Who are you guys?
How did Dirac begin?
Why is it called Dirac?
Why are you releasing Dirac Open Source?
Where can I get Dirac?
What are the license conditions?
Are you going to charge for Dirac?
Do the BBC have patents in Dirac?
Do you infringe any patents?
What will you do if you infringe patents?
Is the BBC going to stream video using
Dirac?
Are you going to broadcast TV using Dirac?
What about Ogg Theora?
How does it work?
Why did you choose the techniques that you're
using?
What's with your picture file format?
How do I play compressed video?
How do I code video?
How can I help?
What needs doing?
When will Dirac be ready?
top
Dirac was originally developed at BBC Research at Kingswood
Warren in the UK. The BBC have specialists in compression
technology, and have been working on digital television and
its ramifications since the 1960s.
Within the BBC, the project leader is Tim Borer, and Thomas
Davies is the guru who understands the algorithm in more
depth than most. The software is managed by Anu Suraparaju.
Today, Peter Bleackley and David Flynn are looking at some of
the hardware aspects of the system. In the recent past,
Andrew Kennedy, Michael Prior-Jones, Chris Bowley and others
have all been involved in the project. Pete Shelswell was the
senior line manager, and more recently acted as editor in
chief for the documentation before his retirement.
top
BBC R&D has always been involved in video coding research
as it's central to what the BBC does.
Thomas began experimenting with compression techniques in
about 2001. The ideas were originally developed as part of an
idea to deliver high definition services as a layered
addition to conventional MPEG-2 broadcasts. As with most
similar proposals, he did not manage to achieve any real
benefit from linking the standard definition and high
definition coding, but the algorithm he used was pretty
efficient in isolation. It was the foundation of Dirac.
top
Dirac is named after the British physicist (of Swiss
extraction) Paul Dirac, who was one of the great figures in
modern physics. We've seen some good explanations on the
'net, suggesting that we chose the name because the Dirac
delta function is fundamental to signal processing and that
Dirac's work underpins wavelets. Sorry, but the stories
aren't true. We thought of hundreds of names, most of them
awful, some of them good, but most of the good ones had been
used already. Tim suggested Dirac. It sounded good. It stuck.
It was going to be called Camel, so maybe we had a lucky
escape.
You can find out more about Paul Dirac here.
top
The BBC has always advocated open standards, and has tried to
use them where possible. So far, streaming has been dominated
by proprietary systems and existing licensing regimes for
standards-based systems have not been as attractive as they
might be for large-scale broadcasting, particularly for
Public Service broadcasters.
top
You can download the latest version from the Sourceforge
project page. You can also look at the developing code on
the CVS there.
top
Dirac is released under the Mozilla triple license (MPL).
This is an Open Source license that allows both free and
commercial use of the software. It also allows for
relicensing under the GPL or the LGPL.
top
No. The terms of the MPL mean that as far as the BBC is
concerned, there will be no charges or royalties for the
Dirac software.
top
Yes. We have patent applications in train for some of the
techniques involved in Dirac, and others that we intend to
put into Dirac in the future. There has been some FUD about
this, so we'll be clear: this does not affect the Open Source
status of Dirac, nor does it affect its royalty-free status.
The conditions of the MPL mean that we're licensing these
patents for use within the Dirac software for free.
top
The short answer is that we don't know for certain, but we
don't think so.
We haven't employed armies of lawyers to trawl through the
tens of thousands of video compression techniques. That's not
the way to invent a successful algorithm. Instead we've tried
to use techniques of long standing in novel ways. Where we
think we're novel, we're in the process of getting patent
protection ourselves, which will invalidate others' claims of
priority.
There are some areas that are more heavily patented than
others. Arithmetic coding is one such, even though the
technique itself has been around for 25 years. We're keeping
an eye on the situation, and we'll adopt alternative
techniques if we have to. Some of the recent changes in the
algorithm have centred on the arithmetic coding - taking it
away from some of the minefields of prior art that we have
uncovered.
top
Code round them, first and foremost. There are many
alternative techniques to each of the technologies used
within Dirac.
