Feature Requests: x264 10 bits
What does “saving bandwidth” mean?
Bandwidth is saved if less bit-rate is needed, keeping the same video quality. When comparing
encoders or encoding technologies, both bit-rate and video quality has to be considered: less bit-rate
keeping the same quality is equivalent to more quality keeping the same bit-rate.
What is a better video quality?
The video quality is better when the decoded video is closer to the source. The most common way of
expressing “closeness” in video processing is stating that there are less errors relative to the pixel bitdepth. For example, a 1 error bit on a 8-bit signal provide the same relative error than 3 bits of error
in a 10-bit signal: 7 bits only are actually meaningful in both cases.
Where are errors introduced in a MPEG encoder?
Video is compressed with a lossy process that introduce errors relative to the source: the
quantization process. Since those errors are relative, they do not depend of the source pixel
bit-depth; but only on encoding tools efficiency when a given bit-rate has to be achieved.
What happens with 10-bit video source when using a “perfect” MPEG encoder?
A 10-bit source carries about 20% more information than an 8-bit one. Assuming the same encoding
tools efficiency, the same bit-rate can be achieved just by quantizing 4 times more (2 bits). Since the
relative error is the same in both cases, the video quality should not change.
So why does a AVC/H.264 10-bit encoder perform better than 8-bit?
When encoding with the 10-bit tool, the compression process is performed with at least 10-bit
accuracy compared to only 8-bit otherwise. So there is less truncation errors, especially in the motion
compensation stage, increasing the efficiency of compression tools. As a consequence, there is less
need to quantize to achieve a given bit-rate.
The net result is a better quality for the same bit-rate or conversely less bit-rate for the same quality:
between 5% and 20% on typical sources.
info :
http://x264.nl/x264/10bit_02-ateme-w..._bandwidth.pdf