COMPARISON OF OPEN AND FREE
VIDEO COMPRESSION SYSTEMS
A Performance Evaluation
Till Halbach
Norwegian Computing Center, Norway
Keywords:
Still-image and video compression, Performance comparison, Dirac, Theora, H.264, Motion JPEG2000.
Abstract:
This article gives a technical overview of two open and free video compression systems, Dirac and Theora I,
and evaluates the rate distortion performance and visual quality of these systems regarding lossy and lossless
compression, as well as intra-frame and inter-frame coding. The evaluation shows that there is a substantial
performance gap of Theora and Dirac when compared to H.264- and Motion JPEG2000-compliant reference
systems. However, an algorithm subset of Dirac, Dirac Pro, achieves a performance comparable to that of
Motion JPEG2000, and which can be less than one dB below the PSNR performance of H.264 with TV-size
and HD video material. It is further shown that the reference implementations of the codecs of concern still
have potential for efficiency improvements.
1 INTRODUCTION
The video compression community has recently been
witness to an interesting development concerning
royalty-free specifications. Both the British Broad-
casting Corporation (BBC) and the Xiph.org Foun-
dation have released video codecs which are open-
technology and open-source and free to use for the
public. They compete therefore with patented codecs
such as MPEG-x and H.26x which are, though spec-
ified in international standards, typically subject to a
license fee, and not entirely open. While the lack of
royalty claims makes these specifications interesting
candidates for use by industry and individuals like-
wise, the technologies provided should offer a perfor-
mance close to or better than today’s state-of-the-art
standards to ensure widespread acceptance.
So far, a detailed performance assessment of both
Dirac and Theora relative to existing standards has
been lacking in the literature; a gap this article wants
to fill. All involved technologies are presented be-
low with technical details. Dirac and Theora are then
compared to each other and to reference standards to
evaluate their performance. Conclusions are drawn at
the end of the article.
2 DIRAC
The BBC have been working on a family of general-
purpose video codecs with the umbrella name Dirac
for some time now. As a result, two major products
have been released to the public at the time of writing.
Dirac Pro, which is version 1.0 of the Dirac fam-
ily, is an intra-frame video codec which targets loss-
less or near lossless image processing with low la-
tency, suitable e.g. for studio and professional appli-
cations (BBC, 2008a). Dirac Pro has been submit-
ted to the Society of Motion Picture and Television
Engineers and is expected to become international
standard VC-2 (W3C, 2008), with Microsoft’s WM-9
being VC-1 (SMPTE, 2006). Version 2.1 of Dirac is
a super-set of Dirac Pro, including motion estimation/
motion compensation (ME/MC) (BBC, 2008b). It is
developed mainly with broadcasting and streaming in
mind, targeting application areas like TV, digital cin-
ema, and Internet. According to the Dirac Pro specifi-
cation, there is the intention to extend VC-2 to include
the full Dirac specification.
In particular for the end user it is of inter-
est that the BBC claim the standards suite to
be patent-/royalty-free and open-technology (Borer,
2005). The reference codec developed is licensed un-
der the Mozilla Public License version 1.1.
74
Halbach T. (2009).
COMPARISON OF OPEN AND FREE VIDEO COMPRESSION SYSTEMS - A Performance Evaluation.
In Proceedings of the First International Conference on Computer Imaging Theory and Applications, pages 74-80
DOI: 10.5220/0001809700740080
Copyright
c
SciTePress
2.1 Technology Overview
Like international video compression standards,
Dirac defines the byte stream and decoding proce-
dure only. However, for illustration purposes, the
discussions below are made with regard to standard-
compliant encoders.
