[x264-devel] Re: Slices in x264

[Måns Rullgård]

Guillaume Poirier <gpoirier at mplayerhq.hu> writes:

> Hi,
>
> List, Peter wrote:
>> Hello everyone,
>> 
>>  
>> 
>> I am new to this group although experienced with H.264. I tried to use
>> the x264 encoder to prepare sequences for an error-resilience test.
>> 
>> I was very surprised, when I discovered, that x264 CAN NOT produce
>> slices! Slices are the basic tool to cope with packet-losses over
>> IP-based networks (without retransmission), and in fact can make a huge
>> difference for subjective quality, in particular at high loss-rates.
>
> Very basic indeed. _I_ think it's better to add more advanced parity
> (which allow the errors to be recovered) infos somewhere in the
> transport layer stream than using slices.

Using error correcting coding at the transmission layer substantially
increases the bitrate.  With slices a transmission error will ruin the
rest of the slice, while other slices still decode properly.
Sometimes a little damage here and there is acceptable if it means you
can keep the bitrate down.  Besides, you don't always have control
over the transmission encoding.

Put another way, slices limit the scope of the damage caused by
whatever transmission errors make it through your error correction
layers.

Anyone who has watched digital TV should appreciate the usefulness of
slices.

>> Did I miss something here, or is it true that x264 can only produce 1
>> slice per frame???
>
> It used until r609 to but it was replaced by a much better and faster
> multi-threaded encoding mode.

Multithreaded encoding and slices are really distinct features.
Slices may be desired, as the OP says, with or without multithreading.

-- 
Måns Rullgård
mru at inprovide.com

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