[MPlayer-DOCS] encoding-guide: inconsistency info about threads in x264enc

Andrew Savchenko Bircoph at list.ru
Thu Jan 11 02:31:51 CET 2007


I've found some contradiction in encoding-guide documentation:

1) Section "Making a high quality MPEG-4 ("DivX") rip of a DVD 
movie", subsection "Choosing the video codec" says:
x264 also allows multi-threaded encoding, which currently speeds up 
encoding by 15-30% (depending on the encoding settings) while 
lowering PSNR by about 0.05dB. 
But in recently added part "Using MEncoder to create 
QuickTime-compatible files", subsection "Encoding example" anyone 
can find:
If you have multi-processor machine, you can add threads=auto. This 
increases encoding speed by about 94% per CPU core, with very 
little quality penalty (about 0.005dB for dual processor, about 
0.01dB for a quad processor machine).

I uderstand, that speedup values are only for rude estimation 
purposes. But they differs very greatly: 15-30% vs ~94%. One of 
them should be fixed... I've no multi-processor system, so I can't 
check this values on my own. 

PSNR impact also varies... Maybe algorithms in x264enc was changed 
since 1st quotation was wrote, or perhaps this value is highly 
dependant on source material.

2) The last quatation mentions about 'threads=auto' option, but 
'auto' is not mentioned in man page as acceptable value:
Split each frame into slices and encode them in parallel (default: 
1).  Also  allows  multithreaded  decoding  if  the decoder 
supports it (lavc doesnot).  This has a slight penalty to 
compression.  Requires that libx264 was compiled  with  pthread 
support; if not, this option will produce a warningand enables 
slices but not multithreading.
I've test option threads=auto, it seems work fine. So man page 
should be updated.
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