What is Slices?

[gelo333]

Help me please. Explain in simple terms what are Slices in video coding? I see this setting in the encoder (264, 265), I see that the size of the resulting file is changing, but I don’t see the difference in the image in the Windows player. The descriptions of the Slices that I have found do not explain anything, as if you need to know something else for this.

[Selur]

here's my try:

!highly! simplified: Sliced encoding basically means that a frame is split into parts (=slices) which are basically frames on their own.
(by default only one slice is used per frame and thus frame and slice are used synonymous)

A bit more abstract (but still far away from being really technical):
In H.265 video encoding, slices are a way to divide a frame of video into smaller regions, or "slices", that can be processed and transmitted independently. Each slice consists of a set of macroblocks, which are the basic units of video encoding.

Dividing a frame into slices can improve parallelism and enable more efficient use of hardware resources, since multiple slices can be processed simultaneously. It can also improve error resilience, since errors in one slice are less likely to affect adjacent slices.

There are several types of slices in H.265, including regular slices, dependent slices, and independent slices, each with their own characteristics and trade-offs. The choice of slice type (I, P, B) and size can have a significant impact on video quality and encoding efficiency.

Usually, using sliced encoding (= using more than one slice per frame) is a way to improve multithreading efficiency (during encoding and decoding), but will lower the encoding efficiency.

Cu Selur

Ps.: side note: Slices are always decoded in the order of the raster scan, so a slice can only consist of connected CTUs.

[gelo333]

thanks for the answer. I correctly understood that slices speed up coding, but increase the file size? and slices are not used to improve video quality.

[Selur]

yes, slicing should:
a. increase encoding (and on playback, decoding) speed assuming your cpu was not utilized fully beforehand.
b. decrease the quality for a given file size (= lower the compression efficiency).


Cu Selur