New Option in Blu-Ray AVC Profiles — Number of Slices?

[Stephen Mann]

Here’s a quote from Jan Ozer, a compression guru:

Finally, though it’s not technically related to B-frames, consider the number of Slices per picture, which can be 1, 2, or 4. At a value of 4, the encoder divides each frame into four regions and searches for redundancies in other frames only within the respective region. This can accelerate encoding on multicore computers because the encoder can assign the regions to different cores. However, since redundant information may have moved to a different region between frames—say in a panning or tilting motion—encoding with multiple slices may miss some redundancies, decreasing the overall quality of the video.

In contrast, at the default value of 1, the encoder treats each frame as a whole, and searches for redundancies in the entire frame of potential reference frames. Since it’s harder to split this task among multiple cores, this setting is slower, but also maximizes quality. Unless you’re in a real hurry, I recommend the default value of 1.

Here’s the full article.

Steve Mann
MannMade Digital Video
http://www.mmdv.com

[Norman Black]

A slice divides the frame into multiple independent pieces. Each encoded independently. So with more slices you lose some compression efficiency.

AFAIK, Blu-ray requires 4 slices in its specification. My only info on this is from years ago when the x264 encoder was not Blu-ray compatible until it supported slices.

It is easier to implement multiple slower very simple decoders in hardware than a faster more complex one. Blu-ray allows for very high bitrates and this (slices) was one way to lower cost of Blu-ray player decoders.

[Norman Black]

Update: Blu-ray levels 4.1 and higher require 4 slices.

In Vegas the Sony AVC encoder cannot go to bitrates high enough for level 4.1. The Mainconcept AVC encoder is allowed to go that high and the high bitrate MC AVC Blu-ray templates default to 4 slices.

[Curtis Lewis]

Thank you both for the info. Best to leave it at 1, then. If I have any resource at my disposal, it’s time. 🙂