycin-raw	libopenraw (Rust)
EXR image (.exr)	glycin-image-rs	exr (Rust)
Minolta MRW raw image¹³	glycin-raw	libopenraw (Rust)
Olympus ORF raw image¹³	glycin-raw	libopenraw (Rust)
Panasonic raw image¹³	glycin-raw	libopenraw (Rust)
Panasonic raw image¹³	glycin-raw	libopenraw (Rust)
Pentax PEF raw image¹³	glycin-raw	libopenraw (Rust)
PNM image	glycin-image-rs	image (Rust)
PBM image	glycin-image-rs	image (Rust)
PGM image	glycin-image-rs	image (Rust)
PPM image	glycin-image-rs	image (Rust)
Sony SRF raw image¹³	glycin-raw	libopenraw (Rust)
TGA image (.tga)	glycin-image-rs	image (Rust)
Windows cursor	glycin-image-rs	image (Rust), bmp (Rust), png (Rust)
XBM image	glycin-image-rs	image-extras (Rust)
XPM image	glycin-image-rs	image-extras (Rust)

1. Support for this format is still experimental the corresponding loader is not built by default
2. The loader for this format depends on GPL-3.0-or-later licenced software
3. The loader for this format depends on LGPL-3.0-or-later licenced software

Image Loader Configuration

Loader configurations are read by the client library from XDG_DATA_DIRS and XDG_DATA_HOME. The location is typically of the form:

<data-dir>/share/glycin-loaders/<compat-version>+/conf.d/<loader-name>.conf

for example:

<data-dir>/share/glycin-loaders/2+/conf.d/glycin-image-rs.conf

The configs are glib KeyFiles of the form:

[loader:image/png]
Exec = /usr/libexec/glycin-loaders/2+/glycin-image-rs

Where the part behind loader is a mime-type and the value of Exec can be any executable path.

Existing Compatibility Versions

Not every new major version of the library has to break compatibility with the loaders. If a glycin version X breaks compatibility, the new compatibility version will be called X+. Only glycin X and newer versions will be compatible with X+ until a new compatibility version is used. The definition of the API of each compatibility version is available in docs/. The following compatibility versions currently exist

compat-version	Compatible With
0+	glycin 0.x
1+	glycin 1.x, 2.x; libglycin 1.x
2+	glycin 3.x; libglycin 2.x

Sandboxing and Inner Workings

This section only applies to Linux. See Limitations for details. By default, glycin spawns one process per image loader. The loaders can handle multiple requests in parallel. Running loaders are terminated after not being used for 30 seconds. All these values can be configured. The communication between glycin and the loader takes place via peer-to-peer D-Bus over a Unix socket.

Glycin supports a sandbox mechanism inside and outside of Flatpaks. Outside of Flatpaks, the following mechanisms are used: The image loader binary is spawned via bwrap. The bubblewrap configuration only allows for minimal interaction with the host system. Only necessary parts of the filesystem are mounted and only with read access. There is no direct network access. Environment variables are not passed to the sandbox. Before forking the process the memory usage is limited via calling setrlimit and syscalls are limited to an allow-list via seccomp filters.

Inside of Flatpaks the flatpak-spawn --sandbox command is used. This restricts the access to the filesystem in a similar way as the direct bwrap call. The memory usage is limited by wrapping the loader call into a prlimit command. No additional seccomp filters are applied to the existing Flatpak seccomp rules.

The GFile content is streamed to the loader via a Unix socket. This way, loaders can load contents that require network access, without having direct network access themselves. Formats like SVG set the ExposeBaseDir = true option in their config. This option causes the original image file's directory to be mounted into the sandbox to include external image files from there. The ExposeBaseDir option has no effect for flatpak-spawn sandboxes since they don't support this feature.

The loaders provide the texture data via a memfd that is sealed by glycin and then given as an mmap to GDK. For animations and SVGs the sandboxed process is kept alive for new frames or tiles as long as needed.

For information on how to implement a loaders, please consult the glycin-utils docs.

Building and Testing

• The -Dloaders option allows to only build certain loaders.
• The -Dtest_skip_ext option allows to skip certain image filename extensions during tests. The option -Dtest_skip_ext=heic might be needed if x265 is not available.
• Running integration tests requires the glycin loaders to be installed. By default, meson test creates a separate installation against which the tests are run. This behavior can be changed by setting -Dtest_skip_install=true, requiring to manually calling meson install before running the tests.

Packaging Status

Software Using Glycin

Apps

• Bazaar
• Camera (Snapshot)
• Contact
• Fotema
• Fractal
• Identity
• Image Viewer (Loupe)
• Nautilus
• Robots
• Shortwave

Other

• Mutter
• gdk-pixbuf

The Name

Glycin (ˈɡlaɪsiːn) is a photographic developing agent. There is no deeper meaning behind the name choice but using a somewhat unique name that is related to images. Glycin is often confused with the amino acid glycine, which is called glycin in other languages, like German.

License

SPDX-License-Identifier: MPL-2.0 OR LGPL-2.1-or-later

The camera raw loader uses the crate libopenraw which is licensed as LGPL-3.0-or-later. The JPEG XL loader uses the jpegxl-rs and jpegxl-sys crates which are licensed as GPL-3.0-or-later. Given these are only separate executables, only the glycin-raw and the glycin-jxl binary falls under said licenses, and doesn't precludes using glycin under MPL-2.0 OR LGPL-2.1-or-later. This is not legal advice.