Dirac Pro (BBC, 2008a) accepts RGB and YUV
video, with chrominance subsampling factors of
4:4:4, 4:2:2, and 4:2:0. There are no restrictions
on the bit resolution, and neither on frame size or
frame rate. Predefined formats defined in the refer-
ence implementation include picture resolutions from
below QCIF size to 4K Digital Cinema. The nature
of the original video can be both interlaced and pro-
gressive. Dirac Pro does not involve any MC. A
compliant encoder comprises three main compression
components. A frame of a video sequence is first
transformed by a wavelet filter bank to exploit spatial
redundancies, before the transform coefficients are
quantized and entropy encoded to exploit statistical
correlation. The filter bank can be implemented in a
very efficient way by means of lifting. The quantizers
used are basically scalar dead-zone quantizers and are
left out for lossless encoding. Entropy encoding can
either mean variable-length coding (VLC) or arith-
metic coding (AC). Concerning data migration, later
transcoding without loss is possible by compressing
video sequences in Dirac Pro’s lossless mode.
Another feature of Dirac Pro is the use of inde-
pendent chunks of data inside the byte stream, which
allows for fast forward and backward functionality.
All data are stored in a single byte stream, such that
the transport in container formats like MPEG trans-
port stream or OGG is possible. Due to the use of a
filter bank hierarchy in the decomposition stage, in-
cremental frame decoding is easily achieved, allow-
ing for scalability in quality and resolution. Temporal
scalability is supported implicitly as no temporal pre-
diction is involved.
Though Dirac Pro defines the from other stan-
dards well known concept of profiles (sets of algo-
rithms) and levels (constraints on decoder resources
like frame and data buffer sizes), none have been de-
fined yet. However, a coding option for high quality
and another one for low latency exists. High quality is
achieved by processing the entire frame at a time in-
stance and implies thus a delay of at least one frame.
Low latency is achieved by processing subregions of
the frame at a time, and by replacing AC by VLC.
This comes typically at the expense of less efficient
compression. By the use of a wavelet filter bank and
predicting subband coefficients, Dirac Pro can be re-
lated to the well know SPIHT video compression al-
gorithm (Said and Pearlman, 1996) and the interna-
tional standard Motion JPEG2000 (ISO/IEC, 2001).
As mentioned before, Dirac 2.1.0 (BBC, 2008b)
deploys MC to exploit temporal redundancies and can
therefore be counted as a member of the family of
block-based hybrid video codecs. Dirac is identical
to Dirac Pro, except that here not the original image
is passed to the filter bank, but a motion compensated
one. As a consequence, the functionality of scalability
is lost. The motion prediction process is quite flexible,
allowing any combination of various unidirectional
and bidirectional modes, including weighting. An ad-
ditional option for global motion estimation exists.
The block sizes utilized in the prediction process can
be of varying size and overlap each other, also known
as overlapped block motion compensation. The accu-
racy can be either half-pel, quarter-pel, or 1/8-pel.
3 THEORA
Theora I is a video codec which is developed by mem-
bers of the Xiph.org Foundation. It is more and
more frequently found on the Web and used by large
sites complying with the Wikimedia Commons like
Wikipedia. However, the main application areas re-
main undefined. Xiph.org claims that Theora con-
tains solely royalty-free and open technology. Its ref-
erence implementation is provided under a BSD-style
license.
3.1 Technology Overview
Also this codec can be classified as belonging to the
family of block-based hybrid video codecs. As with
international standards, the specification details the
decoder operation and data field order in the byte
stream. As a concatenation of data packets, this
stream can easily be encapsulated in any suitable
transport container format. The standard allows pro-
gressive video material with 8bpp accuracy and arbi-
trary dimensions, ranging from below QCIF to sig-
nificantly more than 4K. The sequence may have a
YUV color space with 4:2:0, 4:2:2, and 4:4:4 chroma
subsampling.
A compliant decoder comprises five main compo-
nents. The byte stream data are first entropy decoded
employing Huffman codes. The decoded transform
coefficients are then passed to the inverse quantization
process and, subsequently, to an inverse 8 × 8 DCT,
producing a frame difference signal, or frame delta.
COMPARISON OF OPEN AND FREE VIDEO COMPRESSION SYSTEMS - A Performance Evaluation
75
The frame delta pixels are then added to the predicted
frame to form the reconstructed frame which is pro-
cessed by a deblocking filter before picture display
and storage in the frame buffer. The filter has hence
an in-loop position, and it is applied to block edges.
