Introduction
Dirac is a hybrid motion-compensated video coding standard that can be used without the payment of license fees. Dirac was released at around 2008. Dirac specification
Typical hybrid video coder:
taken from the paper “AV1 and VVC Video Codecs: Overview on Complexity Reduction and Hardware Design”, by MARCEL CORRÊA et al.
Some features of Dirac
- Dirac employs wavelet compression (like JPEG2000), instead of the discrete cosine transforms used in most other codecs. It’s mentioned in literature that Wavelet transform having the advantage of preserving fine details better than block-based transforms. Wavelets operate on the entire picture at once instead focusing on small areas at a time.
- Image motion is tracked and the motion information is used to make a prediction of a later frame. A transform is applied to the prediction error between the current frame and the previous frame aided by motion compensation and the transform coefficients are quantized and entropy coded. Temporal and spatial redundancies are removed by motion estimation, motion compensation and discrete wavelet transform respectively.
- Dirac uses dead-zone quantization technique by making the first set of quantization steps twice as wide and allows Dirac to perform coarser quantization on smaller values. Dirac also provides sub-pixel motion compensation with motion vectors
- Dirac uses overlapped block-based motion compensation (OBMC) to achieve good compression and avoid block-edge artifacts which would be expensive to code using wavelets.
- According to BBC Research, Dirac was developed with a view to optimize its performance with compression ratio and perceptual quality at the forefront.
Technical Literature on Dirac
1) “PERFORMANCE ANALYSIS AND COMPARISON OF THE DIRAC VIDEO CODEC WITH H.264/MPEG-4 PART 10 AVC”, by ARUNA RAVI et al. 2010
2) “Survey of Dirac: A Wavelet Based Video Codec for Multiparty Video Conferencing and Broadcasting”, by Ahtsham Ali, Nadeem et al., 2011
Using ffmpeg to encode in Dirac
ffmpeg by default supports Dirac encoding and decoding, apply ‘ffmpeg -codecs | grep dirac’
DEV.LS dirac Dirac (encoders: vc2 )
To Encode with ffmpeg (100 frames)
ffmpeg -s 352×288 -i .\yuv\akiyo_cif.yuv -c:v dirac cif.drc
To Decode with ffmpeg use:
ffmpeg -f dirac -i cif.drc -vsync 0 -pix_fmt yuv420p raw.yuv
Notes:
for encoding input sources should be divisible by 16
it’s not clear how to set bitrate in Dirac encoding with ffmpeg
23+ years’ programming and theoretical experience in the computer science fields such as video compression, media streaming and artificial intelligence (co-author of several papers and patents).
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