Clutter 1.0 brings stability, new animation API

Version 1.0 of the Clutter graphics library was released on July 29, sporting a stable application program interface (API) and binary interface (ABI), an animation framework, and an OpenGL abstraction library that should prove useful to developers. With Clutter set to take on a more prominent role in the GNOME 3.x series, the announcement should prove to be welcome news to application developers.

Clutter is used to build user interfaces, but unlike traditional toolkits such as GTK+ or Qt, it uses a flexible "scene graph" model with "actors" and "stages" instead of the customary widgets and containers. The free-form actor elements can be placed with fixed positioning on the stage or use managed layout, and they can be easily moved, deformed, and even animated. Clutter is designed to use OpenGL as a back-end, so applications can benefit from hardware accelerated rendering. OpenGL for Embedded Systems (OpenGL ES) is supported, making Clutter a popular choice on slim-CPU mobile devices such as Nokia's Maemo tablets and Moblin's netbook Linux distribution.

The project was started in 2006 at embedded Linux development firm Opened Hand, which was acquired by Intel in late 2008. Clutter has been selected as an official part of the Maemo GUI stack, beginning with the upcoming 5.0 release, called "Fremantle." Following Opened Hand's acquisition by Intel, however, more effort went into integrating Clutter as a core UI library for Moblin, which prior to April 2009, was an Intel-owned effort. The toolkit is also growing in popularity on desktop Linux systems, where it is used by GNOME games, the Mutter window manager, and the GNOME Shell project set to be featured prominently in GNOME 3.0.

Although Clutter is written in C, bindings are available for a wide variety of languages, including C++, C#, Python, Ruby, Vala, JavaScript, and Perl. In addition, applications can embed traditional GTK+ elements, GStreamer video content and Cairo 2-D canvases as Clutter actors. Clutter is developed primarily for usage under X with the GLX extension, but can also use Simple DirectMedia Layer (SDL) or the Linux frame buffer if necessary. Ports to Windows and Mac OS X are also available. Using ClutterScript, Clutter applications can store and load full or partial scene graphs in JavaScript Object Notation (JSON) format.

Changes in 1.0

The Clutter API has undergone several important changes since the last stable series, 0.8, but the development team has declared the 1.0 API frozen for all subsequent 1.x releases. Apart from stabilizing the API, Clutter 1.0 includes a new animation framework, unified handling of all text widgets with Pango, performance improvements based on better tracking which actors in the scene graph will be painted, debugging facilities for application developers, and improvements to the Clutter OpenGL abstraction library COGL. The Clutter and COGL documentation also underwent extensive revision, including a migration guide for developers needing to port their code from Clutter 0.8 to Clutter 1.0.

The Animation API has received the most attention of the changes in 1.0, being the subject of several conference talks this summer. In prior versions of Clutter, animation of actors was handled through two separate features: Behaviors and Effects. Behaviors were to be used when both the starting and ending states of the actor were known (such as moving from one predetermined position to another), and Effects could be used at any time, regardless of the state of the actor. The Effects API proved to be kludgy and difficult for application developers to use, as well as difficult for the Clutter developers to extend, so it has been dropped in favor of the Animation API.

The new API is both simpler and shares base classes, such as Timeline, with the Behaviors API, which should simplify its adoption. Whereas in Effects each transformation was a separate function (e.g., rotate, fade, translate), Animation requires only specifying the desired final state of the actor with a single function call; the animation itself is performed implicitly, with all of the intermediate steps interpolated. This "tweening" behavior is similar to what is possible with JavaScript animation. The Clutter 1.0 migration guide provides some side-by-side example code demonstrating the difference between using Effects and Animation.

COGL is also significant; it began as a purely internal layer for Clutter to abstract away the differences between OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0, but it has since evolved into a library useful for other OpenGL-based projects. COGL attempts to make OpenGL usage as fast as possible by maintaining its own internal store of the scene rather than sending every update to the GPU separately, caching as much as possible, and minimizing the number of validations and state changes.

1.x, 2.0, and more

The Clutter team has expressed its desire to further develop COGL into a more flexible GPU-programming library, providing a modern, object-oriented API for OpenGL programming. As for Clutter itself, the plan is to adopt a six-month release cycle, as used by other projects in the GNOME ecosystem. There will be further 1.x stable releases to improve performance and efficiency, but the guarantee is that no changes made during the 1.x cycle will break API compatibility.

GNOME will reportedly ship Clutter with its 2.28 release in September, but Clutter-based tools like GNOME Shell are not scheduled to arrive until the 3.0 release six months later. The API stability guarantee is more likely to please developers with mobile platform projects like Maemo and Moblin, however, who count on longer product life cycles than those of a typical desktop Linux distribution.

Independent application developers may have to wait a few more weeks before they can begin working with Clutter 1.0, though. The dependent libraries that allow embedding GTK+ widgets, GStreamer content, and Cairo canvases are a bit behind the core Clutter release, as are some of the language bindings. Fortunately, the official packages are built to be installable in parallel with Clutter 0.8, and with the documentation in place — including the migration guide — no one has an excuse to sit idly by in the meantime.