The block-based MC process utilizes forward pre-
diction (P-frames) based on a single reference frame,
as well as simple block copying, but no bi-predictive
(B-) frames. The accuracy is either full pel or half pel.
Otherwise it is of interest that that most algorithms
make use of new data scanning orders in order not to
infringe any patents. Arbitrarily accessing I-frames in
the code stream is possible under the constraint that
the byte stream header must have been transmitted
and processed for decoder initialization previously.
Other than that, it is worth mentioning that Theora
does not provide the possibility for scalable picture
decoding, and it lacks further the option of later loss-
less data migration from data stored in Theora format
to another format, as it does not provide any means
for lossless coding.
4 PERFORMANCE
COMPARISON
In this section, Dirac and Theora are compared to
each other performance-wise, and to Motion JPEG-
2000 and H.264 as reference codecs and international
state-of-the-art video compression standards (ISO/
IEC, 2001; ITU-T, 2003). To the author’s knowledge,
only a single independent evaluation of Dirac has
been undertaken previously (Onthriar et al., 2006),
where the authors found that H.264 outperforms the
at that time emerging Dirac both in PSNR and SSIM
value, especially at low bot rates. Theora remains
untested in that respect.
All encoders are operated in rate distortion
optimization (RDO) mode or near RDO mode (if pro-
vided) and identical or near identical parameter sets
required for fair codec comparisons. The involved
motion compensation implies 250 P- or B-frames be-
tween I-frames and a single pass.
To assess the quality of Dirac 2.1.0, the version
0.9.1 of the reference implementation is used, which
complies with the standard. The encoder is quality
controlled by setting a quality factor similar to that
of the JPEG standard. Near RDO is accomplished
by requiring a full ME search with an area of 32 × 32
pixels. Concerning Theora I, version 0.19 of the refer-
ence implementation with the name libtheora is used.
The software is quality controlled like Dirac with a
quality parameter and is operated in RDO mode. For
compressing video material compliant with H.264,
the implementation named x264 is used. The encoder
is rate controlled but operated without any particular
RDO due to a lack of such an option. Two B-frames
are inserted between two P-frames, for which the ME
process is based on a single reference frame. The
search area is of the size 32 square pixels and has a
hexagonal form.
All video sequences used in the experiment com-
prise 8 bpp image material and a YUV color space
with 4:2:0 subsampling. AKIYO is a 300-frame head-
and-shoulder sequence in QCIF resolution with little
motion, MOBILE has SD/PAL resolution and consists
of 220 frames with a moderate amount of motion,
while CREW comprises HD 720p material with flash-
ing effects.
4.1 Inter-frame Comparison
The comparison of inter-frame coding schemes treats
lossy and lossless coding separately.
4.1.1 Lossy Coding
The original video sequence is encoded and decoded
involving temporal prediction, and the average lumi-
nance PSNR (Y-PSNR, in dB) is recorded, as well as
the achieved bit rate in Kbps. The rate distortion (RD)
curves are drawn over six different codec operation
points, while only ve points are recorded with The-
ora due to its limited allowed range for the quality
factor. The bit rate is measured for raw video data
except Theora, with which the data is encapsulated
inside the OGG container format, implying the use of
(a negligible amount of) overhead data. The size of
the video data is naturally somewhat lower.
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50
50 100 150 200 250 300 350
Y-PSNR / [dB]
Bit rate / [Kbits/s]
Dirac
Theora
H.264
Figure 1: RD comparison for AKIYO (QCIF).
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It can easily be seen in Fig. 1 through Fig. 3 that
there are significant performance gaps among all in-
volved codecs. The reference system H.264 outper-
forms both Dirac and Theora by a substantial amount,
in terms of PSNR, at all rates. The difference between
Dirac and H.264 can be more than 10 dB at low rates
and up to 5 dB at moderate to high rates. Dirac’s tar-
get application range is obviously high bit rate. Be-
sides the aforementioned, Fig. 3 also bears proof of
the fact that Dirac provides only a few coarse quan-
tizers, which limits its RD curve at low RD products.