Dirac is relatively modular (which is one reason why it's a
conventional hybrid codec rather than, say, 3D wavelets) so
removing or adding tools was relatively easy.
Now we are close to a version 1.0. and the bytestream and
decoder functionality are fixed, we have to be a little more
careful. Products are about to come to market and we want to
ensure that they have a useful lifetime. Future changes are
likely to be few and backwards compatible.
top
A good question. Now we are close to a version 1.0, the BBC
has the opportunity to adopt Dirac for operational use. We
have real-time decoding, integration with players, a
bytestream spec and a choice of transport stream formats.
What we need to deliver next is hardware which can be used
live for encoding - without that, there is a problem
delivering streaming video at the volumes that the BBC would
expect.
top
Broadcasting or multicasting over the web is likely to
develop over the next few years, and the technology we use
for this is likely to be the same as for standard streaming.
There is no likelihood at all of replacing our existing
Digital TV infrastructure, based on European DVB standards
and using MPEG-2, with anything new - the installed base of
millions of customers is too large.
However, DVB is starting to
look at a new modulation system for terrestrial broadcasting.
Perhaps there could be an opportunity here. For DVB to adopt
Dirac, they would have to be able to reference Dirac as a
standard. That begs another question.
top
Theora is coming on very nicely, and has an impressive,
well-defined spec. We think we have much better compression
performance, but you can't have too many free codecs. The
Open Source community needs to continue to develop codecs
with increasingly better performance. Theora has helped
create a pool of Open Source compression experts, which is of
benefit to everyone in the field.
We intend to pack the Dirac elementary stream into MXF, which
has lots of useful features. That doesn't preclude it packing
into Ogg (or Matroska, or anything else) as well, and it's
probably a good idea to have a variety of packing formats.
For this the elementary stream needs to be very well defined.
top
See the algorithm
documentation
top
We chose wavelets for a number of reasons. They perform very
well in still image compression, and can be said to be state
of the art there. They also provide a degree of scalability,
so a codec based on wavelets can perform well across a range
of video standards. By scalability we don't mean embedded
bitstreams or the ability to extract lower-resolution video
from higher-resolution bitstreams - Dirac doesn't do this as
it's very complex - only the flexibility to apply the same
tools to a range of resolutions, perhaps with different
parameters. Wavelet coding is also a well-studied and
well-understood field.
Overlapped block motion compensation is used to reduce
blockiness and ease the job of the wavelet transform in
coding motion compensated differences. The wavelet transform
may not be the best tool to do this, and we'd welcome other
contributions, but it does have the benefit of simplicity in
that the tools for all frame types are the same. Dirac
supports more or less any block sizes for motion compensation
and this again helps in scaling the algorithms as larger
blocks can be used for higher resolution pictures and smaller
ones for low resolution pictures.
top
Dirac now just uses raw planar YUV, and there's no need to
create a header anymore. We also have patches for ffmpeg and
transcode, so you can use any of the formats they
support. We've also got some tools for converting formats to
and from the raw format, and there's advice on using these in
the users
README in the distribution.
top
We have a DirectShow filter which you can download, and a
patch for MPlayer.
There are also software decoders - see the SourceForge and
Schrödinger web sites.
top
You can use the encoder directly, using raw video in planar
YUV format. Or you can download and install the patches for
ffmpeg or transcode and use those.
top
By telling people about Dirac. By contributing to the code.
See getting involved or
contact us to find out more.
top
There's still a lot to do. We need optimisation work for the
encoder and decoder - the encoder in particular will always
benefit from speeding up. We also need to integrate tools
like global motion, and optimise other tools. Another thing
we lack is a buffer model and constant bit-rate encoding.
For a full(er) list, see the TODO
list.
top
It depends what you mean by ready.
We already have a specification that can be used for real. We
can code in non-real time in software, and for many
application real-time decoding is now feasible. What we have
not yet delivered is real hardware coders for main-stream
use. That is the next phase of the project.
top
|