AVC/H.264 encoding
May 1, 2009 12:00 PM, By Pierre Larbier
Operators can realize 50 percent efficiency gains over MPEG-2.
However, two PSNR measurements on the same sequence performed with PSNR-optimized configurations can reveal a lot. In this case, the encoder capable of providing the highest PSNR will also be able to provide the best video quality.
When evaluating coding efficiency, it is customary to use the PSNR of the luma component only. If chroma has to be taken into account, a combined PSNR metric is often used:
CombinedPSNR = 0.8*YPSNR + 0.1*UPSNR + 0.1*VPSNR
This metric gives a good idea of the overall coding efficiency while keeping the chroma visual importance.
Encoder configurations
Figure 2. Chroma 4:2:0 subsampling locations
Click to enlarge
AVC/H.264 encoders are configured either in High Profile or High 4:2:2 Profile. AVC/H.264 profiles below the High 4:2:2 Profile process the video as 4:2:0. Because SDI transports 4:2:2 signals, chroma components need to be subsampled vertically prior to encoding and upsampled after decoding. The intent was to simplify the design as well as lower the bit rate needed to transmit compressed video.
A side effect of this process is reduced chroma detail. This is usually not a problem, however, because the human eye is not very sensitive to color information.
Even though the AVC/H.264 standard allows for six possible locations for the chroma samples relative to the luma samples, only the standard MPEG location is widely used. As shown in Figure 2, two schemes are available to handle progressive and interlaced sources.
Artifacts introduced by 4:2:2 to 4:2:0 conversions
Figure 3. Source picture (mobile and calendar)
The AVC/H.264 standard does not precisely define how the chroma subsampling or upsampling should be performed, leaving this decision to manufacturers. Thus, there can be a mismatch between the downsampling filter in the encoder and the upsampling filter in the decoder. Misinterpretation of the progressive or interlaced nature of the video can lead to faulty decoding of the whole chroma plane.
Figure 4. Source picture after five 4:2:2 to 4:2:0 conversions
Video quality has to be kept at the highest possible level to handle several encoding-decoding steps. A mismatch in the chroma sampling can introduce color degradations that worsen with each generation, including color bleeding, loss of color contrast and details, chroma displacement relative to luma, and creation of interlaced (or progressive) color artifacts on progressive (interlaced, respectively) pictures.
It should be noted that an interlaced (alternatively, progressive) chroma artifact might confuse encoders in the cascading process, significantly reducing their coding efficiency and introducing luma degradation.
Figures 3 and 4 give an example of such problems after only five generations. The only defect introduced was a mismatch in the chroma resampling filters — polyphase bicubic downsampler before encoding and simple tent upsampler after decoding.
4:2:2 processing
Figure 5. 4:2:2 vs. 4:2:0 bit rate comparison
Click to enlarge
The only way to avoid those artifacts is to process the video in its original color format (4:2:2). This is possible using the AVC/H.264 High 4:2:2 Profile.
The drawbacks of encoding in 4:2:2 include a moderate bit rate increase (for a given quantizer) relative to 4:2:0 encoding. (See Figure 5.) This bit rate increase does not lead to a loss of video quality with the first generation; in fact, the perceived quality is roughly the same.
As shown in Figure 6, on the next page, objective measurements such as PSNR reflect this subjective impression.
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