Dirac also copes better with SD- and HD-sized video
material than with smaller formats. This can be de-
rived from the fact that the PSNR difference between
Dirac and H.264 is significantly less (roughly 3 dB) in
Fig. 2 and Fig. 3 than in Fig. 1.
Theora performs worse than Dirac. In Fig. 1 and
Fig. 3, the reference implementation does not gain an
improved performance with a bit rate increase, which
is in contrast to what would have been expected intu-
itively. This is, however, likely an implementation is-
sue, as the codec performs as expected with MOBILE.
In either case, it can be concluded that neither The-
ora nor Dirac are suitable codecs for video sequences
with less-than-SD resolution, considering PSNR val-
ues below 30 dB at low bit rates. There are manifold
reasons for H.264’s superiority, and its hierarchical
ME mechanism with variable block sizes and a very
accurate interpolation, the adaptive in-loop deblock-
ing filter, as well as the highly efficient arithmetic en-
coder are among them.
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0 5000 10000 15000 20000 25000 30000
Y-PSNR / [dB]
Bit rate / [Kbits/s]
Dirac
Theora
H.264
Figure 2: RD comparison for MOBILE (SD).
The gap in PSNR between Theora and H.264 can
be as large as 10 dB, when the malfunctioning of the
implementation is ignored. Fig. 2 shows that the gap
decreases with roughly 2dB towards low bit rates.
Theora is obviously designed for low to moderate
bit rates. The container overhead is measured to be
around 1% and is hence without influence on the final
result.
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0 5000 10000 15000 20000 25000 30000 35000
Y-PSNR / [dB]
Bit rate / [Kbits/s]
Dirac
Theora
H.264
Figure 3: RD comparison for CREW (HD).
The efficiency of the involved implementations is
measured in Tab. 1. The respective software is in-
voked 20 times on a single general-purpose CPU with
2.2GHz clock frequency, and the CPU time needed
for encoding (in seconds) is averaged over all runs.
This comparison has to be interpreted with care, as
implementations like the one for Dirac are for a proof
of concept only and not optimized for speed. How-
ever, the comparison serves as a pointer for a rough
assessment of the encoders’ complexity.
Table 1: Implementation efficiency, in fps.
Dirac Theora H.264
Akiyo 21 379 323
Mobile 2 9 13
Crew 0.03 0.3 8
The Dirac implementation appears to be the most
inefficient software of the three measured implemen-
tations. Dirac’s frame processing frequency is lowest
in all cases, while the Theora and H.264 implementa-
tions are alternately best. However, it is of advantage
that the processing frequency of Dirac’s software is
almost constant at varying bit rates, or in other words,
its standard deviation σ
Dirac
is much smaller than 1.
The standard deviation σ
Theora
equals roughly 31, 1,
and 0.07 with AKIYO, MOBILE, and CREW, respec-
tively, and σ
H.264
is roughly equal to 49, 5, and 2,
respectively. Summarizing, the frame processing fre-
quency of Theora’s implementation varies most.
It is concluded that Theora and H.264, which
both deploy a transform instead of a filter bank like
Dirac, can be operated much faster than the latter
mentioned codec, despite the lifting structure as de-
scribed in the Dirac specification. Dirac is close to
real-time performance with QCIF-size video, while
COMPARISON OF OPEN AND FREE VIDEO COMPRESSION SYSTEMS - A Performance Evaluation
77
Figure 4: Original (left) and decoded frame 100 of AKIYO. Middle left: Dirac; Middle right: Theora; Right: H.264. The
images may be slightly scaled due to space considerations.
Theora and H.264 are way above this requirement.
Neither of the mentioned codecs is capable of en-
coding in real time with SD-size video or larger (i.e.,
without additional implementation optimization and
without extra hardware). Concerning Dirac, it should
be mentioned that a high-performance implementa-
tion named Schr
¨
odinger is being developed, and that
specialized hardware exists for coding of HD signals
in real time (Borer, 2007).
