Questions regarding WPP in HEVC decoding

[Rosh]

Hi,

I was just wondering, does anyone know about the following regarding HEVC encoding/decoding:

(a) is WPP optional during encoding ?
- if WPP is enabled when encoding, is it mandatory to use it during decoding ? (for example, hypothetically , what if your system is a single core , and does not support threading ? (thinking about embedded systems)

(b) if WPP is enabled at the encoder, then can the decoder only use WPP for certain frames (say I-frames) and not use WPP for other frames ? (e.g. P/B frames)

(c) Possibly an implementation specific question, but if (b) was not possible by default, then is it a lot of effort to get x265 for example, and change it such that WPP can be optional at the frame-level ?

(d) How does WPP work when it comes to P/B frames (dependency) ? when decoding a certain CTB line in a B-frame, does that line need a dependent CTB line from a P and I frame ?


Sorry for the long list of questions. I have searched online and couldn't find any solid answers. The information is for research purposes. Specifically looking at parallel HEVC decoding.

Many thanks !
Rosh

[Selur]

Normally every algorithm which is multithreaded can be used with a single thread. (main down side of a multithreaded approach normally is the additional memory and processing power to manage the threading, so when running a multithreaded algorithm with one thread against a single threaded algorithm with one thread, the single threaded algorithm will normally win in speed and memory consumption)

WPP is optional during encoding and iirc. even when used it should still be optional during decoding, so i see no reason why one couldn't write a decoder which only uses parallel decoding for during the decoding of some frames. (not really seeing a reason why one should do such a thing.

[Rosh]

Hi Selur,

thank you for replying.

I guess I was wondering, if WPP can be disabled/enabled at the frame-level because it would offer more flexibility to a scheduler of a platform with a large number of cores. I am working on decoding multilpe HEVC streams, so there may exist cases where there might be imbalance in the system. So if there are gaps in the schedule, the resource manager can split up a frame into CTB rows and process the frame in parallel or continue to go on a frame-level parallel path.

Will it be a lot of work say to modify the HM reference decoder to switch on/off WPP at a frame level ? or say specify the number of threads the decoder can use for WPP at the frame-level (e.g. either 1 or n threads)

thanks!

[Selur]

Changing the number of threads the decoder can use would probably not that much work from what I remember, but I haven't looked at the reference decoder that much, so I might be wrong.

Cu Selur