Authors: Jack Jansen, Pablo Cesar, Dick Bultermann (CWI)
Distributed multimedia is a field that depends on many technologies, including networking, coding and decoding, scheduling, rendering and user interaction. Often, this leads to multimedia researchers in one of those fields expending a lot of implementation effort to build a complete media environment when they actually only want to demonstrate an advance within their own field. In 2004 the authors, having gone through this process more than once themselves, decided to design an open source extensible and embeddable multimedia platform that could serve as a central research resource. The NLNet Foundation, www.nlnet.nl, graciously provided initial funding for the resulting Ambulant project.
Ambulant was designed from the outset to be usable for experimentation in a wide range of fields, not only in a laboratory setting but also as a deployed player for end users. However, it was not intended to compete with general end-user playback systems such as the (then popular) RealPlayer, Quicktime or the Windows Media Player. Our goal was to build a glue environment where various research groups could plug in next approaches to media scheduling, rendering and distribution. While some effort was spent on things like ease of installation, multi-platform compatibility and user interface issues, Ambulant has never hoped to usurp commercial media players. The user interface on three different platforms can bee seen in the figure below.
The first deployment of the platform was during the W3C standardization of SMIL 2.1 and 3.0 [2, 3], when Ambulant was used to test the specification and create an open reference implementation. The fact that Ambulant supports SMIL out of the box means that it is not only useful to “low-level” multimedia researchers who want to experiment with replacing systems components, but also to people interested in semantics or server-side document generation: by using SMIL as their output format they can use Ambulant to render their documents on any platform, including inside a web browser.
Design and Implementation
Ambulant is designed so that all key components are replaceable and extensible. This follows from the requirement that it is usable as an experimentation vehicle: if someone wants to replace the scheduler by one of their own design this should be possible, and have little or no impact on the rest of the system.
To ensure wide deployability it was decided to create a portable platform. However, runtime efficiency is also an issue in multimedia playback, especially for audio and video decoding and rendering, so we decided to implement the core engine in C++. This allowed us to use platform-native decoding and rendering toolkits such as QuickTime and DirectShow, and gave us the added benefit of being able to use the native GUI toolkit on each platform, which makes life easier for end users and integrators. Using the native GUI has been a bit of extra effort up front, finding the right spot to separate platform-independent and platform-dependent code, but by now porting to a new GUI toolkit takes about three man-months. About 8 GUI toolkits have been supported over time (or 11 if you count browser plugin APIs as a GUI toolkit). The current version of Ambulant runs natively on MacOSX, Linux, Windows and iOS, and a browser plugin is available for all major browsers on all desktop platforms (including Internet Explorer on Windows). Various old platforms (WM5, Maemo) were supported in the past and, while no longer maintained, the code is still available.
The design of Ambulant is shown in the figure above. On the document level there is a parser which reads external documents and converts them into a representation that the scheduler and layout engine will handle during document playout time. On the intermediate level there are datasources that read documents and media streams and handles them to the playout components. On the lower level there are the machine-dependent implementations of those stream readers and renderers. For each of these components there are multiple implementations, and those can easily be replaced or extended.
The design largely uses factory functions and abstract interfaces, therefore the implementation uses a plugin architecture to allow easy replacement of components at runtime without having to rebuild the complete application.
To make life even more simple, the API to the core Ambulant engine is available not only in C++ but also in Python. The Python bridge is complete and bidirectional: all classes that are accessible from C++ are just as accessible from Python and vice versa, and sending an object back-and-forth through the Python-C++ bridge results in the original object, not a new double-wrapped object. Moreover, not only can C++ classes be subclassed in Python but also the reverse. This means both extending Ambulant through a plugin and embedding Ambulant can be done in pure Python, without having to write any C/C++ code and without having to rebuild Ambulant.
Over the years, Ambulant has extensively been used for experimentation, both within our group and externally. In this section we will highlight some of these applications. The overview is not complete, but it highlights the breadth of applications of Ambulant.
One of the interests of the authors is maintaining the temporal scheduling integrity of dynamically modified multimedia presentations. In the Ambulant Annotator , we experimented with using secondary screens during playback, allowing user interaction on those secondary screens to modify existing shared presentations on the main screen. The modification and sharing interface was implemented as a plugin in Ambulant, which is also used to drive the main screen. In Ta2 MyVideos  we looked at a different form of live modification: a personalized video mashup that was created while the user is viewing it.
Integration of live video conferencing and multimedia documents is another area in which we work. For the Ta2 Family Game project  we augmented Ambulant with renderers to do low delay live video rendering and digitizing, and a Flash engine. The resulting platform was used to play a cooperative action game in multiple locations. We are also using Ambulant to investigate protocols for synchronizing media playback at remote locations.
In a wholly different application area, the Daisy Consortium has used Ambulant as the basis of AMIS, www.daisy.org/projects/amis. AMIS is software that reads Daisy Books, which are the international standard for digital talking books for the visually impaired. For this project Ambulant was only a small part of the solution. The main program allows the end user, who may be blind or dyslectic, to select books and navigate them. Timed playback is then handled by Ambulant, with added functionality to highlight paragraphs on-screen as the content is read out, etc.
At a higher level, an instrumented version of Ambulant has also been deployed to indirectly evaluate social media systems. In 2004, it was submitted to the first ACM Multimedia Open Source Software Competition .
Obtaining and Using Ambulant
Ambulant is available via www.ambulantplayer.org, in three different forms: as a stable distribution (source and installers), as a nightly build (source and installers) and through Mercurial. Unfortunately, the stable distribution is currently lagging quite a bit behind, due to restricted manpower. We also maintain full API documentation, sample documents and community mailing lists.
Ambulant is distributed under the LGPL2 license. This allows the platform to be used with commercial plugins developed by industry partners who provide proprietary software intended for limited distribution. We are considering a switch to dual licensing (GPL/BSD), but a concrete need has yet to arise.
The Bottom Line
Ambulant is a full open source media rendering pipeline. It provides an open, plug-in environment in which researches from a wide variety of (sub)disciplines can test new algorithms and media sharing approaches without having to write mountains of less-relevant framework code. It can serve as an open environment for experimentation, validation and distribution. You are welcome to give it a try and to contribute to its growth.
Bulterman, D. et al. 2004. Ambulant: a fast, multi-platform open source SMIL player. In Proceedings of the 12th annual ACM international conference on Multimedia (MULTIMEDIA ’04). ACM, New York, NY, USA, 492-495. DOI=10.1145/1027527.1027646
Bulterman, D. et al. 2008. Synchronized Multimedia Integration Language (SMIL 3.0). W3C. URL=http://www.w3.org/TR/SMIL/
Bulterman, D. and Rutledge, L. 2008. Interactive Multimedia for the Web, Mobile Devices and Daisy Talking Books. Springer-Verlag, Heidelberg, Germany, ISBN: 3-540-20234-X.
Cesar, P. et al. Fragment, tag, enrich, and send: Enhancing social sharing of video. Transactions on Multimedia Computing, Communications, and Applications (TOMCCAP (2009) vol. 5 (3). DOI=10.1145/1556134.1556136
Jansen, J. et al. 2012. Just-in-time personalized video presentations. In Proceedings of the 2012 ACM symposium on Document engineering (DocEng ’12). ACM, New York, NY, USA, 59-68. DOI=10.1145/2361354.2361368
Jansen, J. et al. Enabling Composition-Based Video-Conferencing for the Home. IEEE Transactions on Multimedia (2011) vol. 13 (5) pp. 869-881. DOI=10.1109/TMM.2011.2159369