A visual comparison of all involved codecs at
a very low bit rate reveals significant artifacts with
both Dirac and Theora. Fig. 4 shows a single de-
coded frame of the AKIYO video. All videos were
encoded as described previously. The bit rates of
Theora and H.264 are 17.8 Kbps and 17.0 Kbps, re-
spectively, while the for the reference implementation
lowest possible bit rate with Dirac is 67.2Kbps. It
can easily be seen that, despite the significant higher
bit rate, Dirac is the codec which performs worst,
i.e. which introduces much more compression arti-
facts than the two competing codecs. The artifacts are
mainly ringing along edges and blurred areas, which
can be attributed to the use of a filter bank instead of
a transform as with Theora and H.264. With Theora,
the decoder outputs a slightly blurred picture when
compared to H.264, and also minor blocking can be
observed (in the women’s face). Dirac and Theora
yield 26.5 dB and 30.5 dB with the frame of con-
cern, respectively, whereas H.264 achieves a PSNR of
34.3dB. Here, the decoded picture is slightly blurred
compared to the original, but no blocking artifacts are
found, despite the approach of block-based MC and
a block-based transform. The explanation lies in the
good performance of H.264’s in-loop deblocking fil-
ter, which obviously outperforms the filter employed
in Theora. Note finally that, while the visual com-
parison (exemplified by a single frame) suggests that
Theora is superior to Dirac at very low bit rates, the
RD points in Fig. 1 are averaged over the entire se-
quence.
4.1.2 Lossless Coding
As already mentioned, Theora does not offer a loss-
less option. Concerning Dirac, it was found that the
reference implementation was not stable enough to
conduct these experiments, hence this topic remains
for future research.
4.2 Intra-frame Comparison
This section investigates the efficiency of Dirac Pro
and Theora when disregarding motion compensation,
i.e. in pure intra-frame coding mode. With Motion
JPEG2000 or short MJ2, an additional international
state-of-the-art standard is taken as reference system.
Lossy and lossless coding are again treated separately.
Version 1.3 of the implementation named Open-
JPEG has been used to encode/decode to and from
MJ2 format. Other implementations with differing ef-
ficiency exist (Pearson and Gill, 2005). The encoding
process generates a single quality layer, and four and
five decomposition levels for AKIYO and MOBILE/
MOBILE, respectively. The lossy scenario involves an
irreversible 9/7 wavelet filter bank, while the lossless
scenario demands reversible 5/3 integer wavelets.
4.2.1 Lossy Coding
The experiments in this section are as described in
Sec. 4.1.1, except that MC is turned off, i.e. the en-
coded bit stream consists solely of I-frames. Fig. 5
through Fig. 7 lack RD curves for Theora as its ref-
erence implementation is not capable of running in
intra-frame mode.
Again, H.264 outperforms the other codecs signif-
icantly at high bit rates, while the difference in PSNR
can be less than 1 dB at low rates, depending on the
video material. It is further observed that Dirac Pro
has roughly the same RD behavior as Motion JPEG-
2000. In fact, Dirac’s intra-frame coding appears to
be optimized for compressing TV and HD material at
low bit rates, given a PSNR close to that of H.264 with
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0 500 1000 1500 2000 2500 3000 3500
Y-PSNR / [dB]
Bit rate / [Kbits/s]
Dirac Pro
Motion JPEG2000
H.264
Figure 5: RD comparison in intra mode for AKIYO (QCIF).
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0 10000 20000 30000 40000 50000 60000 70000
Y-PSNR / [dB]
Bit rate / [Kbits/s]
Dirac Pro
Motion JPEG2000
H.264
Figure 6: RD comparison in intra mode for MOBILE (SD).
MOBILE/CREW. This is in contrast to the inter-frame
RD behavior as seen in Fig. 1 but can be explained by
a different set of quantizers and differing statistical
signal properties.
The latency of the reference implementations is
measured as described in Sec. 4.1.1, and listed in
Tab. 2. It is found that the H.264 software is best with
a substantial margin in terms of frame processing rate,
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0 50000 100000 150000 200000 250000
Y-PSNR / [dB]
Bit rate / [Kbits/s]
Dirac Pro
Motion JPEG2000
H.264
Figure 7: RD comparison in intra mode for CREW (HD).
