Slashdot Mirror
More On The BBC's Codec 'Dirac'
TioHoltzman writes "El Reg is reporting about a new codec that is built on top of wavelet technology and seems to offer performance that is "roughly in line with the Video Codec 9" from Microsoft. The project has been released as open source on SourceForge. This looks like it might be really interesting." (Previously mentioned a few weeks back.)
Last time I checked, wavelet compression methods were burdened by many patents: google search. What does that mean for users of the codec?
"I love my job, but I hate talking to people like you" (Freddie Mercury)
The standard way to compress both audio and video is with the Discrete Cosine Transform, or DCT. MPEG audio and video are based on DCT.
The basic idea of DCT is to transform the data into a series of waves, which tends to concentrate the data. Then you throw away part of the data, and then use lossless encoding on what is left. If you just threw away pixels, the result would be obvious in an image; but if you throw away part of the wave specification data, the results are not as obvious.
With DCT, consistent data sets compress very well (e.g., a blue sky or a white wall). Pictures with lots of sharp little edges (e.g., a field of blades of grass) compress much less well.
My understanding is that potentially wavelets will compress even better than the DCT. However, they are not enough better to be a huge win at the moment.
steveha
lf(1): it's like ls(1) but sorts filenames by extension, tersely
as for wavelet compression being a novel codec, what about apple's pixlet technology?
Some drink at the fountain of knowledge. Others just gargle.
This is from 1998.
http://www.seyboldreports.com/SRIP/wavelet/
steveha
lf(1): it's like ls(1) but sorts filenames by extension, tersely
While wavelets doesn't offer a breathtaking advantage in data rate vs. quality factor, it does appear to lend itself to a simpler implementation than does DCT, and unlike MPEG, which is very intensive on the encoder, wavelets places symmetrical burdens on encoder and decoder.
It was a core assumption in the design of MPEG that the world market for encoders was quite small (where have we heard that theme before???) Clearly, the assumption was false, and one advantage of switching to a wavelets technology would be reduced cost per unit for encoders.
--- Bill
Dirac (pronounced Dih-RAK) was a physicist and mathematician. His name appears in this context because of the "Dirac delta function," otherwise known as an impulse function. It lies at the heart of linear signal theory, including wavelet theory.
Nothing to do with the government. The BBC is granted a charter from Parliment, but is not government run or funded. The BBc is funded by a compulsory license fee for owning equipment capable of recieving and decoding their broadcasts such as a TV or tuner card. Basically it's a tax on virtually every household and business in the UK. There is a discount for black & white TV's, pensioners and those with vision based disabilities. In the 'old days' you used to need a 'wireless licence' as well for radios!
This means that when information is dropped in each block (according to the compression required), the edges of blocks suffer in a way unrelated to the edge of adjacent blocks. The result -- as the quality decreases, the edges between blocks become more and more obvious, and the whole image becomes 'blocky'.
I believe this is one way that wavelet technology improves -- the individual wavelets are spread over the whole image, without regard for any blocks, and so the compression degrades much more gracefully.
As you say, the DCT converts each 8x8 block into a series of cosine waves, both horizontally and vertically in the block. Then, when it needs to reduce the space, it drops the higher-frequency coefficients first -- this is why sharp edges, with lots of high frequency information, suffer most. (You tend to find that lower-frequency coefficients try to compensate, giving the characteristic ripples near sharp edges.) Areas that are relatively smooth, with only low-frequency information to start with, suffer much less.
Another way JPEG loses information is by colour. The human eye is much more sensitive to fine changes in brightness than it is to fine changes in colour; so the picture is transformed from RGB into a brightness channel and two colour channels, and the brightness channel gets a greater share of the limited space. It's quite interesting, if you're, er, interested in that sort of thing...
Ceterum censeo subscriptionem esse delendam.
Pixlet is designed for real-time editing, so it has minimal artifacts and no interframe compression. Dirac is for broadcast, so it is much more agressive about compression and can take advantage of motion compensation and other computationally expensive compression techniques.
You are right, however, that wavelets are not at all a new compression technology. People started playing with it at least 10 years ago and JPEG-2000 uses wavelets for still photo compression. I think that the computational load has prevented their use in video until recently.
The BBC have a long history of supporting computer technology.
Many of us English folks grew up with the BBC Micro computer in schools.
liqbase