PiTiVi video editor previews rewritten core

Video editing stands to get significantly easier for Linux users this year. The GStreamer-based nonlinear video editor (NLE) PiTiVi has released its first public build since undergoing a substantial rewrite. Version 0.13.1 was released May 27, 2009, and adds a handful of user-visible changes but makes substantial improvements under the hood, laying down groundwork for the next development cycle.

Video editing continues to be an unsettled topic among Linux users. Among the handful of applications currently available, each has its share of limitations, user interface quirks, and stability problems. Kino, for example, is very capable at capturing DV video from cameras, but uses an unfamiliar UI for an NLE, without the familiar editing timeline, and requiring a rendering step for every title, transition, or effect. Kdenlive and Cinelerra offer broader feature sets, but are plagued by widely-reported installation and stability problems. For its part, the chief criticism of PiTiVi has always been missing functionality. The rewrite of the 0.13 series, begun in 2008, promises to change that.

PiTiVi (pronounced "P-T-V") uses Python for its user interface and top-level logic, but all audio and video processing — from playback to filters — takes place in GStreamer. That gives the application better performance, the ability to take advantage of GStreamer's multi-threading, and independence of codec and file formats. Any format supported by GStreamer is supported by PiTiVi, which is convenient for users, and distributions can include the application without introducing a package dependency on potentially legally-risky media engines such as FFmpeg. GStreamer's plugin-based abstraction allows PiTiVi to function regardless of whether any particular codec pack — patented, commercial, proprietary, or free — is installed, and does not require maintaining a specially-tailored playback engine to avoid legal conflicts.

Users will notice that PiTiVi 0.13.1 supports multiple layers in the timeline, permitting simple overlay of multiple video and audio sources, support for still images as video tracks, and the ability to trim audio and video clips in the editor. As the before (at left) and after (below, at right) screenshots depict, audio waveforms and video thumbnails are now used to represent tracks in the timeline, making it easier to distinguish between clips while editing. In addition to the visible changes, however, the more substantial work of the 0.13.x series is the refactoring of the core, which will eventually make the editor more powerful.

Refactoring

Lead developer Edward Hervey said that the refactored application core reflects several years of feedback from users and discussions with professional video editors. "The previous design was more the evolution of a certain view I had of video editing and its workflow back in 2003." The old design included a lot of hard-coded features that held the application back in real-world workflows, such as limits on the number of audio and video tracks, a limit on the number of layers, and only being able to have one project open at a time.

One example Hervey cites is the internal representation of the editing timeline. The old PiTiVi core was tied directly to the timeline provided by the Gnonlin library (a dependency of PiTiVi that implements NLE features as GStreamer plugins), which limited it to one object per stream and track. The new core adds an intermediary layer, permitting multiple objects and tracks.

The rewrite is also considerably more modular, making it easier for the developers to add additional features. Features planned for the 0.13.x cycle include critical editing tasks like titling, transitions, and effects — operations which are already possible in GStreamer plugins. Hervey explained:

Most of the user interface work has been taken on by new team addition Brandon Lewis, with assistance from Jean-Francois Tam. As is often the case, the team is trying to maintain a clean separation between the application core and its user interface, but for PiTiVi this choice has implications for its customizability. Hervey explained that the goal is to keep the UI as simple as possible in order to make the program usable by novices, but to leave room for building additional functionality on top of it through the use of plugins.

GStreamer

PiTiVi development is intrinsically tied to GStreamer development. As Hervey explained in an interview with Gnomedesktop.org, he, Lewis, and core developer Alessandro Decina are all employed by Collabora Multimedia, which sponsors ongoing GStreamer work alongside other open source projects. Some of Collabora's revenue comes from consulting jobs that involve building custom GStreamer processing and editing solutions, so building a dependable NLE is an important task, which influences the direction in which GStreamer itself moves.

One such influence was the notion of "segments" that was added in GStreamer 0.10. Segments allow PiTiVi to track in- and out-markers for each video and audio buffer without altering the buffer itself, greatly speeding up the process of rearranging clips in the timeline. "Previously in order to do time-shifting we'd have to modify the timestamps of all buffers, whereas with segments we can give information as to when buffers that follow that event will be displayed, at what speed, etc..." In addition, Hervey added, PiTiVi development tests all GStreamer plugins to ensure that they fully conform to the API, such as making sure that all decoders can do sample-accurate decoding.

Furthermore, as with any GStreamer project, the modular nature of the media framework means that other projects can share in the improvements that originate in PiTiVi, and vice-versa. Because PiTiVi represents a different usage of GStreamer than the considerably more common "media player" paradigm, it stress-tests plugins in different ways.

Up next on PiTiVi

The next milestone in PiTiVi development is scheduled to be released in July, adding essential features like undo/redo, blending and compositing, and video capture. Because it is officially designated a development branch, the team has not pushed the 0.13.x series for inclusion in Linux distributions. Until then, users interested in testing the development releases can either download and compile source code packages, or if using Debian or Ubuntu, access development binaries through the project's Personal Package Archive (PPA). The PPA includes updated packages for GStreamer, GStreamer plugins, Gnonlin, and other key dependencies. Building from source also requires satisfying these dependencies, so instructions are provided on the PiTiVi wiki.

NLEs are notoriously complex beasts, and a stable, capable NLE remains a missing piece of the free software desktop for many users. Because it can leverage GStreamer for the heavy lifting of media processing, PiTiVi stands a better chance than most young projects of reaching dependable usability, and if the first release of the newly-rewritten core in 0.13.1 is any indication, the application is well on its way.