Table 2: Implementation efficiency, in fps.
Dirac Pro Motion JPEG2000 H.264
Akiyo 128 62 266
Mobile 8 3 15
Crew 5 2 11
and the implementation of Motion JPEG2000 is the
most inefficient of the three implementations of con-
cern. However, the latter one performs in the most
uniform manner with a σ
MJ2
less than or equal to 2.
The deviations σ
Dirac Pro
and σ
H.264
are 47 and 79 with
QCIF-size material, 4 and 7 with PAL material, and
2 and 3 with HD material, respectively. I.e., the per-
formance efficiency of H.264’s implementation varies
most.
4.2.2 Lossless Coding
Dirac Pro in lossless intra-coding mode is compared
to both reference codecs. Tab. 3 shows the achieved
bit rate of all coding schemes. Dirac Pro is outper-
formed by both reference codecs. H.264 appears to
draw advantage of its highly efficient intra-frame pre-
diction mechanisms which are superior to those em-
ployed in Dirac Pro. Motion JPEG2000, which in turn
is based on JPEG2000, can rely on superior subband
prediction and a highly efficient arithmetic coding en-
gine. As already mentioned, the filter banks of Dirac
Pro and MJ2 are set to be identical, so the arithmetic
encoder of Dirac Pro has potential for improvement
when compared to Motion JPEG2000. A side result
of the experiments is that Motion JPEG2000 appears
to be the best choice for professional studio applica-
tions.
Table 3: Comparison of lossless intra-frame coding
schemes. The rate is given in Mbps.
Dirac Pro H.264 Motion JPEG2000
Akiyo 4.8 3.6 3.1
Mobile 102.3 86.8 76.5
Crew 335.1 276.2 224.8
5 CONCLUSIONS
Two open and free codecs were presented in this arti-
cle. Their technical details were discussed, and a per-
formance evaluation was given. It is stressed that the
assessment is by no means exhaustive, but rather must
be interpreted as a starting point for further research.
The experiments showed that the reference sys-
tem H.264 outperforms the two codecs significantly,
COMPARISON OF OPEN AND FREE VIDEO COMPRESSION SYSTEMS - A Performance Evaluation
79
both in the objective and the subjective/visual evalu-
ation. The low performance (in terms of PS NR) of
Dirac and Theora surprises, since both codecs have
been designed to compete with current cutting-edge
technology. While Dirac is suitable for compression
of SD/HD-sized video, its reference implementation
is the slowest of the three systems of concern. The
implementation of Theora comes closest to the imple-
mentation of H.264 in terms of efficiency. However,
this system achieves the lowest PSNR of all involved
codecs. Dirac Pro can compete with Motion JPEG-
2000 over the entire rate spectrum and comes with
less than 1dB difference in PSNR close to H.264 with
TV and HD material.
It is concluded that the algorithms of the two
open and free codecs need improvement to be able
to compete with state-of-the-art technology. With to-
day’s performance both standards can, however, be
compared to former video compression systems like
MPEG-2 and H.263+, as the performance gap be-
tween those standards and e.g. H.264 is well known
(Wiegand and Sullivan, 2007). The battle for the
most used video compression format on the Web will
hence continue, and the candidates still have to posi-
tion themselves in a better way, i.e. with improved RD
behavior and other functionality. In particular, The-
ora should offer options for lossless coding and intra-
frame coding, and Dirac should provide for compres-
sion at very low bit rates. At the same time, the encod-
ing/decoding libraries have to be optimized for speed
to increase the possibility for a wide acceptance of the
offered technologies.
It is finally stressed that the comparisons pre-
sented in this article are not only comparisons of spec-
ifications, but also comparisons of encoder implemen-
tations. Though all standard-compliant, different en-
coders typically yield different performances. The re-
sults presented here can therefore not be generalized
to be valid for other implementations or even other
versions than those involved.
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