AVC/H.264 encoding
May 1, 2009 12:00 PM, By Pierre Larbier
Operators can realize 50 percent efficiency gains over MPEG-2.
10-bit versus 8-bit video compression
Figure 6. 4:2:2 vs. 4:2:0 quality comparison
Click to enlarge
All AVC/H.264 profiles above High Profile encode pixels with a bit depth greater than 8 bits:
- High 10 Profile: 8 bits up to 10 bits
- High 4:2:2 Profile: 8 bits up to 10 bits
- High 4:4:4 Predictive Profile: 8 bits up to 14 bits
- High 10 Intra Profile: 8 bits up to 10 bits
- High 4:2:2 Intra Profile: 8 bits up to 10 bits
- High 4:4:4 Intra Profile: 8 bits up to 14 bits
- CAVLC 4:4:4 Intra Profile: 8 bits up to 14 bits
The bit depth increase provides improved accuracy to the AVC/H.264 compression scheme, including motion compensation, intra prediction and in-loop filtering. Figure 7 illustrates the gains that can be achieved using greater than 8-bit processing (measured in 4:2:0 with an 8-bit source upscaled to 10, 12 or 14 bits).
Figure 7. Coding efficiency gain using greater than 8-bit
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Extensive experimentation demonstrates that the coding efficiency gains are highest on videos that contain shallow textures and low noise.
Figure 8 illustrates PSNR improvement gained from increasing the bit depth to 10 or 12 bits on relatively noisy, textured standard sequences.
These curves illustrate that the gain is smaller as the bit rate is reduced, but cannot be considered negligible, making this feature attractive for low bit rate applications.
Figure 8. Coding efficiency gain using greater than 8-bit coding
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Encoding in 10 bits can achieve a PSNR increase of more than 1dB on some natural sequences and measures an average of 0.25dB at 60Mb/s on a varied test set of broadcast HD sequences. This translates to an average savings of about 5 percent and up to 20 percent, while retaining the same video quality.
However, further testing shows that increasing the bit depth to 12 bits (or even 14 bits) provides a much smaller coding efficiency gain (up to about 1 percent in bit rate savings), but again, no loss greater than 8 or 10 bits.
Lastly, there is no relation between 10-bit encoding and the frame format. The advantages are the same whether the source video is HD, SD, progressive or interlaced.
Beyond coding efficiency improvements
A noteworthy benefit of 10-bit processing is perceivable gains in the reduction of three kinds of artifacts: contouring, banding and mosquito noise.
This gives a better aspect to plain surfaces and shallow textured areas (smoke, clouds, sky, sunset, etc.) because it slightly improves object edges. These impairments are otherwise difficult to reduce using traditional tools.
If the source is not too noisy and the plain area is not too large relative to the picture surface, lowering the local quantizer produces an effect close to the one achieved with 10-bit processing. This has several negative impacts, the most important one being a strong reduction of the coding efficiency and a degradation of the rate-control stability.
Another approach is to hide the defects by adding noise during the encoding process, but the amount of added noise needed to achieve the same visual improvement is significant. Even at high bit rates, this can lead to an unacceptable reduction in coding efficiency.
The gains are the result of increased accuracy in internal computations; 8-bit video sources also show improvements. Interestingly, the reduction of artifacts provided by 10-bit processing is perceivable even on standard (8-bit or dithered 6-bit) LCD panels.
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